Harper's Biochemistry Chapter 15 - Saccharides (ie, Carbohydrates) of Physiological Signifcance.PDF

C H A P T E R Saccharides (ie, Carbohydrates) of Physiological Significance Owen P. McGuinness, PhD 15 OBJ E C TI VE S Explain what is meant by the glyco...

C H A P T E R Saccharides (ie, Carbohydrates) of Physiological Significance Owen P. McGuinness, PhD 15 OBJ E C TI VE S Explain what is meant by the glycome, glycobiology, and the science of glycomics. After studying this chapter, Explain what is meant by the terms monosaccharide, disaccharide, you should be able to: oligosaccharide, and polysaccharide. Explain the different ways in which the structures of glucose and other monosaccharides can be represented, and describe the various types of isomerism of sugars and the pyranose and furanose ring structures. Describe the formation of glycosides and the structures of the important disaccharides and polysaccharides. Explain what is meant by the glycemic index of a carbohydrate. Describe the roles of saccharides in cell membranes and lipoproteins. BIOMEDICAL IMPORTANCE Gucose is a major metaboic fue of mammas (except ruminants) an a universa fue of the fetus. It is the precursor for synthe- Carbohyrates (saccharies is the preferre nomencature for sis of a the other carbohyrates in the boy, incuing gly- biochemists) are extremey poar moecues that are wiey cogen for storage, ribose an deoxyribose in nuceic acis, istribute in pants an animas. They have important struc- galactose for synthesis of actose in mik, in gycoipis, an tura an metaboic roes. The wor carbohyrate comes from in combination with protein in gycoproteins (see Chapter 46) the esignation of these moecues as carbon hyrate. A an proteogycans. Sucrose, the most commony use carbo- saccharies are not simpy comprise of C, H, an O or have hyrate in cooking is a naturay sweet moecue, but with it the proportions (CH2O)n; thus saccharie is a more incusive comes caories. With the obesity epiemic sweeteners were term. Saccharies are aso cae sugars, but this esignation eveope that are 600 to 20,000-fo sweeter than sucrose, so focusses on the sweetness of the moecue an misses the one can use them in substantiay ess amount. Sweeteners give many functions of saccharies beyon sweetness. same sweetness as sucrose, but are consiere “caorie free”. In pants, gucose is synthesize from carbon ioxie an Diseases associate with carbohyrate metaboism incue water by photosynthesis an store as starch or use to diabetes mellitus, galactosemia, glycogen storage diseases, synthesize the ceuose of the pant ce was. Animas can fructose intolerance, an lactose intolerance. synthesize carbohyrates from amino acis, but most are erive Glycobiology is the stuy of the roes of saccharies in utimatey from pants they ingest. Glucose is the most impor- heath an isease. The glycome is the entire compement of tant carbohyrate; most ietary carbohyrate is absorbe into saccharies of an organism, whether free or in more compex the boostream as simpe sugars (ie, monosaccharie). Gucose moecues. Glycomics, an anaogous term to genomics an is absorbe after the hyroysis in the intestine of ietary starch proteomics, is the comprehensive stuy of gycomes, incuing an isaccharies. Other simpe sugars are absorbe after genetic, physioogica, pathoogica, an other aspects. igestion an then rapiy converte to gucose in the iver. A very arge number of gycosie inks can be forme between saccharies. Thus, saccharies are the most abunant This was adapted from chapter in 30th edition by David A. Bender, biomoecues both ue to their reactivity an structura PhD, & Peter A. Mayes, PhD, DSc pasticity. For exampe, three ifferent saccharies (eg, hexoses) 147 148 SECTION IV Metabolism of Carbohydrates may be inke to each other to form over 1000 ifferent tri- Poysaccharies are sometimes cassifie as hexosans or saccharies. The conformations of the sugars in oigosaccha- pentosans, epening on the constituent monosaccharies rie chains vary epening on their inkages an proximity to (hexoses or pentoses, respectivey). In aition to starches other moecues with which the