|Lecture Notes: 17 October||
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:
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 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 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)
Can link sugars via acetal bonds, known as glycosidic bonds.
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.
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.
Last modified 17 October 2008