Section 6: Carbohydrate Metabolism

1
Section 6 Carbohydrate Metabolism

• 1. Carbohydrate chemistry

10/14/2005
2
Overview of catabolism
FATS
POLYSACCHARIDES
PROTEINS
Stage 1
amino acids
fatty acids,glycerol
glucose,other sugars
Stage 2
acetyl CoA
CoA
H2O O2
Stage 3
eCO2
Krebscycle
oxidativephosphorylation
v
v
ATP ADP Pi
1
adapted from Fig. 14.12
3
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 (CO) group at end of chain
  aldehyde not at end ketone
• 2 or more OH groups
• OH groups n 1
• oligosaccharides/polysaccharides
• contain a few/many linked monosaccharides

2
4
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
5
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
• example trioses
• simplest monosaccharides
• DHA achiral (no stereoisomerism)
• GA D L forms
• in general,
• possible stereoisomers 2n (n asymmetric
  Cs)
• D forms more common

rotating L D forms
4
6
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 dextroseabbreviation glc
• 6 carbons a hexose
• carbon numbering starts atend nearest CO
• other hexosesfructose (frc)galactose
  (gal)mannose

1 2 3 4 5 6
5
7
Glucose ring formation (cyclization)

• carbonyl C is subject to nucleophilic attack by
  OH groups(arrow), forming a hemiacetal
• 2 isomers form
• 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
8
Glucose ring structure

• cyclic forms of monosaccharides are usually drawn
  as Haworth projections, even though rings not
  actually planar
• tetrahedral geometry around each ring atom
  causes 2 atoms to be out-of-planechair
  conformation
• 6-atom cyclic forms aretermed pyranoses
  afterthe heterocycle pyran

7
9
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
• anomeric C only one that has 2 of its bonds to
  O
• the reactions are rapid, reversible dont
  need catalysts
• forms in rapid-equilibrium with each other
• form at equil.
• open-chain lt 1 a 36 b 63

8
10
Glucosea 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
b glc
b gal
9
11
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

10
12
Oxidation

• carbohydrates with a free carbonyl group are
  easily oxidized, i.e., they are good reducing
  agents
• termed reducing saccharides/sugars
• oxidation reaction is used to measure glc in
  blood other fluids
• glc oxidase more reliable than Cu2
• color developmentproportional toH2O2

11
13
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 factorin long-termdamage causedby
  high bloodglc in diabetesmellitus

12
14
Next2. Glycosides Polysaccharides