Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online


A:  Mono- and Disaccharides


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."

A4.  Formation of Hemiacetals, Acetals, and Disaccharides

Monosaccharides that contain aldehydes can cyclize through intramolecular nucleophilic attack of an OH at the carbonyl carbon in an addition reaction to form a hemiacetal (hemiketal if attack on a ketone). On the addition of acid (which protonates the anomeric OH,  forming water as a potential leaving group), another alcohol can add forming an acetal (or ketal from a ketone) with water leaving.

If the other alcohol is a second monosaccharide, a dissacharide results. The acetal (or ketal) link bonding to the two monosaccharides is called a glycosidic link. Links between the two sugars can be either a (if the OH on C1 involved in the glycosidic link is pointing down) or b (if the O on C1 involved in the glycosidic link is pointing up). Since sugars contain so many OH groups which can act as the "second" alcohol in acetal (or ketal) formation, links between sugars can be quite diverse. These include a and b forms of 1-2, 1-3, 1-4, 1-5, 1-6, 2-2, etc. links. For example:

The glycosidic (acetal or ketal) link can be cleaved by hydrolysis, just as the peptide bond in proteins.

Figure:  A closer look at reducing and nonreducing sugars:  lactose and maltose

JmolD Glucose    Jmol Acetal Formation



Return to Chapter 3A:  Monosaccharide Sections

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Archived version of full Chapter 3A:  Mono- and Disaccharides


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