Chapter 8 - Carbohydrates (Sugars and Polysaccharides)-- PowerPoint

Carbohydrates or Saccharides (from Greek meaning sweet): have basic formula (CH2O)n where n3. Many derivatives of carbohydrates deviate from this basic formula by modification of the constituent groups. Basic units are monosaccharides or simple sugars, which combine to form oligosaccharides with a few simple sugar units or polysaccharides with many units. Oligosaccharides are often combined witgh proteins to produce glycoproteins or with lipids to produce glycolipids.

A. Monosaccharides--aldehyde or ketone derivatives of straightchain polyhydroxyl alcohols

1. Classification Fig. 8-1 & 8-2
a. nature of their carbonyl group
  • aldehyde --> aldose
  • ketone --> ketose

b. number of C atoms

  • 3 C's - triose
  • 4 C's - tetrose
  • 5 C's - pentose
  • 6 C's - hexose
  • 7 C's - heptose

c. or by both

  • glucose = aldohexose
  • fructose = ketohexose

d. stereochemistry: except for trioses, all sugars have more than one asymmetric (chiral) C

L-Glyceraldehyde

D-Glyceraldehyde

  • most sugars have D conformation in Fischer nomenclature ==> when drawn in linear form with carbonyl at top, the -OH on next to last C is on the right
  • epimers differ by orientation around only one asymmetric C; note that except for glyceraldehyde, sugars which are epimers are not optical isomers (enantiomers) because they have the same configuration about all of the other asymmetric C's

 2. Configurations and Conformations

a. alcohols react with carbonyls Fig. Page 199
  • -OH + aldehyde --> hemiacetal
  • -OH + ketone --> hemiketal

b. monosaccharides react internally to form hemiacetals or hemiketals forming cyclic structures Fig. 8-3

  • pyranose: 6-membered ring similar to Pyran; most stable
  • furanose: 5-membered ring similar to Furan; also stable
  • usually drawn as Haworth Projection depicting the ring as simple hexagon or pentagon, although conformation is more complex (see below and Fig. 8-5)

c. Anomers-depending upon which side of the planar carbonyl group the -OH binds, one of two different isomers are formed; these are called anomers and belong to a class of sterioisomers called diastereomers because they are not optical isomers

a-D-Glucopyranose

b-D-Glucopyranose

  • a-anomer: -OH is on the opposite side of the sugar ring from the CH2OH
  • b-anomer: -OH is on the same side of the sugar ring from the CH2OH
  • mutarotation: the anomeric forms can interconvert in water by going through the inetermediate straight chain form Fig. 8-4
  • complete name of a particular sugar includes stereochemistry: e.g. a-D-Glucopyranose

d. furanose and pyranose rings can adopt various conformations. e.g. pyranose.. Fig. 8-5

  • chair conformation: two forms, most stable has bulky substituents in equitorial positions
  • boat conformation: less stable than chair because it puts bulky substituents close together.

3. Sugar Derivatives: chemistry of sugars involves primarily their -OH and carbonyl groups

a. Glycosidic Bonds between anomeric C and an alcohol; these connect sugar residues of oligosaccharids and polysaccharides Fig. 8-7

b. oxidation-reduction reactions

  • aldoses - oxidation of carbonyl forms an aldonic acid e.g. oxidation of glucose --> Gluconic acid; aldoses are called reducing sugars because the aldehyde is easily oxidized reducing another compound
  • oxidation of primary alcohol of an aldose forms an uronic acid e.g. glucuronic acid
  • both aldonic and uronic acids esterify internally to form 5- and 6-membered lactone
  • aldoses and ketoses can be reduced to form acyclic polyhydroxy alcohols, alditols

c. deoxy-sugars: -OH replaced by -H e.g. D-2-deoxyribose in DNA

d. amino sugars; one or more -OH's replaced with amino group which is often acetylated

  • glucosamine and galactosamine / N-Acetylglucosamine Fig. p 205
  • N-acetylmuramic acid (NAM)
  • N-acetylneuraminic acid aka sialic acid Fig 8-6

4. disaccharides: two simple sugars linked by glycosidic bond a or b

  • sucrose: O-a-D-glucopyranosyl-(1-->2)-b-D-fructofuranoside
  • lactose: O-b-D-galactopyranosyl-(1-->4)-D-glucopyranose
  • maltose: O-a-D-glucopyranosyl-(1-->4)-D-glucopyranose
  • isomaltose
  • cellobiose

5. Structural Polysaccharides: Cellulose and Chitin

a. cellulose: linear polymer of up to 15,000 D-glucose residues linked by b-(1-->4) glycosidic bonds; gives plant cell walls great strength Fig. Page 204
  • b glycosidic bond alternates orientation of glucopyranose rings along the chain giving it an extended conformation
  • extended cellulose chains are connected by interchain H-bonds in three dimensions excluding many water molecules. and making the aggregate strong Fig. 8-9
  • can only be hydrolyzed by some microorganisms (bacteria) which have cellulase; even these hydrolyze very slowly because the enzyme has difficulty gaining access to the bonds

b. chitin: linear polymer of D-N-acetylglucosamine linked by b-(1-->4) glycosidic bonds; forms exoskeletons of insects, crustaceans, and spiders; structure is similar to cellulose. Fig. page 205

6. Storage Polysaccharides: Starch and Glycogen

a. Starch: polymer of D-glucose linked by a(1-->4) glycosidic bonds
  • a-amylose-linear polymer with no branch points; although it is an isomer of cellulose it has very different physical properties resulting from a very different conformation, a left-handed helix which forms many H-bonds with water Fig. 8-10
  • amylopectin-linear polymer like amylose but with branches every 24-30 residue at a(1-->6) glycosidic bonds
  • degraded by a-amylase at 1-->4 bonds an by debranching enzyme (a-dextrinase) at branchpoints

b. Glycogen (animal starch)

  • structure is similar to amylopectin but with branches every 8-12 glucose residues
  • degraded by glycogen phosphorylase which removes glucose units from nonreducing ends forming glucose-1-phosphate.

7. Glycosaminoglycans Fig. 8-12

a. Haluronate - extracellular matrix along with collagen and elastin

b. Chondroitin Sulfates (at carbons 4 and 6): in cartilage

c. Keratan Sulfate - proteolglycans

B. Glycoproteins

1. Proteoglycans
a. core protein - with covalently linked glycosaminoglycans (keratan sulfate and chondroitin sulfate)

b. link proteins (40-60 kD) connect N-terminal globular end of core protein noncovalently to..

c. hyaluronic acid.

2. Bacterial Cell Walls - Peptidoglycan Fig. 8-15

a. linkear polymers of alternating NAG (N-acetylglucosamine) and NAM connected by b(1-->4) glycosidic bonds

b. NAM's lactic acid linked to tetrapeptide conaining D-Ala

c. neighboring strands are crosslinked via the tetrapeptide by other short peptides, pentaglycine in Staphylococcus aureus