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7: Carbohydrates and Glycobiology

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• Page ID 14961

• Henry Jakubowski and Patricia Flatt
• College of St. Benedict/St. John's University and Western Oregon University
• 7.1: Monosaccharides and Disaccharides Carbohydrate or glycan biochemistry is very complex and difficult, owing to the stereochemical complexity of simple sugars, the large number of positions on the sugars used to form linkages between other sugars to create polymers, the large number of chemical modification to base sugars, and the lack of a genetic template to instruct glycan polymer formation. It is no wonder that our understanding of complex glycans has developed after that of the chemically simpler polymers like nucleic acids a
• 7.2: Polysaccharides Polysaccharides contain many monosaccharides in glycosidic links, and may contain many branches. They serve as either structural components or energy storage molecules. Polysaccharides consisting of single monosaccharides are homopolymers. Among the most common are starch, glycogen, dextran, cellulose and chitin. We'll discuss these grouped together based on whether the acetal link is alpha or beta.
• 7.3: Glycoconjugates - Proteoglycans, Glycoproteins, Glycolipids and Cell Walls
• 7.4: The Sugar Code and Lectin Decoding
• 7.5: Working with Carbohydrates
• 7.6: Chapter 7 Problems - Answer Key
• 7.7: Chapter 7 Problems

Thumbnail: Cellulose molecular structure (CC BY-SA 3.0 Unported; Pintor4257 via Wikipedia )

6.1: Structure and Function- Carbohydrates

Carbohydrates are commonly described as sugars, or saccharides, from the Greek word for sugar. The simplest carbohydrates are called monosaccharides. An example is glucose. Monosaccharides can be joined to make larger molecules. Disaccharides contain two monosaccharides. Sucrose is a disaccharide, containing both fructose and glucose. Mono and disaccharides are sometimes referred to as simple sugars. Polysaccharides are chains of many sugar subunits. Examples include glycogen and cellulose, both of which are polymers of glucose (configured differently).

Carbohydrates are literally “hydrates of carbon.” This name derives from the generalized formula of simple monosaccharides, which can be written in the form of C x (H 2 O) x , where x is a digit typically between 3 and 8. Not all sugars have this formula, however. Deoxyribose, the sugar found in every nucleotide in a DNA molecule lacks one oxygen and thus has the formula C 5 H 10 O 4 .

Carbohydrates are important in cells as energy sources (especially glucose, glycogen, and amylose), as markers of cellular identity (oligosaccharides on the surface of cells of multicellular organisms), as structural components (cellulose holding up plants), and as constituents of nucleotides (ribose in RNA, deoxyribose in DNA).

The building blocks of all carbohydrates are the monosaccharides.

Shown below are Fischer projection formulas for a group of common monosaccharides. Fischer projection formulas are similar but not identical to organic structural formulas. Carbons in the sugar are represented with the elemental symbol C at the end of the chain, but also are represented by vertices (such as carbon 1 in D-Ribose below) and by intersecting perpendicular lines (carbons 2, 3, and 4 in D-Ribose).

The tetrahedral arrangement around the carbons in the chain of a monosaccharide are represented as flat, with 90 degree bond angles, in the Fischer projection. This is not an accurate representation of the three-dimensional molecules. To interpret these structures as 3D models, each carbon within the chain can be considered in sequence. The bonds shown vertically in the Fischer projection are oriented back, away from the viewer, while the horizontal bonds (to H and OH) emerge forward, out of the plane of view.

Fischer projections make for easy drawing and comparison of carbohydrate structure but their interpretation is prone to error.

Figure 2.148 – Common sugar structures

Monosaccharides

The most common monosaccharides include glucose, fructose, galactose, ribose, and mannose. Of these sugars, all but one (fructose) exists as an aldehyde. Fructose and some other less well known sugars are ketones. Figure 2.148 shows the structure of these sugars.

By convention, the letters ‘ose’ at the end of a biochemical name flags a molecule as a sugar. Thus, there are glucose, galactose, sucrose, and many other ‘-oses’. Other descriptive nomenclature involves use of a prefix that tells how many carbons the sugar contains. For example, glucose, which contains six carbons, is described as a hexose. The following list shows the prefixes for numbers of carbons in a sugar:

Other prefixes identify whether the sugar contains an aldehyde group (aldo-) or a ketone (keto-) group. Prefixes may be combined. Glucose, which is a 6-carbon sugar with an aldehyde group, can be described as an aldohexose. The list that follows gives the common sugars and their descriptors. •Ribose = aldo-pentose

•Glucose = aldo-hexose

•Galactose = aldo-hexose

•Mannose = aldo-hexose

•Fructose = keto-hexose

Diastereomers

Figure 2.149 – Diastereomers

Sugars may have multiple chiral carbons and thus differ from each other in the configuration of groups around those asymmetric carbons. Two sugars having the same chemical form (aldoses, for example) and the same number of carbons, but that differ only in the stereochemical orientations of their carbons are referred to as diastereomers (Figure 2.149). For example, glucose, galactose, and mannose all have the formula of C6H12O6, but are chemically distinct from each other in the orientation of groups around the carbons within them.

Figure 2.150 – Epimers – D-Galactose and D-Glucose differ only in the configuration of carbon #4

Enantiomers and epimers

Figure 2.151 – Enantiomers – D-Glucose (left) and L-Glucose (right) are mirror

If two sugars are identical except for having one chiral carbon arranged differently (such as images glucose and galactose – Figure 2.150), they are considered epimers of one another. If two sugars are mirror images of each other, they are enantiomers (Figure 2.151). Biochemical notation uses the letters D and L to describe monosaccharide stereochemistry in a very particular way. As a result, one enantiomer will be given an L designation while the other is D. So L-glucose is the mirror image of D-glucose.

Movie 2.6 – Conversion of glucose from a straight chain form to a ring form

Figure 2.152 – Conversion of D-fructose between furanose (top right), linear (left), and pyranose (bottom right) forms Image by Pehr Jacobson

Sugars with five and six carbons can readily cyclize (Figure 2.152, Movie 2.6) in solution. When they do, a new asymmetric carbon is created that didn’t exist in the same sugars when they were in the straight chain form, as the carbon to oxygen double bond converts to an alcohol. This carbon has a special name – it is called the anomeric carbon and (like the other asymmetric carbons in sugars) it can have the hydroxyl in two different positions. These positions are referred to as α and β . Sugars, such as α-D-glucose and β-D-glucose that differ only in the configuration of the anomeric carbon are referred to as anomers (Figure 2.153).

