Biochemistry and Molecular Biology Resource

Web Lessons

Eric C. Niederhoffer, Ph.D.

Associate Professor of Biochemistry and Molecular Biology

Southern Illinois University School of Medicine
600 Agriculture Drive, Carbondale, IL 62901-6503
Rm 112 Lindegren, 618-453-6467
eniederhoffer@siumed.edu
Copyright 2000- , E.C. Niederhoffer.
All Rights Reserved. All trademarks and copyrights are the property of their respective owners.

Saccharides

Monosaccharides are simple sugars (single polyhydroxy aldehyde or ketone unit). They exist as linear or cyclic molecules. Monosaccharides may also be modified to a variety of important derivatives. Two or more monosaccharides are connected through glycosidic bonds. Monosaccharides are one size class of what we term carbohydrates. The other two size classes are oligosaccharides (short chains of monosaccharides) and polysaccharides (generally more than 20 monosaccharide units). The longer (> 3 monosaccharide units) chains are typically linked to either proteins or lipids (glycoconjugates).

Simple monosaccharides (aldoses and ketoses)

Generic monosaccharide representations
aldose	ketose

Monosaccharides are illustrated as Fischer projections, which puts all atoms and bonds in the same plane.

Common monosaccharides
aldose	ketose
D-glyceraldehyde	dihydroxyacetone
D-erythrose	D-erythrulose
D-ribose	D-ribulose
D-glucose	D-fructose

Common hexoses
D-galactose	D-glucose	D-mannose

D-Glucose, D-galactose, and D-mannose are related to each other, differing by the configuration about one chiral center (epimers, note the OH and H groups when comparing D-Gal, D-Glc, and D-Man).

Cyclic monosaccharides (furanoses and pyranoses)

Monosaccharides possessing more than five carbon atoms predominantly exist in solution as cyclic (ring) molecules. One of the hydroxyl groups attacks the carbonyl carbon to form the ring. Five-membered rings as based upon furan, while six-membered rings are based upon pyran. Formation of the cyclic structures can result in two configurations about the carbonyl (anomeric) carbon, either or anomers. The cyclic molecules are represented as Hayworth projections below (note the OH group attached to the anomeric carbon)..

Common furanoses and pyranoses
-D-fructofuranose	-D-fructofuranose	-D-ribofuranose	-D-ribofuranose
-D-glucopyranose	-D-glucopyranose	-D-galactopyranose	-D-galactopyranose

Hexose derivatives

Sugar derivatives exist where a hydroxyl group is replaced with an amino group (hexosamine) and the amino group condensed with acetic acid (N-acetylhexosamine). The carbonyl group may be oxidized to generate aldonic acids while C-6 oxidation results in uronic acids.

Common hexose derivatives
-D-glucose	-D-glucosamine	N-acetyl--D-glucosamine
-D-glucuronate	D-gluconate cyclizes to D-glucono--lactone	N-acetylneuraminic acid

Common abbreviations
Name	Abbreviation
Fructose	Fru
Galactose	Gal
Glucose	Glc
Mannose	Man
Ribose	Rib
Glucuronic acid	GlcA
Glucosamine	GlcN
N-acetylglucosamine	GlcNAc
N-Acetylneuraminic acid	Neu5Ac

Disaccharides

Monosaccharides may be joined by reaction of a hydroxyl group of one monosaccharide with the anomeric carbon of another monosaccharide. This linkage is denoted as a glycosidic bond, which may be cleaved by dilute acid but otherwise resists base.

Common disaccharides
-D-glucopyranosyl-(14)-D-glucopyranose maltose
-D-galactopyranosyl-(14)--D-glucopyranose Gal(14)Glc lactose
-D-fructofuranosyl-D-glucopyranoside Fru(21)Glc sucrose

Glycosyl linkages

Monosaccharides or their derivatives may be linked to specific amino acids in glycoproteins. Serine and threonine form O-linkages and asparagine forms N-linkages.