1. Biochem Block Handout #8: Saccharides (sugars)
Cycle of Carbon

2. Aldose vs Ketose
Aldoses can isomerize to ketoses via an enediol intermediate. In vivo,
this requires enzymes to proceed at a reasonable rate.
Has aldehyde group
Has ketone group

3. Tetroses: Example of Stereochemistry
For chiral molecules: # of stereoisomers = 2n, where n = # of chiral carbons.
Recall that a chiral carbon must have 4 different groups bound to it.
Thus carbons 2 & 3 in the sugars below are chiral; carbons 1 & 4 are not

4. Stereochemical relationships of the D-aldoses:

5. Cyclization of sugars
Furanose: a 5-membered (pentagonal) ring form of the sugar, related to furan.
Pyranose: a 6-membered (hexagonal) ring form of the sugar, related to pyran.

6. Conformations of Sugars
Hexoses can be in either the “chair” or “boat” conformation.
The chair is much more stable.
Conformations are not isomers.

7. Isomeric Forms of Sugars
a: OH points down
b: OH points up

8. The glycosidic bond
Formation of the glycosidic bond between two monomers in an oligosaccharide is a condensation reaction, involving the elimination of water. See below for lactose:
Like the phosphodiester bond in nucleic acid and amide bond in proteins, the glycosidic bond is metastable and enzymes control its hydrolysis.
Another example below: maltose, with an α(14) glycosidic bond

9. Amylose Helix Starch is made of 2 polymers: Amylose and amylopectin.
Amylose: a linear glucose polymer with a-(1,4) linkages. Amylose can form a stable helix. The helix can bind iodine to form a bright blue complex. Used for long-term energy storage.
Amylopectin: a highly branched
polymer with a-(1,4) linkages.
The branches come from
a-(1,6) linkages.

10. Branching in Amylopectin
Glycogen, found in animal flesh, has a similar structure. Cleavage of glucose
residues occurs from non-reducing ends. Reducing ends can undergo oxidation.

11. Oxidation of Sugars
Oxidation of monosaccharides can proceed in several ways, for example, mild oxidation of an aldose with alkaline Cu(II) (Fehling’s solution)

12. Blood group antigens: oligosaccharide signaling
The ABO blood group antigens:
The blood group substances are a set of antigenic oligosaccharides attached to the surfaces of red cells.
The O oligosaccharide does not elicit antibodies in most humans.
The A and B antigens are formed by addition of GalNAc or Gal, respectively, to the O oligosaccharide.
Each of the A and B antigens can elicit a specific antibody.
R can represent either a protein molecule or a lipid molecule.

13. Cartilage: a glycopolymer

14. Peptidoglycan synthesis
Penicillin inhibits the crosslinking step. Other antibiotics act on different steps.