Figure 2.153 – Anomers – α-DGlucose and β-D-Glucose differ only in the configuration of the anomeric carbon #1

Sugars cyclizing to form rings with five atoms in them (see fructose in Figure 2.128) are referred to as furanoses (named for furan) and those forming rings with six atoms, such as glucose in the same figure, are called pyranoses (named for pyran). The carbonyl carbon becomes the anomeric carbon in the ring by binding to the oxygen of a hydroxyl elsewhere in the chain. α- and β- forms of a given sugar can readily “flip” between each form in solution, so long as the anomeric hydroxyl is free, because the bonding in cyclic forms is unstable, so molecules interconvert in solution. Most pentoses and hexoses can form both furanose and pyranose structures (Figure 2.152).

Figure 2.154 – A furanose (left) and a pyranose (right)

Linking the anomeric hydroxyl to another group will create a structure called a glycoside which will remain locked in whichever α- or β- configuration they were in when the anomeric hydroxyl was altered.

Boat/chair conformations

Figure 2.155 – Chair and boat forms of glucose

Orbitals of carbon prefer to be in tetrahedral conformations and this means that the bonds between carbons in a ring do not lie flat. Indeed, rings “pucker” to try to accommodate this tendency, giving rise to different 3D forms for any given sugar. Some of these forms resemble boat structures, which others resemble chairs or envelopes (Figure 2.155). The stablest (and thus most abundant) of these forms have all of the hydoxyls in the equatorial positions, resulting in less steric hindrance.

Modified monosaccharides

Figure 2.156 – Modified sugars. Locations of glycosidic carbon indicated with red asterisks. All are glycosides except N-acetylglucosamine

Many chemical modifications can occur on sugar residues (Figure 2.156). Common ones include oxidation, reduction, phosphorylation, and substitution of an amine or an acetylamine for a hydroxyl. The ones that affect the anomeric hydroxyl group make glycosides (Figure 2.157), whereas modifications that don’t affect the anomeric hydroxyl, (glucose-6-phosphate, for example), do not.

Figure 2.157 – Formation of a glycosidic bond

Oxidation/reduction

Figure 2.158 – A positive Benedict’s test starting at left and moving right Wikipedia

The last considerations for simple sugars relative to their structure are their chemical reactivity and modification. Sugars that are readily oxidized are called ‘reducing sugars’ because their oxidation causes other reacting molecules to be reduced. A test for reducing sugars is known as Benedict’s test. In it, sugars are mixed and heated with an alkaline solution containing Cu++. Reducing sugars will donate an electron to Cu++, converting it to Cu+, which will produce cuprous oxide Cu2O, as an orange precipitate (Figure 2.158). Since Cu++ solution is blue, the change of color provides an easy visual indication of a reducing sugar.

Figure 2.159 – Reducing and non-reducing sugars

The aldehyde group of aldoses is very susceptible to oxidation, whereas ketoses are less so, but can easily be oxidized if, like fructose, they contain an α-hydroxyl and can tautomerize to an aldose. Most monosaccharides are reducing sugars. This includes all of the common ones galactose, glucose, fructose, ribose, xylose, and mannose. Some disaccharides, such as lactose and maltose are reducing sugars since they have at least one anomeric carbon free, allowing that part of the sugar to linearize and yield an aldose. Sucrose, on the other hand has no anomeric carbons free – both are involved in a glycosidic linkage, so they cannot linearize and thus it is not a reducing sugar.

Oxidation and reduction of sugars can occur in cells. As we will see, phosphorylation of sugars occurs routinely during metabolism.

Glucuronic acid

Figure 2.160 – Glucuronic acid

One oxidation product of glucose is glucuronic acid, a six carbon molecule where the CH 2 OH on carbon six is oxidized to a carboxylic acid (Figure 2.160). Related oxidized sugars include galacturonic acid and mannuronic acid. Glucuronic acid is commonly conjugated to other molecules in the liver/bile by UDP-glucuronyltransferase enzymes to make the molecules more water soluble for excretion, since the carboxyl group of glucoronic acid ionizes readily at physiological pH. The reactions are usually done starting with glucuronic acid linked to UDP (UDPGlucuronic Acid). In addition, glucuronic acid is made from a UDP-glucose precursor. Glucuronic acid is a common constituent of glycosaminoglycans, proteoglycans, and glycoglycerolipids. Glucuronic acid is found in heparin, dermatan sulfate, chondroitin sulfate, hyaluronic acid, and keratan sulfate. Glucuronic acid is also a precursor of ascorbic acid (Vitamin C) in organisms that synthesize this compound.

Sugar alcohols

Figure 2.161 – Sorbitol (also called glucitol)

Reduction of aldoses or ketoses by hydrogenation produces the corresponding sugar alcohols. The compounds are widely used as thickeners of food or as artificial sweeteners, due to their ability to stimulate sweet receptors on the tongue. Common sugar alcohols (sugar progenitor in parentheses) include glycerol (glyceraldehyde), xylitol (xylose), sorbitol (Figure 2.161 – from glucose), galactitol (galactose), arabitol (arabinose), and ribitol (ribose). Most of these compounds have a sweetness of between 0.4 and 1.0 times as sweet as sucrose, but provide considerably fewer calories per weight. Xylitol is the sweetest of them with a sweetness equal to that of sucrose.

Figure 2.162 – Structure of sucralose

Sugar alcohols are used sometimes to mask the aftertaste of other artificial sweeteners. Many of them also produce a cooling sensation upon dissolving, due to that being an endothermic process for them, resulting in a pleasant mouth sensation. Last, they are poorly absorbed by intestines, and so have a low glycemic index.

Artificial sweeteners

Figure 2.163 – Common disaccharides – glycosidic bonds in rectangles

Artificial sweeteners are compounds that stimulate taste receptors for sweetness, but are metabolized for energy inefficiently at best. Such compounds frequently are many times sweeter than table sugar (sucrose) on a weight/weight basis and are referred to as “intensely sweet.” Most of the artificial sweeteners are not carbohydrates, but rather are able to stimulate the same sweet receptors that sugar does. Seven such compounds are approved for use in the U.S. – stevia, aspartame, sucralose, neotame, acesulfame potassium, saccharin, 1 1 1 2 4 4 and advantame. The sugar alcohol known as sorbitol is also sometimes used as an artificial sweetener.

Disaccharides

Disaccharides (Figure 2.163) are made up of two monosaccharides. The most common ones include sucrose (glucose and fructose), lactose (galactose and glucose), and maltose (glucose and glucose). All of the common disaccharides contain at least one glycosidic bond. We name the disaccharides according to which carbons are linked to each other and the how the anomeric carbon of the glycosidic bond is configured. Lactose, for example, is described as β-Dgalactopyranosyl-(1→4)-D-glucose, or more succinctly as having an α-1,4 glycosidic bond.

