Chapter 10 – Carbohydrates

Most carbohydrates are produced photo-synthetically by bacteria, algae and plants

Carbohydrates (“hydrate of carbon”) have the empirical formulaof (CH2O)n , where n ≥ 3.

- Monosaccharides: one monomeric unit

- Oligosaccharides: ~2-20 monosaccharides

- Polysaccharides: > 20 monosaccharides

- Glycoconjugates: linkages to proteins or lipids

Saccharides of varying lengthAldoses and Ketoses

Trioses – three carbon sugars

AldotrioseKetotriose

Tetroses – 4 carbon sugars

Saccharides of varying lengthAldoses and Ketoses

Chiral designation comes fromthe most distant chiral carbonfrom the carbonyl

D-sugars dominate in nature

Saccharides of varying lengthAldoses and Ketoses

Pentoses – 5 carbon sugarsHexoses – 6 carbon sugars

Figure 10.2

Chiral designation comes fromthe most distant chiral carbonfrom the carbonyl

Figure 10.1 Isomeric forms of carbohydrates

Epimers

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

example: D-Mannose is an epimer of D-Glucose

Figure 10.4 Cyclization ofD-fructose to form a- and b-fructofuranose

anomeric carbon

Fig 10.3 Cyclization of D-glucose to form glucopyranose

In aqueous solutions hexosesand pentoses will cyclizeforming alpha (a) and beta (b)forms C1 is called the anomeric carbon

In aqueous solutions, it is the ring structures that dominateThese rings are NOT planar

Figure 10.5

Figure 10.7 modified monosaccharides

Important in metabolic pathways

Important in structuralglycans

6-deoxy-L-galactose

Disaccharides and other Glycosides

Glycosidic bond – the primary structural linkage in all polymersof monsaccharides

Glycosides – glucose provides the anomeric carbon

Figure 10.8 Structures of disaccharides: maltose

More structures of disaccharides: lactose and sucrose

Major carbohydratein milk

Most abundant sugar

Comes from the breakdown of starch and glycogen

Sucrase, lactase andmaltase are foundon the outer surfaceof the epithelial cellslining the small intestine.

Reducing and Nonreducing ends of sugars

- In linear polymeric chains of monosaccharides thereis usually one reducing end (containing the free anomericcarbon) and one nonreducing end

- Branched polysaccharides have a number of nonreducingends, but only one reducing end

Reducing end(anomeric carbon)

Nonreducing end

Read Clinical Insight pg 161

Polysaccharides

- Homoglycans – homopolysaccharides containing only onetype of monosaccharide.

- Heteroglycans – heteropolysaccharides containing residuesof more than one type of monosaccharide

- The lengths and compositions of a polysaccharide may varywithin a population of these molecules

e.g.: starch and glycogen – storage polysaccharidescellulose and chitin – structural polysaccharides

Starch- D-glucose is stored intracellularly in polymeric forms

- plants and fungi store glucose as starch- Animals store glucose as glycogen

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

(a) Amylose is a linear polymer containing only a-1,4-glycosidic bonds.

(b) Amylopectin is a branchedpolymer also contains a-1,6-bonds

Figure 10.12

Prentice Hall c2002 Chapter 8 16

Starch is stored by plants and used as fuel.

a-amylase on amylopectin

- a-amylase is found in plants and animals and is a hydrolaseit is an endoglycosidase – hydrolyzes internal a-(14) glycosidic bonds

Prentice Hall c2002 Chapter 8 18

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 connectedby a-(14) linkages with a-(16) branches (one branch every8 to 12 residues)

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

Cellulose – a structural polysaccharide that is aMajor component of cell walls of plants

Cellulose has b-(14) glycosidic bonds

Each glucoseresidue is rotated180o relative to the next residue

Extended hydrogen bondingbetween chains leads to bundles or fibrils

Figure 10.14 Glycosidic bondDetermines polysaccharideStructure.

Bent structure

Straight structure

- Humans digest starch and glycogens ingested in their dietusing amylases, enzymes that hydrolyze a-(14) glycosidic bond

- Humans cannot hydrolyze b-(14) linkages of cellulose. Thereforecellulose is not a fuel source for humans. It is fiber.

- Certain microorganisms have cellulases, enzymes that hydrolyzeb-(14) linkages of cellulose.

- cattle have these organisms in their rumen- termites have them in their intestinal tract

Carbohydrates attached to proteins form glycoproteins

Many glycoproteins are found as components of cell membranes (chapter 11) and take part in cell adhesion and binding.

Two methods to anchor the protein to carbohydrates

or Thr

Mucins or mucoproteins are proteins which has N-

acetylgalactosamine attached. This glycoprotein is found in mucus and is a

lubricant.

Figure 10.15 Glycosidic bonds between proteins and carbohydrates

asparagine serine

Two methods to anchor the protein to carbohydrates

Thr is also used to makeO-linkages

Figure 10.16 N-linked oligosaccharides

All N-linkages have a common core shown in grey.

Protein residue Protein residue

High mannoseComplex structure

Additional sugars can be attached to these cores to make diverse and unique structures

Proteoglycans are glycoproteins were the protein is bound to aspecial class of polysaccharides called a glycosaminoglycan.

This class of glycoprotein are used as structural components and lubricants.

-In proteoglycans nearly 95% of the mass comes from the polysaccharide.

-Proteoglycans function as lubricants, structural components in tissue and mediate the adhesion of cells to the extracellular matrix.

Assignment

Read Chapter 10Read Chapter 11

Topics not covered:Section 10.4 Lectins