oigosaccharies may interact. an extrans (which are hexosans), foos contain a wie Oigosaccharie chains encoe biological information that variety of other poysaccharies that are coectivey known epens on their constituent sugars, sequences, an inkages. as nonstarch poysaccharies; they are not igeste by human enzymes, an are the major component of ietary fiber. Exampes are ceuose from pant ce was (a gucose poymer; see Figure 15–13) an inuin, the storage carbohy- SACCHARIDES ARE ALDEHYDE rate in some pants (a fructose poymer; see Figure 15–13). OR KETONE DERIVATIVES OF POLYHYDRIC ALCOHOLS Carbohyrates (saccharies) are cassifie as foows: BIOMEDICALLY, GLUCOSE 1. Monosaccharides are those sugars that cannot be hyro- IS THE MOST IMPORTANT yze into simper saccharies. They may be cassifie as trioses, tetroses, pentoses, hexoses, or heptoses, epening MONOSACCHARIDE on the number of carbon atoms (3-7), an the ocation of the carbony group C=O. If it is a termina aehye, it is an The Structure of Glucose Can Be aldose. If it is not termina, then there is a ketone an is es- Represented in Three Ways ignate as ketose. For trioses there are ony two possibiities— The straight-chain structura formua (aohexose;Figure 15–1A) gyceraehye an ihyroxyacetone. Exampes are iste can account for some of the properties of gucose, but a cycic in Table 15–1. In aition to aehyes an ketones, the structure (a hemiacetal forme by reaction between the ae- poyhyric acohos (sugar acohos or polyols), in which hye group an a hyroxy group) is thermoynamicay favore the aehye or ketone group has been reuce to an aco- an accounts for other properties. The cycic structure is nor- ho group, can occur naturay in foos. They are aso syn- may rawn as shown in Figure 15–1B, the Haworth projection, thesize by reuction of monosaccharies for use in in which the moecue is viewe from the sie an above the the manufacture of foos for weight reuction an for pane of the ring; the bons nearest to the viewer are bo an iabetics. They are poory absorbe, an have about haf thickene, an the hyroxy groups are above or beow the the energy yie of sugars. The biggest sie effect is fatu- pane of the ring. The hyrogen atoms attache to each carbon ence; bacteria in the intestine ferment the sugar acoho are not shown in this figure. The ring is actuay in the form of that was not absorbe. a chair (Figure 15–1C). 2. Disaccharides are conensation proucts of two mono- 1 saccharie units, for exampe, actose, matose, isomatose, HC O A 2 HC OH sucrose, an trehaose. 3 HO CH 4 3. Oligosaccharides are conensation proucts of 3 to 10 mono- HC OH 5 saccharies. Most are not igeste by human enzymes. HC OH 6 CH2OH 4. Polysaccharides are conensation proucts of more than 10 monosaccharie units; exampes are the starches an B 6 extrans, which may be inear or branche poymers. CH2OH 5 O 4 1 OH 2 TABLE 15–1 Classification of Important OH OH 3 Monosaccharides OH Aldoses Ketoses C H 6 Trioses (C3H6O3) Glycerose Dihydroxyacetone 4 CH2OH (glyceraldehyde) (dihydroxyacetone) HO 5 O Tetroses (C4H8O4) Erythrose Erythrulose H H 2 H Pentoses (C5H10O5) Ribose Ribulose 3 OH HO 1 Hexoses (C6H12O6) Glucose, galactose, Fructose H OH mannose Heptoses (C7H14O7) — Sedoheptulose FIGURE 15–1 d-Glucose. (A) Straight-chain form. (B) α-d-glucose; Haworth projection. (C) α-d-glucose; chair form.  CHAPTER 15 Saccharides (ie, Carbohydrates) of Physiological Significance 149 1 1 HC O HC O 2 2 HO CH HC OH 3 3 CH2OH CH2OH L-glycerose D-glycerose (L-glyceraldehyde) (D-glyceraldehyde) 1 1 HC O HC O 2 2 HC OH HC OH 3 3 HO CH HO CH 4 4 HC OH HC OH 5 5 HO CH HC OH 6 6 CH2OH CH2OH L-Glucose D-Glucose FIGURE 15–3 Pyranose and furanose forms of glucose. FIGURE 15–2 d- and l-isomerism of glycerose and glucose. 