Oligosaccharides

Figure 2.164 – A Branched oligosaccharide attached to an RGroup

As their name implies, oligosaccharides (Figure 2.164) are comprised of a few (typically 3 to 9) sugar residues. These often, but not always contain modified sugars. Unlike all of the other saccharides, oligosaccharides are not typically found unattached to other cellular structures. Instead, oligosaccharides are found bound, for example, to sphingolipids (making cerebrosides or gangliosides) or proteins (making glycoproteins).

Oligosaccharides in membrane glycoproteins play important roles in cellular identity/ recognition. The patterns of oligosaccharides displayed on the extracellular face of the plasma membrane acts as a sort of barcode that identifies specific cell types. The immune system recognizes these identity tags in the body. “Foreign” oligosaccharide structures trigger the immune system to attack them. While this provides a very good defense against invading cells of an organism, it also can pose significant problems when organs are transplanted from one individual into another, with rejection of donated organs, in some cases.

Organelle targeting

The oligosaccharides that are attached to proteins may also determine their cellular destinations. Improper glycosylation or errors in subsequent sugar modification patterns can result in the failure of proteins to reach the correct cellular compartment.

For example, inclusion cell disease (also called I-cell disease) arises from a defective phosphotransferase in the Golgi apparatus. This enzyme normally catalyzes the addition of a phosphate to a mannose sugar attached to a protein destined for the lysosome. In the absence of a functioning enzyme, the unphosphorylated glycoprotein never makes it to the lysosome and is instead exported out of the cell where it accumulates in the blood and is excreted in the urine. Individuals with Icell disease suffer developmental delays, abnormal skeletal development, and restricted joint movement.

Glycosylation

Figure 2.165 – N-linked glycosylation in various organisms Wikipedia

Sugars are commonly attached to proteins in a process called glycosylation. Typically the attachment is to a hydroxyl or other functional group. The majority of proteins synthesized in the endoplasmic reticulum are glycosylated.

N-glycans on cell surfaces play roles in the immune system. The immunoglobulin types (IgG, IgA, IgE, IgD, and IgM) have distinct glycosylation patterns that confer unique functions by affecting their affinities for immune receptors. Glycans also are important in self/non-self identity is tissue rejection and autoimmune diseases.

Glycoproteins

Figure 2.170 – Erythropoietin Wikipedia

Glycoproteins are a very diverse collection of saccharide-containing proteins with many functions. Attachment of the saccharide to the protein is known as glycosylation. Secreted extracellular proteins and membrane proteins with exposed extracellular regions are often glycosylated. Saccharides attached to these may be short (oligosaccharides) or very large (polysaccharides). Glycoproteins play important roles in the immune system in antibodies and as components of the major histocompatibility complex (MHC). They are important for interactions between sperms and eggs, in connective tissues and are abundant in egg whites and blood plasma. Two glycoproteins (gp41 and gp120) are part of the HIV viral coat and are important in the infection process. Some hormones, such as erythropoietin, human chorionic gonadotropin, follicle-stimulating hormone and luteinizing hormone are also glycoproteins.

Glycation is a chemical process (nonenzymatic) that occurs when a protein or lipid covalently binds to a sugar, such as glucose or fructose. Glycation differs from glycosylation in that the latter process is controlled by enzymes and results in specific attachment of specific sugars to biomolecules. Glycation, by contrast, is driven by two properties of monosaccharides 1) their chemistry and 2) their concentration. Glycations may be endogenous (occurring in an organism) or exogenous (occurring external to an organism).

Exogenous glycation arises most commonly as a result of cooking of food and this results in attachment of sugars to lipids and/or proteins to form advanced glycation endproducts (AGEs). At temperatures above 120°C, AGE production occurs readily and contributes to the taste and the appearance of the food we eat.

Browning of food, for example, is a product of glycation and is enhanced as the sugar content of a food increases. Browning of french fries is often enhanced, for example, by adding sugar to them. The formation of a crust of bread or the toasting of bread are other examples. These glycations are products of the Maillard reaction in which a reactive sugar carbonyl group combines with a nucleophilic amine of an amino acid. The process is favored in an alkaline environment, when amines are less protonated. The formation of the harder shell of a pretzel, for example, results from addition of lye to the exterior. At higher temperatures, though, a carcinogen known as acrylamide can be formed by reactions involving asparagine.

Figure 2.171 – Repeating unit of amylose

Endogenous glycation, on the other hand, arises with a frequency that is proportional to the concentration of free sugar in the body. These occur most frequently with fructose, galactose, and glucose in that decreasing order and are detected in the bloodstream. Both proteins and lipids can be glycated and the accumulation of endogenous advanced glycation endproducts (AGEs) is associated with Type 2 diabetes, as well as in increases in cardiovascular disease (damage to endothelium, cartilage, and fibrinogen), peripheral neuropathy (attack of myelin sheath), and deafness (loss of myelin sheath).

The formation of AGEs increases oxidative stress, but is also thought to be exacerbated by it. Increased oxidative stress, in turn causes additional harm. Damage to collagen in blood cells causes them to stiffen and weaken and is a factor in hardening of the arteries and formation of aneurysms, respectively. One indicator of diabetes is increased glycation of hemoglobin in red blood cells, since circulating sugar concentration are high in the blood of diabetics. Hemoglobin glycation is measured in testing for blood glucose control in diabetic patients.

Polysaccharides

Long polymers of sugar residues are called polysaccharides and can be up to many thousands of units long. Polysaccharides are found free (not attached to other molecules) or bound to other cellular structures such as proteins. Some polysaccharides are homopolymers (contain only one kind of sugar). Others are heteropolymers (glycosaminoglycans, hemicellulose). Polysaccharides function in energy storage (nutritional polysaccharides, such as glycogen, amylose, amylopectin, e.g.), structure enhancement (chitin, cellulose, e.g.), and lubrication (hyaluronic acid, e.g.). These individual categories of polysaccharides are discussed below.

Nutritional polysaccharides

This group of polysaccharides is used exclusively for storage of sugar residues. They are easily easily broken down by the organism making them, allowing for rapid release of sugar to meet rapidly changing energy needs.

Figure 2.172 – Another view of amylose

Amylose has the simplest structure of any of the nutritional polysaccharides, being made up solely of glucose polymers linked only by α-1,4 bonds (Figure 2.171 & 2.172). (Note that the term ‘starch’ is actually a mixture of amylose and amylopectin). Amylose is insoluble in water and is harder to digest than amylopectin (see below). The complexing of amylopectin with amylose facilitates its water Figure 2.172 – Another view of amylose solubility and its digestion. Amylose is produced in plants for energy storage and since plants don’t have rapidly changing demands for glucose (no muscular contraction, for example), its compact structure and slow breakdown characteristics are consistent with plants’ needs.

