CHAPTER 3 - CARBOHYDRATES/GLYCANS
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
- differentiate between enantiomers, diastereomers and anomers
of the hexoses
- explain the difference between reducing and nonreducing
- draw the structure and name common sugar derivatives of the
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
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:
- lactose: Gal(b 1->4)Glc Since Glc is attached to Gal through
the OH on C4, its anomeric carbon, C1, could revert to the noncyclic
aldehyde form. This aldehyde is susceptible to oxidation by reagents
(Benedicts Solution - with citrate,
Fehlings reagent - with tartrate) which are subsequently reduced.
In both reagents, reducing sugars reduce a basic blue solution of CuSO4
(Cu2+) to form a brick red precipate of Cu2O (Cu+). Sugars (monosaccharides,
dissacharides and polysaccharides which can form an aldehyde at C1 or
have an a-hydroxymethyl ketone group which can
isomerize to an aldehyde under basic conditions (such as fructose) are
called reducing sugars. These oxidizing agent are mild and react with
aldehydes and not ketones.
- sucrose: Glc(a 1->2)Fru. Since Fru is attached through the anomeric
OH of this ketose, the Fru is not in equilibrium with its straight chain
keto form, and hence sucrose is a nonreducing sugar.
The glycosidic (acetal or ketal) link can be cleaved by hydrolysis, just
as the peptide bond in proteins.
A closer look at reducing and nonreducing sugars: lactose and maltose
Chapter 3A: Monosaccharide Sections
Biochemistry Online Table of Contents
Archived version of full Chapter 3A:
Mono- and Disaccharides
Biochemistry Online by Henry Jakubowski is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.