4. Alpha- and beta-anomers: The ring structure of an aose is a Monosaccharides Exhibit Various hemiaceta, since it is forme by reaction between an aehye Forms of Isomerism an an acoho group. Simiary, the ring structure of a ketose is a hemiketa. Crystaine gucose is α-d-gucopyranose. Gucose, with four asymmetric carbon atoms, can form 16 The cycic structure is retaine in the soution, but isomer- isomers. The more important types of isomerism foun with ism occurs about position 1, the carbony or anomeric car- gucose are as foows: bon atom, to give a mixture of α-gucopyranose (38%) an 1. d- and l-isomerism: The esignation of a sugar isomer as β-gucopyranose (62%). Less than 0.3% is represente by the d form or its mirror image as the l form is etermine by α- an β-anomers of gucofuranose. its spatia reationship to the parent compoun of the carbo- 5. Epimers: Isomers iffering as a resut of variations in configu- hyrates, the three-carbon sugar gycerose (gyceraehye). ration of the —OH an —H on carbon atoms 2, 3, an 4 of The l an d forms of this sugar, an of gucose, are shown in gucose are known as epimers. Bioogicay, the most impor- Figure 15–2. The orientation of the —H an —OH groups tant epimers of gucose are mannose (epimerize at carbon 2) aroun the carbon atom ajacent to the termina acoho an gaactose (epimerize at carbon 4) (Figure 15–5). carbon (carbon 5 in gucose) etermines whether the sugar beongs to the d or l series. When the —OH group on this 6. Aldose-ketose isomerism: Fructose has the same moecu- carbon is on the right (as seen in Figure 15–2), the sugar is ar formua as gucose but iffers in that there is a potentia the d-isomer; when it is on the eft, it is the l-isomer. Most keto group in position 2, the anomeric carbon of fructose, of the naturay occurring monosaccharies are d sugars, whereas in gucose there is a potentia aehye group an the enzymes responsibe for their metaboism are spe- in position 1, the anomeric carbon. Exampes of aose cific for this configuration. an ketose sugars are shown in Figures 15–6 an 15–7, respectivey. Chemicay, aoses are reucing compouns, 2. The presence of asymmetric carbon atoms aso confers an are sometimes known as reucing sugars. This provies optical activity on the compoun. When a beam of pane- the basis for a simpe chemica test for gucose in urine in poarize ight is passe through a soution of an optical isomer, it rotates either to the right, extrorotatory (+), or to the eft, evorotatory (−). The irection of rotation of poarize ight is inepenent of the stereochemistry of the sugar, so it may be esignate d(−), d(+), l(−), or l(+). For exampe, the naturay occurring form of fructose is the d(−) isomer. Confusingy, extrorotatory (+) was at one time cae -, an evorotatory (−) -. This nomencature is obsoete, but may sometimes be foun; it is unreate to d- an l-isomerism. In soution, gucose is extrorotatory, an gucose soutions are sometimes known as dextrose. 3. Pyranose and furanose ring structures: The ring struc- tures of monosaccharies are simiar to the ring structures of either pyran (a six-membere ring) or furan (a five- membere ring) (Figures 15–3 an 15–4). For gucose in soution, more than 99% is in the pyranose form. FIGURE 15–4 Pyranose and furanose forms of fructose. 150 SECTION IV Metabolism of Carbohydrates FIGURE 15–5 Epimers of glucose. CHO CHO CHO CHO CHO CHO CHO H C OH HO C H H C OH CHO HO C H H C OH HO C H H C OH HO C H HO C H HO C H CHO H C OH HO C H HO C H H C OH H C OH HO C H H C OH H C OH H C OH H C OH H C OH H C OH H C OH H C OH H C OH H C OH H C OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH D-Glycerose (D-glyceraldehyde) D-Erythrose D-Lyxose D-Xylose D-Arabinose D-Ribose D-Galactose D-Mannose D-Glucose FIGURE 15–6 Examples of aldoses of physiological significance. CH2OH CH2OH C O CH2OH CH2OH C O HO C H C O C O HO C H H C OH CH2OH HO C H H C OH H C OH H C OH C O H C OH H C OH H C OH H C OH CH2OH CH2OH CH2OH CH2OH CH2OH Dihydroxyacetone D-Xylulose D-Ribulose D-Fructose D-Sedoheptulose FIGURE 15–7 Examples of ketoses of physiological significance. poory controe iabetes meitus by reuction of an aka- Saccharides Form Glycosides With ine copper soution (see Chapter 48). Other Compounds & With Each Other Glycosides are forme by conensation between the hyroxy Many Monosaccharides Are group of the anomeric carbon of a monosaccharie, an a sec- Physiologically Important on compoun that may be another monosaccharie (enote Derivatives of trioses, tetroses, an pentoses an