Amylopectin and glycogen

Figure 2.173 – Structure of glycogen

More complicated homopolymers of glucose are possessed by amylopectin in plants and glycogen (Figure 2.173) in animals. Both compounds contain long glucose chains with α-1,4 bonds like amylose, but unlike amylose, these long chains have branches of α-1,6 bonds. Amylopectin is the less-branched of the two, having such bonds about every 25-30 residues, whereas glycogen has branches about every 8-12 residues.

Branching plays important roles in increasing water solubility and in providing more “ends” to the polymer. In animals, glycogen is broken down starting at the ends, so more ends means more glucose can be released quickly. Again, plants, which have a lower need for quick release of glucose than animals get by with less branching and fewer ends.

Structural polysaccharides

Figure 2.174 – Cellulose with β-1,4 links between glucose sugars

An additional function of polysaccharides in cells relates to structure. Cellulose, which is a polymer of glucose with exclusive β-1,4 linkages between the units (Figure 2.174) is an important structural component of plants and fungi cells. Notably, most non-ruminant animals are unable to digest this polymer, as they lack the enzyme known as cellulase.

Ruminants, such as cattle, however, contain in their rumen a bacterium that possesses this enzyme and allows them to obtain glucose energy from plants. Another group of polysaccharides found in plant cell walls is the hemicelluloses. This class of molecules encompasses several branched heteropolymers of (mostly) D-pentose sugars along with a few hexoses and L-sugars as well. Hemicelluloses are shorter than cellulose (500-3000 sugars versus 7000-15,000 sugars).

Monomer sugars of polysaccharides besides glucose include xylose, mannose, galactose, rhamnose, and arabinose. Xylose is usually present in the greatest amount (Figure 2.175).

Figure 2.175 Xylose

Figure 2.176 – Chitin with β-1,4 links between N-acetylglucosamine sugars

Chitin (Figure 2.176) is another structural polysaccharide, being comprised of N-acetylglucosamine units joined by β-1,4 linkages. It is a primary component of the cell walls of fungi and is also prominent in the exoskeletons of arthropods and insects, as well as the beaks and internal shells of cephalopods (Figure 2.177). Chitin’s structure was solved by Albert Hofmann in 1929. It is like cellulose except for the acetylamine group replacing the hydroxyl on position 2. This change allows hydrogen bonding to occur between adjacent polymers, thus providing greater strength.

Another group of structural polysaccharides is the pectins (Figure 2.178). These com pounds are present in most primary plant cell walls and are abundant in non-woody parts of terrestrial plants. They are rich in galacturonic acid (α-1,4 links with no branches – Figure 2.179) and are used commercially as a gelling agent in jams/jellies, as well as a stabilizer in fruit juices and milk drinks. Pectin consumption may result in reduced blood cholesterol levels due to its tendency to 1) bind cholesterol and 2) to increase viscosity in the intestinal tract, thus reducing absorption of cholesterol from food. Pectins also trap carbohydrates in the digestive system and reduce their rate of absorption.

Figure 2.177 – Chitin in the wing of a sap beetle Wikipedia

Figure 2.178 – A powdered form of pectin

Lectins are not carbohydrates, but proteins that specifically bind to carbohydrate molecules found in animals and plants (where they are known as phytohemagglutinins) and are each highly specific for certain sugars. They function in cellular and molecular recognition, as well as cell adhesion. One lectin recognizes hydrolytic enzymes containing mannose-6-phosphate and targets them to be delivered to lysosomes. In the innate immune system, a mannose binding lectin helps defend against invading microbes. Other lectins have roles in inflammation and autoimmune disorders.

Figure 2.179 – α-DGalacturonic acid – An important component of pectin polymers

Some viruses and bacteria use lectins to recognize and bind specific carbohydrate residues on the surface of target cells. Flu virus, for example, carries a lectin known as hemagglutinin (Figure 2.180) that binds to sialic acid and is essential for entrance of the virus into the target cell. After binding, the viral particle enters by endocytosis after the hemagglutinin has been cleaved by a protease.

Figure 2.180 – Hemagglutinin

After replication of the virus inside of the cell, hemagglutinin and and a viral enzyme known as neuraminidase cluster in the cell membrane. Viral RNA and associated viral proteins cluster near this membrane site and new viruses bud off in a portion of the cell’s membrane after the hemagglutinin-sialic acid link to the infected cell is released by the neuraminidase cutting the bond between the sialic acid and the rest of the cell surface carbohydrate. Drugs, such as tamiflu, that interfere with neuraminidase work by preventing release of the viral particle. Unreleased particles will tend to aggregate and not function.

Some viral glycoproteins from hepatitis C virus may attach to lectins on the surface of liver cells in their infectious cycle. The bacterium Helicobacter pylori uses a cell surface lectin to bind oligosaccharides on epithelial cells lining the stomach. One lectin known as ricin is a very powerful toxin. It is produced in the endosperm of seeds of the castor oil plant and is of concern as a bioterrorism weapon as a result of its acute toxicity when inhaled or ingested.

Lectins were discovered originally in plants and have been most studied in legumes, but lectins are now known to be widely dispersed in nature. In the immune system, a mannan binding lectin (MBL) helps mediate the first defenses against microorganisms. Other immune system lectins are thought to modulate inflammatory processes and probably play a role in self/non-self recognition that is at the root of rejection of transplanted organs.

Figure 2.186 – Repeating sulfated disaccharide in heparin

Heparin (Figures 2.186 & 2.187) is a modified polysaccharide whose biological function is unclear, but whose ability to prevent clotting of blood is used for medical purposes. Heparin does not dissolve blood clots. Rather, it acts to prevent conversion of fibrinogen to fibrin.

Whether or not heparin is actually used by the body for its anticoagulation property is uncertain. It is stored in the secretory granules of mast cells and released at the point of injury and it has been proposed it is a protection against bacteria and other foreign materials.

Figure 2.187 – Two structures for heparin

Heparin has abundant sulfates and is, in fact, the molecule with the highest negative charge density known. Its size varies from 3 kDa to 30 kDa, with an average of about 15 kDa. The repeating disaccharide of 2-Osulfated iduronic acid and 6-O-sulfated, N- sulfated glucosamine, occupies about 85% of the molecule. Copper salts of heparin help stimulate the synthesis of blood vessels (angiogenic).

