Prentice Hall c2002Chapter 81 Chapter 8 - Carbohydrates Carbohydrates (“hydrate of carbon”) have...

Prentice Hall c2002 Chapter 8 1

Chapter 8 - Carbohydrates

• Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH2O)n , where n ≥ 3

• Monosaccharides one monomeric unit

• Oligosaccharides ~2-20 monosaccharides

• Polysaccharides > 20 monosaccharides

• Glycoconjugates linked to proteins or lipids

• Trioses - 3 carbon sugars

• Tetroses - 4 carbon sugars

• Pentoses - 5 carbon sugars

• Hexoses - 6 carbon sugars


•Trioses - 3 carbon sugars

Fig. 8.3


•Tetroses - 4 carbon sugars

Fig. 8.3


•Pentoses - 5 carbon sugars

Fig. 8.3


•Hexoses - 6 carbon sugars

Fig 8.3


Enantiomers and epimers

• D-Sugars predominate in nature

• Enantiomers - pairs of D-sugars and L-sugars

• Epimers - sugars that differ at only one of several chiral centers

• Example: D-galactose is an epimer of D-glucose


Fig 8.7 (a) Pyran and (b) furan ring systems

• (a) Six-membered sugar ring is a “pyranose”

• (b) Five-membered sugar ring is a “furanose”


Fig 8.8 Cyclization of D-glucose to form glycopyranose

In aqueous solution hexoses and pentoses will

cyclize, forming alpha () and beta

() forms


Fig 8.9 Cyclization of D-ribose to form -

and -D-ribopyranose and

- and -D-ribofuranose


8.4 Derivatives of Monosaccharides

• Many sugar derivatives are found in biological systems

• Some are part of monosaccharides, oligosaccharides or polysaccharides

• These include sugar phosphates, deoxy and amino sugars, sugar alcohols and acids


Sugar Phosphates

Fig 8.13 Some important sugar phosphates


Deoxy Sugars

• In deoxy sugars an H replaces an OH

Fig 8.14 Deoxy sugars


Amino Sugars

• An amino group replaces a monosaccharide OH

• Amino group is sometimes acetylated

• Amino sugars of glucose and galactose occur commonly in glycoconjugates


Sugar Alcohols (polyhydroxy alcohols)

• Sugar alcohols: carbonyl oxygen is reduced

Fig 8.16 Several sugar alcohols


Sugar Acids

• Sugar acids are carboxylic acids

Fig 8.17 Sugar acids derived from glucose


Sugar Acids

• L-Ascorbic acid (Vitamin C) is derived from D-glucuronate

Fig 8.18 L-Ascorbic acid

L-Ascorbic acid

(Vitamin C)


Disaccharides and Other Glycosides

• Glycosidic bond - primary structural linkage in all polymers of monosaccharides

• Glucosides - glucose provides the anomeric carbon

Fig 8.20 Structures of disaccharides (a) maltose, (b) cellobiose


Fig 8.20 Structures of disaccharides (c) lactose, (d) sucrose


Polysaccharides

• Homoglycans - homopolysaccharides containing only one type of monosaccharide

• Heteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharide

• Lengths and compositions of a polysaccharide may vary within a population of these molecules


Starch• D-Glucose is stored intracellularly in polymeric forms

• Plants and fungi store glucose as starch

• Starch is a mixture of amylose (unbranched) and amylopectin (branched every 25 sugars)

(a) Amylose is a linear polymer

Figure 8.22

(a) Amylopectin is a branched polymer

Figure 8.23


Amylose and Amylopectin form helical structures in starch granules of plants


Starch is stored by plants and used as fuel.


Glycogen is is stored by animals and used as fuel.


Glycogen

Glycogen is the main storage polysaccharide of humans.

Glycogen is a polysaccharide of glucose residues connected by -1-4) linkages with -(1-6) branches (one branch per 10 sugars).

Glycogen is present in large amounts in liver and skeletal muscle.


Cellulose, a structural polysaccharide in plants has -(1-4) glycosidic bonds

Fig 8.25 Structure of cellulose


Fig 8.26 Cellulose fibrils

• Intra- and interchain Hydrogen bonds give strength


Humans digest starch and glycogen ingested in theirdiet using amylases, enzymes that hydrolyze -1-4) glycosidic bonds.

Humans cannot hydrolyze -1-4) linkages ofcellulose. Therefore cellulose is not a fuel sourcefor humans. It is fiber.

Certain microorganisms have cellulases, enzymes thathydrolyze -1-4) linkages of cellulose.

Cattle have these organisms in their rumen.Termites have them in their intestinal tract.


Fig 8.27 Structure of chitinThe exoskeleton of arthropods

• Repeating units of -(1-4)GlcNAc residues

GlcNAc = N-acetylglucosamine


Glycoconjugates

• Heteroglycans appear in 3 types of glycoconjugates:

1. Proteoglycans

2. Peptidoglycans

3. Glycoproteins


Proteoglycans

• Proteoglycans - glycosaminoglycan-protein complexes

• Glycosaminoglycans - unbranched heteroglycans of repeating disaccharides of amino sugars

(D-galactosamine or D-glucosamine)


Fig 8.28 Repeating disaccharide of hyaluronic acid, a glycosaminoglycan

• GlcUA =D-glucuronate

• GlcNAc= N-acetylglucosamine


Fig 8.29 Proteoglycan aggregate of cartilage


Peptidoglycans

• Peptidoglycans - heteroglycan chains linked to peptides

• Major component of bacterial cell walls

• Heteroglycan composed of alternating N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc)

• -(1-4) linkages connect the units


Fig 8.30 Glycan moiety of peptidoglycan


Fig 8.31 Structure of the peptidoglycan of the cell wall of Staphylococcus aureus

(a) Repeating disaccharide unit,

(b) Cross-linking of the peptidoglycan macromolecule


Penicillin inhibits a transpeptidase involved in bacterial cell wall formation

• Fig 8.32 Structures of penicillin and -D-Ala-D-Ala

• Penicillin structure resembling -D-Ala-D-Ala is shown in red


Glycoproteins

• Proteins that contain covalently-bound oligosaccharides, either to serine (O-Glycosidic linkage) or asparagine (N-glycosidic linkage)

• Oligosaccharide chains exhibit great variability in sugar sequence and composition

Fig. 8.33 O-Glycosidic and N-glycosidic linkages


Fig 8.34 and 8.35. Types of glycosidic linkages

Prentice Hall c2002Chapter 81 Chapter 8 - Carbohydrates Carbohydrates (“hydrate of carbon”) have...

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