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Biochemistry: Carbohydrates & Glycobiology

Carbohydrate Roles, Structure, Stereochemistry, Glycosidic Bonds

Carbohydrates are found in:

polymers stores in muscle and liver cells

Bacterial cell walls (PG)

Attached to specific proteins found in cellular membranes

3 different roles of carbs:

nutrition/energy-glycogen & starch (amylopectin)

structure polymers- cellulose, PG, chitin

informational molecules-DNA, RNA, & glycoproteins

Carbohydrates participate in covalently link bonds & are primarily hydrophilic molecules

Typically six carbons in a typical monosaccharide, having five chiral centers ( five hydroxyl groups)

There is 2^n stereoisomers for a sugar with n chiral centers.

An epimers are monosaccharides which differ in configuration around a single carbon

Look at the furthest chiral carbon from the carbonyl and use its OH group, if OH group is on the right it is assigned a D but if its on the left it is assigned an L when determining L or D configuration

An anomeric carbon is the carbon atom in a sugar at the new stereo center formed when a sugar cyclizes to form a hemiacetal/hemiacetal, this is the carbonyl carbon of aldehydes and ketones

Mutarotation is the change is specific rotation of a pyronose or furanose sugar or glycoside accompanying the equilibration of its alpha or beta anomeric forms

Pyronose=six membered ring compounds

Furanoses= five membered ring

By "reducing" end of a di-, oligo- or polysaccharide we mean end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) commonly known as the reducing end

3 Major Size Classes

1.)monosaccharides (contain 1 mono. unit)

2.)oligosaccharides (contain 2 -20 mono. units)

3.)polysaccharides (>20 mono. units)

Polysaccharides

a.k.a. glycans , differ in the identity of their:

-recurring monosaccharides units

-in the length of their chains

-in the types of bonds linking their units

Homopolysaccharides=contain only a single monomeric species

Heteropolysaccharide= cotanin two or more different kinds

Some of the common sugars are Maltose, Lactose, and Sucrose

Advantage of a cell storing glucose as a polymer vs. individual molecules-