Hyaluronic acid

Figure 2.188 – Repeating disaccharide of hyaluronic acid

Hyaluronic acid (also known as hyaluronan or hyaluronate) is a glycosaminoglycan found in connective, epithelial, and nerve tissues. It is an unusual glycosaminoglycan (Figure 2.188), lacking sulfate, is made by hyaluronan synthases on the inner face of the plasma membrane and has a molecular weight in the millions. An average adult body contains about 15 grams of HA, one third of which is replaced every day. The repeating unit in hyaluronic acid is a disaccharide structure of D-glucuronic acid joined to D-N-acetylglucosamine. The compound, which can have upwards of 25,000 units of the disaccharide, is delivered directly into the extracellular matrix by enzymes from its plasma membrane site of synthesis.It is an important component of the extracellular matrix, where it assists in cell proliferation and migration. The polymer provides an open hydrated matrix to facilitate general cell migration whereas directed cell migration occurs via the interaction between hyaluronic acid and specific cell surface receptors. HA interaction with the receptor RHAMM (Receptor for Hyaluronan Mediated Motility) has been shown to be involved in wound repair as well as tumor progression.

Synovial Fluid

Figure 2.189 – Synovial fluid in joint lubrication Wikipedia

The function of hyaluronic acid has traditionally been described as providing lubrication in synovial fluid (the lubricating material in animal joints – Figure 2.189). Along with the proteoglycan called lubricin, hyaluronic acid turns water into lubricating material. Hyaluronic acid is present as a coat around each cell of articular cartilage and forms complexes with proteoglycans that absorb water, giving resilience (resistance to compression) to cartilage. Aging causes a decrease in size of hyaluronans, but an increase in concentration.

Function in skin

Hyaluronic acid is a major component of skin and has functions in tissue repair. With exposure to excess UVB radiation, cells in the dermis produce less hyaluronan and increase its degradation.

For some cancers the plasma level of hyaluronic acid correlates with malignancy. Hyaluronic acid levels have been used as a marker for prostate and breast cancer and to follow disease progression. The compound can to used to induce healing after cataract surgery. Hyaluronic acid is also abundant in the granulation tissue matrix that replaces a fibrin clot during the healing of wounds. In wound healing, it is thought that large polymers of hyaluronic acid appear early and they physically make room for white blood cells to mediate an immune response.

Breakdown of hyaluronic acid is catalyzed by enzymes known as hyaluronidases. Humans have seven types of such enzymes, some of which act as tumor suppressors. Smaller hyaluronan fragments can induce inflammatory response in macrophages and dendritic cells after tissue damage. They can also perform proangiogenic functions.

each of two isomers with different configurations of atoms around one of several asymmetric carbon atoms present

Introductory Biochemistry Copyright © by Carol Higginbotham is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Biochemistry : Monosaccharides and Carbohydrates

Study concepts, example questions & explanations for biochemistry, all biochemistry resources, example questions, example question #1 : monosaccharides and carbohydrates.

Compared to a molecule of galactose, a molecule of glucose has __________  number of hydrogen atoms and __________  number of oxygen atoms. 

the same . . . the same

a greater . . . the same

a greater . . . a greater

the same . . . a greater

The chemical structures of glucose and galactose differ in the C4 atom. The hydroxyl group is oriented differently at this position, altering the stereochemistry at C4. All other carbon atoms in glucose and galactose have the same stereochemistry. This means that glucose and galactose are a special type of structural isomer called epimers.

Disaccharidases are enzymes found in the small intestine that participate in degradation of disaccharides. Which of the following molecules can be broken down by these enzymes?

I. Fructose

II. Sucrose

III. Starch

Sucrose is a disaccharide that is made up of a glucose and a fructose molecule, bound by a glycosidic linkage. A disaccahridase, called sucrase, breaks down sucrose molecules into their component monosaccharides (glucose and fructose), which can then by absorbed by the enterocytes in the small intestine.

Fructose is a monosaccharide that can be directly absorbed by enterocytes. Starch is a complex carbohydrate (polysaccharide) with many glucose molecules attached via glycosidic bonds. 

Example Question #3 : Monosaccharides And Carbohydrates

Which of the following is true regarding the polysaccharides glycogen and cellulose? 

Only glycogen molecules have branching regions

Both molecules involve a glycosidic bond between the 1-carbon and the 6-carbon

Cellulose can be found as either amylose or amylopectin

Humans can digest glycogen because it has beta glycosidic bonds

Example Question #4 : Monosaccharides And Carbohydrates

A researcher is analyzing a compound. He finds that it has the same structure as glucose, but has an altered configuration at one of the stereogenic centers. What can the researcher conclude about the compound?

I. It is an epimer of glucose

II. It is an aldose

III. It could be fructose

Recall that aldoses are carbohydrates that have an aldehyde group at one of the carbons. Glucose and all of its epimers have an aldehyde group at the first carbon; therefore, this compound is an aldose.

Example Question #91 : Macromolecule Fundamentals

Which of the following carbohydrates is most likely to be found in an open chain?

Only 1% of all sugars that have five or more carbons are found in an open chain, thus any sugar that has five or more carbons will be most likely found in its cyclic form. Of the four choices a triose is the only one that has less that five carbons (it has three) the others have 5 (pentose), 6 (hexose), and 7 (heptose).

If a monosaccharide has a single carbonyl group situated between two carbon atoms, which of the following best describes that monosaccharide's classification?

Given no other information about how many carbons are in the chain, any monosaccharide with a carbonyl group on a carbon between two others and not at the terminal carbon of the chain is called a ketose. Aldoses are when the carbonyl group is at the end of the chain. Ribose and aldohexose are incorrect because 1) ribose is too specific, we do not know anything about the structure and ribose indicates a specific monosaccharide, and 2) aldohexose has an aldehyde group, not a ketone group. 

What is a furanose?

A sugar that contains a six-membered ring as part of its cyclical structure

A six-carbon open-chain sugar

A five-carbon open chain sugar

A sugar that contains a five-membered ring as part of its cyclical structure

A furanose is defined as a cyclical sugar structure with a five-membered ring. By contrast, a pyranose is a cyclical sugar structure with a six-membered ring.

Example Question #5 : Monosaccharides And Carbohydrates

What two sugars is lactose composed of?

Galactose and sucrose

Glucose and sucrose

Glucose and galactose

Glucose and fructose

Glucose and glucose

Three common simple sugars are: glucose, fructose, and galactose. Combining these simple sugars leads to the formation of more complex sugar molecules. Glucose and fructose make sucrose. Glucose and galactose make lactose. Two glucose molecules make maltose. 

What is a pyranose?

A sugar that contains a six-membered ring as part of its cyclic structure

A five-carbon open-chain sugar

A sugar that contains a five-membered ring as part of its cyclic structure

A pyranose is a carbohydrate that includes a ring. It is not an open-chain carbohydrate. Additionally, this term is reserved for six-membered, not five-membered carbohydrate ring structures. A sugar which contains a five-membered ring as part of its cyclic structure is called a furanose.

Example Question #6 : Monosaccharides And Carbohydrates

What type of process is occurring as carbohydrates are broken down to carbon dioxide?