of the seven- glycone) or, a nonsaccharie group (enote aglycone). If carbon sugar seoheptuose are forme as metaboic interme- the secon group is aso a hyroxy, the O-gycosiic bon iates in gycoysis (see Chapter 17) an the pentose phosphate is an acetal ink because it resuts from a reaction between pathway (see Chapter 20). Pentoses are important in nuceo- a hemiaceta group (forme from an aehye an an —OH ties, nuceic acis, an severa coenzymes (Table 15–2). group) an another —OH group. If the hemiaceta portion is Gucose, gaactose, fructose, an mannose are the physi- gucose, the resuting compoun is a glucoside; if gaactose, oogicay important hexoses (Table 15–3). The biochemicay a galactoside; an so on. If the secon group is an amine, an important aoses an ketoses are shown in Figures 15–6 an N-gycosiic bon is forme, for exampe, between aenine 15–7, respectivey. an ribose in nuceoties such as ATP (see Figure 11–4). In aition, carboxyic aci erivatives of gucose are Gycosies are wiey istribute in nature; the agycone important, incuing d-gucuronate (for gucuronie for- may be methano, gycero, a stero, a pheno, or a base such mation an in gycosaminogycans), its metaboic eriva- as aenine. The gycosies that are important in meicine tive, l-iuronate (in gycosaminogycans, Figure 15–8) an because of their action on the heart (cardiac glycosides), a l-guonate (an intermeiate in the uronic aci pathway; see contain sterois as the agycone. These incue erivatives of Figure 20–4). igitais an strophanthus such as ouabain, an inhibitor of the  CHAPTER 15 Saccharides (ie, Carbohydrates) of Physiological Significance 151 TABLE 15–2 Pentoses of Physiological Importance Sugar Source Biochemical and Clinical Importance d-Ribose Nucleic acids and metabolic intermediate Structural component of nucleic acids and coenzymes, including ATP, NAD(P), and flavin coenzymes d-Ribulose Metabolic intermediate Intermediate in the pentose phosphate pathway d-Arabinose Plant gums Constituent of glycoproteins d-Xylose Plant gums, proteoglycans, glycosaminoglycans Constituent of glycoproteins l-Xylulose Metabolic intermediate Excreted in the urine in essential pentosuria TABLE 15–3 Hexoses of Physiological Importance Sugar Source Biochemical Importance Clinical Significance d-Glucose Fruit juices, hydrolysis of starch, cane or beet The main metabolic fuel for tissues; Excreted in the urine (glucosuria) in sugar, maltose and lactose “blood sugar” poorly controlled diabetes mellitus as a result of hyperglycemia d-Fructose Fruit juices, honey, hydrolysis of cane or beet Rapidly metabolized by the liver Hereditary fructose intolerance sugar and inulin, enzymic isomerization of leads to accumulation of glucose syrups for food manufacture metabolites of fructose that impair glucose production and induce hypoglycemia d-Galactose Hydrolysis of lactose Readily metabolized to glucose; Hereditary galactosemia as a result synthesized in the mammary gland for of failure to metabolize galactose synthesis of lactose in milk. A constituent leads to cataracts of glycolipids and glycoproteins d-Mannose Hydrolysis of plant mannan gums Constituent of glycoproteins Na+–K+-ATPase of ce membranes. Other gycosies incue Amino Sugars (Hexosamines) antibiotics such as streptomycin. Are Components of Glycoproteins, Deoxy Sugars Lack an Oxygen Atom Gangliosides, & Glycosaminoglycans Deoxy sugars are those in which one hyroxy group has been The amino sugars incue d-gucosamine, a constituent repace by hyrogen. An exampe isdeoxyribose (Figure 15–9) of hyauronic aci (Figure 15–10), d-gaactosamine (aso in DNA. The eoxy sugar l-fucose (see Figure 15–15) occurs in known as chonrosamine), a constituent of chonroitin, an gycoproteins. In cinica meicine the tissue accumuation of a d-mannosamine. Severa antibiotics (eg, erythromycin) contain tracer quantity of 2-eoxygucose (18F 2-fuoro-2-eoxygucose) amino sugars, which are important for their antibiotic activity. is use to etect metaboicay active tumors, which have very high rates of gucose uptake. 