Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431

Biochemistry

Fall 2008

Lecture Notes: 17 October

© R. Paselk 2008
 
PREVIOUS  

NEXT

Carbohydrates, cont.

Last time looked at the aldoses up tp the aldohexoses. In paticular we focussed on the linear forms of D-glucose, D-mannose and D-galactose:

structures of open Fischer structures of D-glucose, D-mannose, D-galactose and the closed ring form of glucose, alpha-D-glucopyranose

which commonly occur in the cyclic pyranose form (as shown for glucose) (text Figure 7-7), and the six carbon ketohexose, fructose.
Fischer structures of the open form of  D-fructose
which commonly occurs in a cyclic furanose form. The important ketoses include dihydroxyacetone, D-Xylulose, D-Ribulose, and D-Fructose (text Figure 7-3b-2). Note the relationship between the Fischer projections and the cyclic Haworth projections, using the example of glucose.
diagram demonstrating the conversion of the open form of D-glucose to its closed, ring form

The ring is then sealed via a hemiacetal bond. (text Figure 7-6) This would normally be quite unstable, however the closeness of the two reacting centers in the same chain makes them poor leaving groups, thus the hemiacetal is in fact the stable form of the six carbon aldoses. Thus the expected aldehyde chemistry for glucose is not seen (glucose is stable to oxygen etc.). Note that if drawn in the proper conformations (text Figure 7-8), or if constructed as models it will be seen that the chair conformation should be more stable. In addition, the Beta configuration of the hemiacetal -OH will be equatorial and should thus be preferred stereochemically as is in fact the case. Interestingly organisms can generally only use the Alpha form, so isomerases are provide to interchange the two. 

Note that these sugars with hemiacetal groups are "reducing sugars." (text Figure 7-10) That is the hemiacetal can open up and be oxidized as an aldehyde.

Variety of modified sugars important in biological systems. (text Figure 7-9)

Disaccharides

Can link sugars via acetal bonds, known as glycosidic bonds.

diagram showing the formation of a glycosidic bond between two sugars

There are four common disaccharides:

The first two are reducing sugars, that is they have "free" aldehyde groups, whereas sucrose and trehalose have both carbonyl groups tied up in the relatively stable glycosidic bond. Maltose, fructose and trehalose are joined in alpha-glycosidic bonds. In general the alpha-glycosidic bond is easily cleaved (it is less stable chemically and organisms have enzymes to cleave it) whereas the beta-glycosidic bond is very difficult to break down.

An exception for mammals is the ability of nursing animals to digest lactose, for which the special enzyme Lactase is provided. Note that this ability is generaly lost at the age of weaning, at which time the animal becomes lactose intolerant.

Sucrose, Sucrase (Invertase), and the magic of liquid filled chocolate covered cherries.

Polysaccharides

Can have both homo- and heteropolysaccharides. We will focus on homopolysaccharides as most central, but will mention some heteropolysaccharides to illustrate their functions. Homopolysaccharides have a single type of residue. Most common polysaccharides contain glucose. Used for energy (food) storage (starches and glycogen) and structure (cellulose).

Starch (energy storage in plants). Two kinds

Glycogen: animal starch. Just like amylopectin, but more highly branched (every 8-12 residues). This allows more free ends for more rapid breakdown-important in animals.


Pathway Diagrams

C431 Home

Lecture Notes

Last modified 17 October 2008