Proteolysis

Substitution

Elimination

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Course info, instructors.

• Prof. Michael Yaffe
• Prof. Matthew Vander Heiden

Departments

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• Biochemistry
• Analytical Chemistry
• Organic Chemistry
• Physical Chemistry

Learning Resource Types

General biochemistry.

The first half of this course, taught by Prof. Yaffe, is available on the MITx platform as 7.05x Biochemistry: Biomolecules, Methods, and Mechanisms . The exams below are from the second half with Prof. Vander Heiden, which focuses on metabolism.

Exam 3 (PDF - 2MB)

Exam 4 (PDF - 2.2MB)

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• Chemistry Concept Questions and Answers

Biochemistry Questions

The study of chemical reactions in live beings, including but not limited to living matter, is known as biochemistry. All living organisms and processes are governed by biochemistry. Biochemical processes manage the flow of information through biochemical signalling and the flow of chemical energy through metabolism, resulting in life’s tremendous complexity.

Biochemistry is concerned with the structures, activities, and interactions of biological macromolecules such as proteins, nucleic acids, carbohydrates, and lipids, which create cell structures and perform many of the functions associated with life. The cell’s chemistry is also influenced by the reactions of smaller molecules and ions. These can be inorganic, such as water and metal ions, or organic, such as the amino acids utilised in protein synthesis.

Biochemistry Chemistry Questions with Solutions

Q1: Mention the importance of Biochemistry.

The following topics require a basic understanding of biochemistry.

• The chemical reactions convert food into molecules unique to the cells of a given species.
• The catalytic features of enzymes.
• Utilising the possible energy derived from foodstuff oxidation consumed for the living cell’s many energy-demanding operations.
• The qualities and structure of chemicals make up tissues and cells’ framework.
• To solve fundamental medical and biological challenges.

Q2: How are amino acids classified?

Amino acids are divided into basic and non–essential amino acids.

Non-essential amino acids, including asparagine, glycine, and others, can be produced in the body.

Essential amino acids are amino acids that the body cannot produce and should be received from the diet. Histidine with lysine, for example.

Q3: What do you understand about the secondary structure of proteins?

The secondary structure of a protein is the shape in which a long polypeptide chain can persist due to regular folding of the polypeptide chain’s backbone due to hydrogen bonding between > C = O and the polypeptide chain, – N‑H group.

Q4: Glucose or sucrose are soluble in water, but cyclohexane or benzene (simple six-membered ring compounds) are insoluble. Explain.

A glucose molecule includes five -OH groups, whereas a sucrose molecule includes eight -OH groups. As a result, glucose and sucrose form a lot of H-bonds with water. As a result, they are water-soluble.

On the other hand, Cyclohexane and benzene do not have -OH groups. As a result, they cannot form H-bonds with water and are hence water-insoluble.

Q5: Write the structure of the product obtained when glucose is oxidised with nitric acid.

The structure of the product when glucose is oxidised with nitric acid.

Q6: Glycogen is a branched-chain polymer of α-D-glucose units in which the chain is formed by C1—C4 glycosidic linkage whereas branching occurs by the formation of C1-C6 glycosidic linkage. Structure of glycogen is similar to _____________________ .

(i) Amylose

(ii) Glucose

(iii) Cellulose

(iv) Amylopectin

Answer: (iv) Amylopectin

Explanation: Polysaccharides are made up of several monosaccharide units linked by glycosidic bonds. These are the most frequent carbohydrates found in nature. Amylopectin is a water-insoluble starch that accounts for 80-85% of total starch. It’s an alpha-D-glucose-based branched-chain polymer with C 1 —C 4 glycosidic linkage for the chain and C 1 —C 6 glycosidic linkage for the branching.

Q7: Proteins are found to have two different types of secondary structures viz. α-helix and β-pleated sheet structures. α-helix structure of a protein is stabilised by:

(i) Hydrogen bonds

(ii) van der Waals forces

(iii) Peptide bonds

(iv) Dipole-dipole interactions

Answer: (i) Hydrogen bonds

Explanation: α-helix and β-pleated sheet structures emerge due to the usual folding of the backbone of the polypeptide chain due to hydrogen bonding between >C—O and —NH— group of the peptide bond.

α-Helix is one of the most common methods in which a polypeptide chain constructs all potential hydrogen bonds by twisting into a right-handed screw (helix) with the -NH group of every amino acid residue hydrogen bonded to the >C=O of an adjacent turn of the helix.

Q8: Which of the following acids is a vitamin?

(i) Aspartic acid

(ii) Adipic acid

(iii) Ascorbic acid

(iv) Saccharic acid

Answer: (iii) Ascorbic acid

Explanation: Vitamin C is also known as Ascorbic acid.

Q9: Give one example of each- Monosaccharide, disaccharide and polysaccharide.

• Monosaccharide – Glucose, Fructose etc.
• Disaccharide – Sucrose, maltose etc.
• Polysaccharide – Cellulose, starch etc.

Q10: Name the reagents used to check the reducing nature of carbohydrates.

Tollen’s reagent and Fehling’s solution can be utilised to confirm the reducing nature of sugars.

Q11: Which of the following B group vitamins can be stored in our body?

(i) Vitamin B1

(ii) Vitamin B12

(iii) Vitamin B6

(iv) Vitamin B2

Answer: (ii) Vitamin B12

Explanation: Water-soluble vitamins must be consumed regularly because they are excreted in urine and cannot be stored in the body (except for vitamin B12).

Q12: Describe what you understand by primary structure and secondary structure of proteins.

Primary structure of proteins: Proteins could have one or more polypeptide chains. The basic structure is made up of amino acids linked together in a precise sequence in each polypeptide.

Secondary structure of proteins : The secondary structure of a protein refers to the conformation that polypeptide chains take due to hydrogen bonding.

The following two secondary structures are possible, depending on the size of the R groups:

• α-Helix structure: Intramolecular H-bonds observed between the C = O of one amino acid and N – H of the fourth amino acid.
• β-Pleated sheet structure: The two neighbouring polypeptide chains are retained together by intermolecular H-bonds.

Q13: Differentiate between fibrous proteins and globular proteins. What is meant by the denaturation of a protein?

Denaturation of protein: The native shape of the protein is destroyed, and biological function is lost due to globular protein coagulation under the influence of temperature, pH, and other factors. The produced protein is called denatured proteins, and the phenomenon is denaturation.

Q14: Define the following as related to proteins:

(i) Peptide linkage

(ii) Primary structure

(iii) Denaturation

(i) Peptide linkage: A peptide linkage is an amide linkage created by the loss of a molecule of water between the – COOH group of one amino acid and the NH2 group of the second a-amino acid. The CO–NH bond constituted is termed peptide linkage.

