Biochemistry Online: An Approach Based on Chemical Logic

CHAPTER 3 - CARBOHYDRATES/GLYCANS

A: Mono- and Disaccharides

03/14/16

Learning Goals/Objectives for Chapter 3A: After class and this reading, students will be able to

define a sugar
apply knowledge about reactions at carbonyl carbons of aldehyde and ketones in the formation of hemiacetals/hemiketals and acetals/ketals from organic chemistry to draw mechanics showing cyclization of aldoses and ketoses and (hemiacetal/hemiketal formation) and of polymerization of aldoses and ketones to form dissacharide and polysaccharide (acetal/ketals).
draw open chain, Haworth (planar), and puckered (nonplanar) representations of 6 member cyclic sugars
know the names of common trioses, ribose (5C) and hexoses and draw open chain, Haworth, and puckered representations of each
differentiate between enantiomers, diastereomers and anomers of the hexoses
explain the difference between reducing and nonreducing dissacharides
draw the structure and name common sugar derivatives of the hexoses

The link below is an extraordinary and free resource on glycobiology. It defines the word "glycan" as a "generic term for any sugar or assembly of sugars, in free form or attached to another molecule" and "is used interchangeably ... with saccharide or carbohydrate."

Essentials of Glycobiology, 2nd edition, is available online.

A3. Monosaccharide Derivatives

Many derivatives of monosaccharides are found in nature. These include

oxidized forms in which the aldehyde and/or alcohol functional groups are oxidized to carboxylic acids
phosporylated forms in which phosphate is added by ATP to form phosphoester derivatives
amine derivatives such as glucosamine or galactosamine
acetylated amine derivatives such as N-Acetyl-GlcNAc (GlcNAc) or GalNAc
lactone forms (intramolecular esters) in which an OH group attacks a carbony C that was previously oxidized to a carboxylic acid
condensation products of sugar derivatives with lactate (CH3CHOHCO2-) and pyruvate, (CH3COCO2- ), both from the glycolytic pathway, to form muramic acid and neuraminic acids, (also called sialic acids), respectively.

Figure: Sugar Derivatives

Sugar Derivatives

In the above figure, N-acetylmuramic acid, found in bacterial cell walls, consists of GlcNAc in ether link at C3 with lactate, while N-acetylneuraminic acid results from an intramolecular cyclization of a condensation product of ManNAc and pyruvate.

Figure: Is sialic acid the big difference between humans and chimps?

What happens when non-vegan humans eat animal products (meat, milk) with N-glycoyl neuraminic acids (Neu5Gc)? Varki et al found that some gets incorporated into human membrane glycans (see next Chapter). Sialic acids on surface proteins can serve as "receptors" that allowing binding of self-cells as well as foreign cells or proteins that have evolved to bind them. Byres et al discovered that a toxin, SubAB, secreted by E. Coli 0157, can bind Neu5Gc. Hence eating meat products can make us more susceptible to bacteria that recognize Neu5Gc.

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