2-eoxygucose after being phos- Maltose, Sucrose, & Lactose phoryate by hexokinase cannot be further metaboize, so Are Important Disaccharides it accumuates. This accumuation is etecte using positron The isaccharies are sugars compose of two monosac- emission tomography. charie resiues inke by a gycosie bon (Figure 15–11). COO– 5 HO CH2 O O O OH – COO 4 1 OH OH 3 OH OH OH OH 2 OH OH OH FIGURE 15–8 α-d-Glucuronate (left) and β-l-iduronate (right). FIGURE 15–9 2-Deoxy-d-ribofuranose (β-form). 152 SECTION IV Metabolism of Carbohydrates CH2OH (80-87%), which consists of branche chains, consisting of O 24 to 30 gucose resiues with α1 → 4 inkages in the chains OH an by α1 → 6 inkages at the branch points (Figure 15–12). OH OH The extent to which starch in foos is hyroyze by amy- NH3+ ase is etermine by its structure, the egree of crystaization or hyration (the resut of cooking), an whether it is encose FIGURE 15–10 Glucosamine (2-amino-d-glucopyranose) in intact (an inigestibe) pant ce was. Iniviuas with (α-form). Galactosamine is 2-amino- d-galactopyranose. Both glucos- amine and galactosamine occur as N-acetyl derivatives in complex iabetes or preiabetes have troube controing their gucose carbohydrates, for example, glycoproteins. concentration an are encourage to eat compex carbohy- rates that o not rapiy increase gucose eves on ingestion. The physioogicay important isaccharies are matose, The glycemic index of a starchy foo is base on the extent to sucrose, an actose (Table 15–4). Hyroysis of sucrose yies which it raises the boo concentration of gucose compare a mixture of gucose an fructose cae “invert sugar” because with an equivaent amount of gucose or a reference foo such fructose is strongy evorotatory an changes (inverts) the as white brea or boie rice. An important eterminant of weaker extrorotatory action of sucrose. the gycemic inex is its igestibiity. Gycemic inex ranges from 1 (or 100%) for starches that are reaiy hyroyze in the sma intestine to 0 for those that are not hyroyze at a. POLYSACCHARIDES SERVE Glycogen is the energy storage poysaccharie in animas STORAGE & STRUCTURAL an is sometimes cae anima starch. It is a more highy branche structure than amyopectin, with chains of 12 to FUNCTIONS 15 α-d-gucopyranose resiues (in α1 → 4 gucosiic inkage) Poysaccharies incue a number of physioogicay impor- with branching by means of α1 → 6 gucosiic bons. Musce tant carbohyrates. They can provie one of the three func- gycogen granues (β-partices) are spherica an contain up to tions (1) structura or mechanica protection, (2) energy 60,000 gucose resiues; in iver there are simiar granues an storage, an (3) bin water to resist ehyration. aso rosettes of gycogen granues that appear to be aggregate Starchis a homopoymer of gucose forming an α-gucosiic β-partices. Gycogen can be reaiy mobiize by the iver to chain, cae a glucosan or glucan. It is the most important support gucose nees of periphera tissues. ietary carbohyrate in cereas, potatoes, egumes, an other Inulin is a poysaccharie of fructose (a fructosan) foun in vegetabes. The two main constituents are amylose (13-20%), tubers an roots of ahias, artichokes, an aneions. It is not which has a nonbranching heica structure, an amylopectin hyroyze by intestina enzymes, so has no nutritiona vaue. CH2OH CH2OH OH O O O HO CH2 CH2OH OH OH OH OH OH O OH OH O O OH OH OH OH CH 2 OH Sucrose (glucosyl-fructose) Trehalose (glucosyl-glucoside) CH2OH CH2OH CH2OH CH2OH OH O O O O OH O OH OH OH OH OH O OH OH OH OH OH Lactose (galactosyl-glucose) Maltose (glucosyl-glucose) CH2OH O OH OH O Isomaltose OH CH2 O OH OH OH OH FIGURE 15–11 Structures of nutritionally important disaccharides.  