(ii)The primary structure of a protein refers to the exact sequence in which distinct amino acids are present inside it, i.e. the sequence of amino acid linkages in a polypeptide chain. Each protein has a particular order in which amino acids are organised. A change in the sequence results in the production of a different protein.

Primary structure:

(iii) Denaturation: A protein is determined to have a distinct 3-dimensional structure and biological activity in a biological system. In this instance, the protein is considered a native protein. When the native protein is exposed to physical changes, such as temperature changes, or chemical changes, such as pH changes, the H-bonds are disrupted. The globules are uncoiling, and the helix is unfolding due to this disruption. As a result, the protein ends up losing its biological activity. Denaturation refers to a protein’s loss of biological activity. The secondary and tertiary structures of the protein are destroyed during denaturation, while the fundamental structure is unaltered.

Q15: (a) Write the structural and functional differences between DNA and RNA

(b) Name two starch components.

(a) Structural differences:

Adenine (A), the prevalent base in both DNA and RNA is:

• Guanine (G)
• Cytosine (C)

Functional difference: The primary function of DNA is to regulate cell activity, such as telling each organ what to manufacture and do. The primary function of RNA is to produce protein.

(b) Components of starch: Amylose and Amylopectin.

Practise Questions on Biochemistry

Q1: How are enzymes named? Give an example.

Q2: What is glycogen? How is it different from starch?

Q3: (i)Which one of the following is a disaccharide:

starch, maltose, fructose, glucose?

(ii) What is the difference between acidic and basic amino acids?

(iii) Write the name of the linkage joining two nucleotides.

Q4: The two strands in DNA are not identical but are complementary. Explain.

Q5: (i) Which one of the following is a polysaccharide: starch, maltose, fructose, glucose

(ii) Write one difference between α-helix and β-pleated sheet structures of protein.

(iii) Write the name of the disease caused by the deficiency of vitamin B12

Click the PDF to check the answers for Practice Questions. Download PDF

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MCQ on Carbohydrates Part-1

Biochemistry MCQ-01: Carbohydrates, one of the most essential classes of biomolecules, are critical for providing energy, building cell structures, and storing information. Understanding the properties and functions of carbohydrates is therefore essential for any student of biochemistry. The present a set of MCQ on Carbohydrates is designed to challenge and expand your knowledge of this fundamental class of this biomolecule.

Learn more : Carbohydrates MCQ Part-2   |  Part-3   |

1. The general chemical formula of carbohydrate is a.      (CH2O)n b.      (CH2O)2n c.       (CHO)n d.      CnH2nO

2. Which of the following is an aldotriose? a.      Dihydroxyacetone b.      Glyceraldehyde c.       Ribulose d.      Erythrose

3. What is the molecular formula of sucrose? a.      C12H22O11 b.      C10H20O10 c.       C6H12O6 d.      C12H20O11

4. The glycosidic linkage between glucose molecule in maltose is a.      β 1 – 4 b.      α 1 – 2 c.      α 1 – 4 d.      β 1 – 2

Note on Carbohydrates  |   Monosacchrides  |   Disaccharides  |   Polysaccharides  |   Sugar Derivates   |   Glycosaminoglycans  | Glycoconjugates  |

5. A keto pentose will have _____ sterioisomers.

a.      4 b.      6 c.      8 d.     10

6. The reserve food material of green algae is

a.      Laminarin b.      Chrysolaminarin c.      Floridian starch d.      Starch

7. The only carbohydrate which is not having any chiral carbon atom is

a.      Glyceraldehyde b.      Erythrose c.      Dihydroxyacetone d.      Erythrulose

8. Select the odd one from the following.

a.      Arabinose b.      Xylose c.      Lyxose d.      Erythrose

9.  A pentose sugar reported to be present in heart cells

a.      Xylose b.      Arabinose c.      Lyxose d.      Xylulose

10. Which of the following is an epimeric pair?

a.      D-glucose and D-mannose b.      D-glucose and D-galactose c.      D-glucose and L-glucose d.      Both A and B

11.  Select the odd one from the following

a.      Xylulose b.      Dihydroxyacetone c.      Glyceraldehyde d.      Ribulose

12.  Which of the following sugar give a positive result with Seliwanoff test

a.      Sucrose b.      Glucose c.      Galactose d.      Mannose

13.  Which of the following is a keto tetrose?

a.      Erythrulose b.      Xylulose c.      Sorbose d.      Psicose

14.  The glycosidic linkage between two glucose molecules in isomaltose is

a.      α 1 – 4 b.      β 1 – 4 c.      α 1 – 6 d.      β 1 – 6

15.  Which of the following is an alpha lactone

a.      Vit. C b.      Vit. D c.      Vit. A d.      Vit. K

16.  A sweetener used in sugar less gums and candies

a.      Ribitol b.      Xylitol c.      Inositol d.      Mannitol

17.  The glycosidic linkage in cellobiose is

18.  Pick out the odd one from the following

a.      Deoxyribose b.      Rhamnose c.      Fucose d.      Altrose

19.  Lectins are_________.

a.      Sugars specific to proteins b.      Proteins specific to sugars c.      Enzymes specific to carbohydrates d.      Carbohydrates specific to enzymes

20.  Which of the following is a keto triose?

a.      Dihydroxyacetone b.      Glyceraldehyde c.      Ribulose d.      Erythrose

21.  Maltose is a disaccharide of ____

a.      Glucose and galactose b.      Glucose and glucose c.      Glucose and lactose d.      Fructose and lactose

22.  Glycosidic bond in sucrose is____

a.      α 1 – 4 b.      β 1 – 4 c.      α 1 – 2 d.      β 1 – 2

23.  Minimum number of carbon required for a ketose sugar to have cyclic structure is

a.      3 b.      4 c.      5 d.      6

24.  An aldo hexose will have _____ stereoisomers

a.      8 b.      10 c.      14 d.      16

25.  Minimum number of carbon required for a monosaccharide

a.      1 b.      2 c.      3 d.      4

Answers with Explanations

1. Ans. (a) (CH2O)n

Majority of carbohydrates follow this general formula

2. Ans. (b). Glyceraldehyde

3. Ans. (a). C12H22O11

4. Ans. (c). α 1 – 4

Maltose is a disaccharide of two glucose molecules linked by α 1 – 4 glycosidic linkage.

5. Ans. (a). 4

This can be calculated by a formula 2 n, where n will be the number of chiral carbon in the molecule. A keto pentose will have a total of five carbons, among which two carbons will be chiral.

