Carbohydrate Chemistry 2a5d3v

Section 6: Carbohydrate Metabolism 1. Carbohydrate chemistry

10/14/2005

Overview of catabolism FATS

POLYSACCHARIDES

PROTEINS

Stage 1 fatty acids, glycerol

glucose, other sugars

amino acids Stage 2

acetyl CoA

oxidative phosphorylation

1

CoA

O2

ATP ADP + Pi

e– CO2

Stage 3

v

H2O

Krebs cycle

v

adapted from Fig. 14.12

Carbohydrates    

aka saccharides (suffix: -ose) elements: C, H, O common empirical formula: (CH2O)n n 3 monosaccharides aka simple sugars  carbon chain unbranched  1 carbonyl (C=O) group at end of chain: aldehyde not at end: ketone  2 or more OH groups # OH groups = n – 1 



2

oligosaccharides/polysaccharides 

contain a few/many linked monosaccharides

Monosaccharides: properties 

very polar (hydrophilic) functional groups OH: H-bond donor & acceptor =O: H-bond acceptor



chemically reactive, especially the carbonyl group, which reacts with: OH groups, forming rings oligosaccharides polysaccharides  oxidants & reductants  amino groups on proteins (glycation) 

3

Stereoisomerism (chirality) 

stereoisomers differ only in arrangement of atoms or groups  any C with 4 different groups attached (asymmetric or chiral C) 

enantiomers: a pair of molecules that are nonsuperimposable mirror images of one another H2C OH  example: trioses C O  simplest monosaccharides H2C OH  DHA: achiral (no stereoisomerism) dihydroxyacetone DHA (a ketose)  GA: D & L forms O O H H  in general, C C OH HO H H  # possible stereoisomers C C n = 2 (n = # asymmetric Cs) H2C OH H2C OH  D forms more common L-glyceraldehyde D-glyceraldehyde 

4

rotating L & D forms

GA (an aldose)

Glucose 







most important monosaccharide  the major monosaccharide in most organisms  main form in which carbohydrate transported in blood main roles: precursor of many biomolecules energy source (fuel) aka dextrose O H C H2C OH abbreviation: glc 1 H C OH C O 2 6 carbons: a hexose 

carbon numbering starts at end nearest C=O

other hexoses: fructose (frc) galactose (gal) 5 mannose



HO C H H C OH H C OH H2COH D-glucose (an aldohexose)

3 4 5 6

HO C H H C OH H C OH H2COH D-fructose (a ketohexose)

Glucose: ring formation (cyclization) 

carbonyl C is subject to nucleophilic attack by OH groups (arrow), forming a hemiacetal H2COH 2 isomers form: O H O 6



5

1

 a: OH projects on side of ring opposite from C6 (trans to C6)

b: OH projects on same side of ring as C6 (cis to C6) 6

C

H

4

2

H C OH 3

HO C H 4

H C OH 5

H C OH 6

H2COH D-glucose

H2COH C OH H H H C OH H C HO C C O H OH

open-chain form

HO

H

H OH

1

H

3

2

H

OH

OH

a ring form

H2COH H HO

H OH H

O OH H H OH

b ring form

Glucose: ring structure 



cyclic forms of monosaccharides are usually drawn as Haworth projections, even though rings not actually planar tetrahedral geometry H2COH H CH OH around each ring atom O H 2 H H O HO causes 2 atoms to be H H OH H H OH H O out-of-plane: HO OH OH H H OH chair conformation "chair" conformation a-D-glucopyranose of a-isomer 6-atom cyclic forms are Haworthprojections termed pyranoses after  H2COH the heterocycle pyran 5

1

4

3



H HO

7

H OH

2

O OH H H

H OH b-D-glucopyranose

O pyran

Glucose isomers







the a&bisomers are called anomers; the former carbonyl C (C1 in this case) is called the anomeric C anomer formation: aka anomerization 6 anomeric C: H2COH only one that has 2 of its bonds to O O H H H O H the reactions are: OH H 1C OH H O rapid, reversible 2 H C OH H OH & don’t need catalysts 3 a-D-glucopyranose HO C H forms in rapid-equilibrium 4 H C OH with each other H2COH 5 H C OH form % at equil. O OH 6 5



1

4

3



8



open-chain < 1 a  36 b  63

H2COH

open-chain form

H

HO

H OH H

2

H H OH

b-D-glucopyranose

Glucose: a & b ring forms 

ball & stick representations of  the a & b anomers  galactose, an epimer of glc at C-4

1

a glc

4

4

1 9

b gal

b glc

Fructose isomers  

 

like glc, fructose forms pyranoses it also forms 5-atom rings called furanoses furanoses are major forms in frc derivatives (e.g., sucrose) C2 is the anomeric carbon 1

H2C OH C O

OH OH HO C H H2C OH CH2 C C H C OH H H OH O C C H C OH HO H O H2COH D-fructose 10

open-chain form

furan

OH H2C

O

OH 1CH2

C C H H HO OH C C HO H a-D-fructofuranose

Oxidation 

 

carbohydrates with a free carbonyl group are easily oxidized, i.e., they are good reducing agents termed reducing saccharides/sugars – O O oxidation reaction is O H C C used to measure [glc] H C OH H C OH in blood & other HO C H HO C H + 2 Cu+ fluids + 2 Cu2+ H C OH H C OH  glc oxidase more H C OH H C OH reliable than Cu2+ H2COH H2COH  color development D-glucose D-gluconate proportional to gluconate + H2O2 glucose + O2 glucose [H2O2]

11

oxidase

Glycation of protein amino groups 



 

carbonyl groups react slowly & reversibly with amines, e.g., protein side chains (abbreviation: Pr-NH2) the resulting aldimine rearranges very slowly & irreversibly to a ketimine function of long-lived proteins can be impaired probable factor H O Pr N Pr N H H in long-term C C CH2 damage caused H C OH H C OH C O by high blood HO C H HO C H HO C H Pr NH2 [glc] in diabetes H C OH H C OH H C OH mellitus H C OH

12

H2COH glucose

H C OH

H2COH aldimine adduct

H C OH H2COH ketimine

Next: 2. Glycosides & Polysaccharides