CHAPTER 15 Saccharides (ie, Carbohydrates) of Physiological Significance 153 TABLE 15–4 Disaccharides of Physiological Importance Sugar Composition Source Clinical Significance Sucrose O-α-d-glucopyranosyl-(1→2)-β-d- Cane and beet sugar, sorghum, and some Rare genetic lack of sucrase leads to sucrose fructofuranoside fruits and vegetables intolerance—diarrhea and flatulence Lactose O-β-d-galactopyranosyl-(1→4)-β- Milk (and many pharmaceutical Lack of lactase (alactasia) leads to lactose d-glucopyranose preparations as a filler) intolerance—diarrhea and flatulence; may be excreted in the urine in pregnancy Maltose O-α-d-glucopyranosyl-(1→4)-α-d- Enzymic hydrolysis of starch (amylase); glucopyranose germinating cereals and malt Isomaltose O-α-d-glucopyranosyl-(1→6)-α-d- Enzymic hydrolysis of starch (the branch glucopyranose points in amylopectin) Lactulose O-α-d-galactopyranosyl-(1→4)-β- Heated milk (small amounts), mainly Not hydrolyzed by intestinal enzymes, but d-fructofuranose synthetic fermented by intestinal bacteria; used as a mild osmotic laxative Trehalose O-α-d-glucopyranosyl-(1→1)-α-d- Yeasts and fungi; the main sugar of insect glucopyranoside hemolymph It is use cinicay to assess kiney function. When infuse it consists of β-d-gucopyranose units inke by β1 → 4 bons is excrete by the kiney. The abiity of the kiney to remove to form ong, straight chains strengthene by cross-inking inuin is a refection of the gomeruar fitration rate. Dextrins hyrogen bons. Mammas ack any enzyme that hyroyzes are intermeiates in the hyroysis of starch. Cellulose is the β1 → 4 bons, an so cannot igest ceuose. It is the the chief constituent of pant ce was. It is insoube an major component of ietary fiber. Microorganisms in the gut FIGURE 15–12 The structure of starch and glycogen. Amylose is a linear polymer of glucose residues linked α1→4, which coils into a helix. Amylopectin and glycogen consist of short chains of glucose residues linked α1→4 with branch points formed by α1→6 glycoside bonds. The glycogen molecule is a sphere ~21 nm in diameter that can be seen in electron micrographs. It has a molecular mass of ~107 Da and consists of polysaccharide chains, each containing about 13 glucose residues. The chains are either branched or unbranched and are arranged in 12 con- centric layers. The branched chains (each has two branches) are found in the inner layers and the unbranched chains in the outermost layer. The blue dot at the center of the glycogen molecule is glycogenin, the primer molecule for glycogen synthesis. 154 SECTION IV Metabolism of Carbohydrates FIGURE 15–13 The structures of some important nonstarch polysaccharides. of ruminants an other herbivores can hyroyze the inkage in ce membranes (see Chapters 40 an 46) an many proteins an ferment the proucts to short-chain fatty acis as a major are gycosyate. The sialic acids are N- or O-acy erivatives energy source. Recent ata suggest that the avaiabiity of these short-chain fatty acis, which is etermine by the microbia composition in the intestine, can impact metaboic heath in at-risk iniviuas (iabetes, obesity, irritabe bowe isease). By manipuating the microbia composition using prebiotics, one can shift the microbia popuation an possiby improve metaboic heath. There is some bacteria metaboism of ce- uose in the human coon. Chitin is a structura poysaccha- rie in the exoskeeton of crustaceans an insects, an aso in mushrooms. It consists of N-acety-d-gucosamine units joine by β1 → 4 gycosiic bons. Pectin occurs in fruits; it is a poymer of gaacturonic aci inke α1 → 4, with some gaactose an/or arabinose branches, an is partiay methy- ate (Figure 15–13). Glycosaminoglycans (mucopoysaccharies) are compex carbohyrates containing amino sugars an uronic acids. They may be attache to a protein moecue to form a proteoglycan. Proteogycans provie the groun or packing substance of connective tissue (see Chapter 50). They ho arge quanti- ties of water an occupy space because of the arge number of —OH groups an negative charges on the moecue, which, by repusion, keep the carbohyrate chains apart. They serve to cushion or ubricate other structures, such as connective tis- sue in a joint an cartiage. Exampes are hyaluronic acid an chondroitin sulfate. Heparin is an important anticoagulant (Figure 15–14). Glycoproteins (aso known as mucoproteins) are proteins containing branche or unbranche oigosaccharie chains FIGURE 15–14 Structure of some complex polysaccharides (Table 15–5), including fucose (Figure 15–15). They occur and glycosaminoglycans.  