Chiral carbon: An asymmetric carbon, i.e., the four valences of the carbon are satisfied by four different groups or atoms

6. Ans. (d). Starch

Laminarin: reserve food material of brown algae

Chrysolaminarin: reserve food material of diatoms

Floridian starch: reserve food material of red algae

7. Ans. (c). Dihydroxyacetone

All carbohydrates except dihydroxyacetone (a keto triose) will have at least one chiral centre.

8.Ans. (d). Erythrose

Erythrose is an aldo tetrose

Arabinose, Xylose and Lyxose are aldo pentoses

9. Ans. (c). Lyxose

10. Ans (d). Both (a) and (b)

Epimer: Two isomers that differs only in the configuration around a carbon atom.

11. Ans. (c). Glyceraldehyde

Glyceraldehyde is an aldose sugar

Xylulose, Dihydroxyacetone and Ribulose are ketose sugars

12. Ans. (a). Sucrose

Seliwanoff test is used to distinguish aldoses and ketoses. Ketoses give positive test for Seliwanoff test. Sucrose gives Seliwanoff test positive since it contains a fructose moiety which is a ketose sugar.

13. Ans. (a). Erythrulose

Xylulose is a keto pentose

Sorbose and Psicose are keto hexoses

14. Ans. (c). α 1 – 6

Isomaltose is a disaccharide of two glucose molecules connected by α 1 – 6 glycosidic linkage rather than the α-1 – 4 glycosidic linkage in maltose

15. Ans. (a). Vit. C

16. Ans (b). Xylitol

Xylitol, Ribitol, Inositol and Mannitol are sugar alcohols

17. Ans. (b). β 1 – 4

Cellobiose: a reducing disaccharide of glucose molecules connected by β 1 – 4 glycosidic linkage.

18. Ans. (d). Altrose

Altrose: is an aldo hexose sugar

Deoxyribose, Rhamnose and Fucoses are deoxysugars

19. Ans. (b). Proteins specific to sugars

20. Ans. (a). Dihydroxyacetone

21. Ans. (b). Glucose and glucose

22. Ans. (c). α 1 – 2

Sucrose is a non-reducing disaccharide of glucose and fructose connected by α 1 – 2 glycosidic linkage.

23. Ans. (c). 5

Aldose sugars with 4 carbons onwards can form cyclic structures

24. Ans. (d). 16

Use the formula 2 n, where n is the number of chiral carbon atoms. In aldo hexoses there will be four chiral carbons, thus 2 4 = 2 x 2 x 2 x 2 = 16. Of these 16 isomers 8 will be D form and the remaining 8 will be L form.

25. Ans. (c). 3

It is not possible to form a carbohydrate with 2 carbon atom. Simplest carbohydrates are with three carbon back bone and they are glyceraldehyde (an aldo triose) and dihydroxyacetone (a keto triose).

<< Back to BIOCHEMISTRY MCQ

Related posts:

• MCQ on Carbohydrates Part-2
• MCQ on Amino Acids (Part-3)
• MCQ on Carbohydrates Part-3
• MCQ on Amino Acids (Part-1)
• MCQ on Amino Acids (Part-4)

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Chemistry of carbohydrates- multiple choice questions- revision- set-1.

Q.1- Which of the following is a non- reducing disaccharide?

a) Galactose                                                                      

c) Trehalose                                                                      

d) Sucrose                                             

Q.2- Which of the following is a true statement about glucose?

a) It cannot be utilized by red blood cells

b) It has 4 asymmetric carbon atoms

c) It is stored as starch in animals

d) It is oxidized to form glycerol                                                                                                

Q.3-Sucrose is composed of which of the following two sugars?

a) Glucose and Glucose                                                

b) Glucose and Fructose

c) Glucose and Galactose                                            

d) Fructose and Galactose            

Q.4- Which of the following is not a homopolysaccharide?

a) Starch                                                             

c) Glycogen                                                       

d) Cellulose                     

Q.5-Which out of the following is a fructosan?

a) Glycogen                                                                       

c) Inulin                                                               

d) Cellulose                                         

 Q.6- Choose a sugar abundantly present in honey :

a) Maltose                                                          

b) Fructose

c) Ribulose                                                         

d) Lactose                                          

Q.7- Choose an Aldo pentose :

a) Glyceraldehyde                                                          

b) Dihydroxyacetone

c) Erythrose                                                                       

d) Arabinose                                     

Q.8- Which of the following is a keto tetrose?

a) Xylose                                                             

b) Erythrulose

c) Fructose                                                                         

d) Sedoheptulose                            

Q.9- α –D-Glucose and β-D- Glucose are :

a) Stereoisomers                                                            

c) Keto- Aldose Isomers                                               

d) Optical isomers                             

Q.10- All tests are positive for Lactose except-

a) Benedict                                                                        

c) Molisch                                                                           

d) Osazone                                          

Q.11- Which out of the following is a carbohydrate with 6 carbon atoms and a keto group as the functional group?

a) Glyceraldehyde                                                          

d) Galactose                      

Q.12- Mucic acid and Gluconic acids are

a) Glycosides                                                                     

b) Sugar acids

c) Amino sugar acids                                                      

 d) Sugar alcohols                            

Q.13- Sorbitol and Mannitol are-

a) Optical isomers                                                           

c) Stereoisomers                                                            

d) Epimers                                           

Q.14- Which of the following tests is not based on the reaction of carbohydrates with strong acids?

a)Molisch                                                                           

b) Benedict’s

c) Bial’s                                                                                 

d) Seliwanoff’s                 

Q.15- Which out of the following does not form osazone crystals?

a) Galactose                                                                       

c) Lactose                                                                           

d) Sucrose                           

Q.16- N- Acetyl Neuraminic acid is a-

a) Sugar acid                                                                      

b) Amino sugar-acid

c) Amino sugar                                                                 

d) Sugar alcohol                                 

Q.17- Which of the following gives a negative reaction with Barfoed’s test

a) Glucose                                                                          

d) Fructose                                        

Q.18- A polysaccharide formed by β1→4 Glycosidic linkages between glucose residues is

a) Inulin                                                                                               

c) Agar                                                                                  

d) Cellulose                                          

Q.19- Which of the following sugars is laevorotatory predominantly?

c) Sucrose                                                                          

d) Glucose                                                                                                                                                                         

Q.20- Which of the following Mucopolysaccharides is non-sulfated and most abundant in tissues?

 a) Hyaluronic acid                                                           

b) Keratan sulfate

c) Heparin                                                          

d) Dermatan sulfate   

 Key to answers

1)-c, 2)-b, 3)-b, 4)-b, 5)- c, 6)-b, 7)-d, 8)-b, 9)-b, 10)-b, 11)-c,12)-b, 13)-d, 14)-b, 15)-d, 16)-b, 17)-b, 18)-d,19)b, 20)-a .      

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14: Carbohydrates (Structure and Function): Questions

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• Page ID 44599