1) Metabolic

- Fuel (e.g., glucose, glycogen)
- Biosynthetic precursors (e.g., amino acids, nucleotides and nucleic acids)

- structural glycans (e.g., chitin, cellulose)
- glycoconjugates (proteo- and peptidoglycans, glycoproteins)

Monosaccharides: e.g., glucose, mannose, fructose, etc.

Oligosaccharides (2-~ 20): e.g., sucrose, "CHO" in glycoproteins

Polysaccharides (homo- and hetero-glycans): e.g., amylose, glycogen, cellulose, chitin

Synthesis is "enzymatic":

1) Unlike proteins and nucleic acids, sequence + length of polymers is not template-dependent/genetically-determined

2) Polymers may be linear (e.g., amylose) or branched (amylopectin, glycogen)
 

Basic formula" = (CH₂O)_n
Structural diversity is based on:

1) Carbon number:

- trioses (Glyceraldehyde, Dihydroxyacetone)
- tetroses (e.g., erythrose, threose)
- pentoses (Ribose, Ribulose, arabinose, etc.)
- hexoses (Glucose, Mannose, Galactose, Fructose)

3) Stereoisomers: (2)ⁿ

• For linear sugars, # of chiral centers (n) = (C2) for aldoses and (C3) for ketose
• D/L determined by highest numbered carbon: on Fischer projection, D= right, L=left (converion from D- to L- requires inverion of every center)
• Epimers (e.g. D-glucose vs. D-mannose) have inversion of a single carbon

Aldoses: aldehyde + alcohol = hemiacetal
Ketoses: ketone + alcohol = hemiketal

Furanose: five-membered ring (e.g, ribofuranose)
Pyranose: six-membered ring (e.g, glucopyranose, ribopyranose)

OH oxygen is incorporated into ring
C=O oxygen becomes OH that projects from anomeric carbon
The "C=O" carbon becomes chiral in the ring: -OH group is either down (the anomer) or up ( anomer)
As long as it does not participate in further covalent linkages (e.g., to other sugars), the anomeric carbon is subject to:

• rapid equilibrium: transient ring opening results in interconversion between & (and furanose/pyranose)
• oxidizing agents (polysaccharide chains have reducing and non-reducing ends)

Converting between Fischer and Haworth projections:

1) D means "down" for -OH: groups on right of Fisher projection point down in Haworth projection

2) D means up for carbon: in cases where C6 is not incorporated into the ring, it will project above the ring in "D" sugars

Furanoses:

Twist (3 atoms are co-planar)
Envelope (4 coplanar atoms)

Subsituents are either axial or equatorial:

- more stable when bulky groups are equatorial
- in chair conformation of -glucose (-D-glucopyranose), all non-H groups can be equatorial

Sugar phosphates (esters)

Deoxy sugars

Amino sugars (e.g., glucosamine, GlcNAc)

Sugar alcohols (e.g., inositol, ribitol)

Sugar acids (e.g., gluconate, glucuronate)
 

Condensation reaction (with elimination of water) involving the anomeric carbon

Anomeric carbon becomes fixed (alpha or beta) and is no longer subject to oxidation (i.e., is "non-reducing")
Allow polymerizaton of sugars: hemi-acetal/hemi-ketal becomes linked to -OH of another sugar
 

Glucose polymers (homoglycans):

1) Amylose: alpha1->4 links (linear)

2) Glycogen and amylopectin: alpha1->4 links with 1->6 branchpoints (more frequent in glycogen than amylopectin)

(Key digestive enzymes:
alpha-amylase is a digestive endoglycosidase that hydrolyzes alpha1->4 links between internal glucose molecules
beta amylase is an exoglycosidase of plants that cleaves maltose from non-reducing ends
Cleavage of alpha 1->6 links requires "debranching enzyme")

3) Cellulose: Beta1->4 links

Resistant to mammalian enzymes: indigestible
Interchain H-bonding stabilizes strong, insoluble fibrils

Glycosaminoglycans

e.g., Hyaluronic acid: glucuronic acid and GlcNAc, beta1->3 and beta 1->4 links

In addition to an amino sugar and (often) an alduronic acid, some other glycosaminoglycans (e.g., keratan sulfate, chondroitin sulfate) have additional (charged) sulfates

Highly hydrated: important for tensile and compressile strength of cartilage and other connective tissue