CHAPTER 15 Saccharides (ie, Carbohydrates) of Physiological Significance 155 TABLE 15–5 Saccharides Found in Glycoproteins CH3 C O Hexoses Mannose (Man), Galactose (Gal) NH O O COO– Acetyl N-Acetylglucosamine (GlcNAc), CH3 (CHOH)2 hexosamines N-acetylgalactosamine (GalNAc) OH CH2OH Pentoses Arabinose (Ara), Xylose (Xyl) OH OH OH OH Methyl pentose l-Fucose (Fuc, see Figure 15–15) Fucose N-Acetylneuraminic acid Sialic acids N-Acyl derivatives of neuraminic acid; the predominant sialic acid is N-acetylneuraminic FIGURE 15–15 β-l-Fucose (6-deoxy-β-l-galactose) and acid (NeuAc, see Figure 15–15) N-acetylneuraminic acid, a sialic acid. of neuraminic aci (see Figure 15–15). Neuraminic acid is oigosaccharie is ae to the protein. Aing oigosaccha- a nine-carbon sugar erive from mannosamine (an epimer ries to ce surface proteins make them an antigen. Iniviu- of gucosamine) an pyruvate. Siaic acis are constituents of as with boo type O o not have this moification are thus both glycoproteins an gangliosides. universa boo onors. Proteins can unergo gycation, which is the nonenzymatic aition of a saccharie (eg, gucose) to a protein. Gycation increases as gucose eves increase. A gycate form of hemo- gobin (hemogobin A1c) increases in iniviuas with iabetes. SUMMARY It is use for the iagnosis of iabetes. It is aso monitore The gycome is the entire compement of sugars of an uring meica treatment of iabetes to etermine how we organism, whether free or present in more compex moecues. gucose is manage. Gycomics is the stuy of gycomes, incuing genetic, physioogica, pathoogica, an other aspects. Carbohyrates are major constituents of anima foo an CARBOHYDRATES OCCUR IN CELL anima tissues. They are characterize by the type an number of monosaccharie resiues in their moecues. MEMBRANES & IN LIPOPROTEINS Gucose is the most important carbohyrate in mammaian Approximatey 5% of the weight of ce membranes is the biochemistry because neary a carbohyrate in foo is carbohyrate part of gycoproteins (see Chapter 46) an gy- converte to gucose for metaboism. coipis. Their presence on the outer surface of the pasma Saccharies have arge numbers of stereoisomers because they membrane (the glycocalyx) has been shown with the use of contain severa asymmetric carbon atoms. pant lectins, proteins that bin specific gycosy resiues. For The physioogicay important monosaccharies incue exampe, concanavalin A bins α-gucosy an α-mannosy gucose, the “boo sugar,” an ribose, an important constituent resiues. Glycophorin is a major integra membrane gyco- of nuceoties an nuceic acis. protein of human erythrocytes. It has 130 amino aci resi- The important isaccharies incue matose (gucosy– ues an spans the ipi membrane, with poypeptie regions gucose), an intermeiate in the igestion of starch; sucrose outsie both the externa an interna (cytopasmic) surfaces. (gucosy–fructose), important as a ietary constituent Carbohyrate chains are attache to the amino termina por- containing fructose; an actose (gaactosy–gucose), in mik. tion outsie the externa surface. Carbohyrates are aso pres- Starch an gycogen are storage poymers of gucose in pants an ent in apoipoprotein B of pasma ipoproteins. ABO boo animas, respectivey. Starch is the major metaboic fue in the iet. groups are efine by ifferent immunogenic moecues on Compex carbohyrates contain other sugar erivatives such erythrocytes. ABO boo group antigens are oigosaccharie as amino sugars, uronic acis, an siaic acis. They incue chains that are attache to proteins. Poymorphisms in the proteogycans an gycosaminogycans, which are associate enzyme ABO glycosyltransferase etermine which singe with structura eements of the tissues, an gycoproteins, saccharie is pace in the oigosaccharie chain. The boo which are proteins containing oigosaccharie chains; they are group A expresses A-transferase that paces N-acetygaactos- foun in many situations incuing the ce membrane. amine. The boo group B expresses the B-transferase which Oigosaccharie chains encoe bioogica information, paces gaactose. Iniviuas with boo type O have a muta- epening on their constituent sugars an their sequence an tion that prouces an inactive ABO gycosytransferase; no inkages.