Biochemistry and Molecular Biology Resource

 

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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.
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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 α or β anomers. The cyclic molecules are represented as Hayworth projections below (note the OH group attached to the anomeric carbon)..

Common furanoses and pyranoses

α-D-fructofuranose

β-D-fructofuranose

α-D-ribofuranose

β-D-ribofuranose

α-D-glucopyranose

β-D-glucopyranose

α-D-galactopyranose

β-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

β-D-glucose

β-D-glucosamine

N-acetyl-β-D-glucosamine

β-D-glucuronate

D-gluconate cyclizes to D-glucono-δ-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

α-D-glucopyranosyl-(14)-D-glucopyranose

maltose

β-D-galactopyranosyl-(14)-D-glucopyranose

Gal(β14)Glc

lactose

β-D-fructofuranosyl α-D-glucopyranoside

Fru(β 21α)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.

Glycosyl linkages

O-linkages

N-linkages

GalNAc to Ser (or Thr)

GalNAc to Asn

 

 

 

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eniederhoffer@siumed.edu