CHEM 237 - Carbohydrates

Monosaccharides

aldose

ketose

aldose

polyhydroxy aldehyde (end/terminal carbonyl)

ketose

polyhydroxy ketone (internal carbonyl)

Common stereochemistry of sugars in nature

D and L

D is more common

Glyceraldehyde

3 carbon sugar (triose)

aldose

D or L designation

D: -OH is on the right

L: -OH is on the left

determined by the stereochemistry at the furthest chiral carbon from carbonyl

enantiomers

mirror images

same physical properties

diastereomers

stereoisomers that differ at 1 or more chiral centre

different physical properities

epimers

diastereomers that differ at ONLY ONE stereocenter

D-glucose

is an aldose

D-fructose

is a ketose

D-glyceraldehyde

is an aldotriose

D-ribose

is an aldopentose

Aldohexoses

D-glucose

D-mannose

D-galactose

how do linear sugars cyclize

intramolecular nuc attack by -OH on carbonyl group

forms new chiral center

Furanose

5 membered ring

4Cs

Pyranose

6 membered ring

5Cs

anomeric carbon

carbon attached to O and OH

most reactive carbon

Haworth convention

ring O at back, anomeric C at right

α: -OH points down

β: -OH points up

anomers

diastereomers that differ at the anomeric carbon

α-anomer

OH and CH2OH are opposite

β-anomer

OH and CH2OH are same

sugars prefered conformation

chair conformation

put bulky substituents in equatorial>axial

mutarotation

reopening and reclosing of a cyclic sugar to convert into α or β

aldonic acid

oxidation of an aldehyde to a carboxylic acid

produces ester

uronic acid

oxidation form pimary OH group to a carboxylic acid

produces ester

Reducing sugars

have a terminal aldehyde group that can be reduced by NADP+ to NADPH

deoxy sugars

sugars missing an OH group

β-D-Deoxyribose (DNA)

β-D-Ribose (RNA)

amino sugars

hydroxyl groups replaced by amino groups

can create N-acetyl substituent

reduced sugars

carbonyl group reduced to alcohol

e.g. gylceraldehyde gives glycerol

glycosidic bonds

formation of bonds from the anomeric carbon

*cannot undergo mutarotation at neutral pH

head-to-tail linkage

anomeric carbon linked to a non-anomeric carbon

head to head linkage

2 anomeric carbons linked together

linear 1-4 linkage

has one reducing end

polysaccharides

can be homopolymeric or heteropolymeric

homopolymeric polysaccharides

polygluose

• starch

• glycogen

• cellulose

poly-N-acetylglucosamine

• chitin

homopolymers with α-linkages

starch

glycogen

α-amylose

starch

mixture of glucose polymers that plants make as their food reserve

deposited in cytoplasm of plants as insoluble

2 major sugar polymers

• α-amylose (linear)

• amylopectin (branched)

glycogen

storage of sugars in animals

found especially in muscle and liver

similar to amylopectin but more branched

α(1-4) linked polyglucose with α(1-6) branches every 8-14

one reducing end

α-amylose (starch)

α(1-4) linkage

linear unbranched chains, thousands of glucose long

helical shape

one non reducing end

amylopectin

α(1-4) linked with α(1-6) branches every 24-30 glucose residues

main chain has one reducing end

branch has no reducing ends

Homopolymers with b-linkage

cellulose

chitin

cellulose

mixture of glucose polymers joined by b1-4 linkage

main component of plant cell walls

long extended structure, highly H-bonded

bonds within strands, between strands, between sheets

hard to metabolize

1. extensive H-bonding

2. inaccessibility of linkages

chitin

similar to cellulose but sugar is N-acetylglucosamine (NAG)

components of exoskeleton of crustaceans and insects

structural polysaccharides

b(1-4) linkages

storage polysaccharides

a(1-4) linkages

glycosaminoglycans

unbranched polysaccharides with alternating uronic acid and hexosamine

proteoglycans

diverse group of molecules when core proteins has at least 1 glycosaminoglycan chain attached to O-(ser/thr) and N-(Asn) linkages

peptidoglycan

specialized structural polysaccharide in bacterial cell walls

consists of NAG and NAM covalently linked to tetrapeptide and pentaGly bridges

highly crosslinked = high stability to cell wall

glycoproteins

proteins have sugars covently linked to AA side chains

often use b-turns

2 types:

1. O linked

2. N linked

O-linked glycoproteins

linked via OH group of Ser or Thr

N-linked glycoproteins

linked through amide group of Asn

secreted and membrane proteins

generally glycosylated

carbohydrates on cell surface

play a role in cell-cell recognition

protein glycosylation is highly heterogeneous

same proteins can differ based on what/how much sugar is added

ABO blood group

Type A has extra GalNAC

Type B has extra Gal

glycolipids

various oligosaccharides attached to lipids