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.

A2. Isomers

Sugars can exists as either configurational isomers (interconverted only by breaking covalent bonds) and conformational isomers. The figure below reviews different types of isomers.

The configurational isomers include enantiomers (stereoisomers that are mirror images of each other), diastereomers (stereoisomers that are not mirror images), epimers (diastereomers that differ at one stereocenter), and anomers (a special form of stereoisomer, diastereomer, and epimer that differ only in the configuration around the carbon which was attacked in the intramolecular nucleophilic attack to produces the a and b isomers).

Sugars can also exist as conformational isomers, which interchange without breaking covalent bonds. These include chair and boat conformations of the cyclic sugars.

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