Humboldt State University ® Department of Chemistry

Richard A. Paselk
Chem 431

Biochemistry

Fall 2007
Lecture Notes: 10 October

© R. Paselk 2007
  
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Example Enzymes, cont.

Chymotrypsin: (digestive enzyme: zymogens-precursor protein has peptide covering active site, activated by having it hydrolyzed off).

CARBOHYDRATES

  The carbohydrates, or sugars, are our third group of biomolecules. They are characterized by having a carbonyl carbon (aldehyde or ketone) and multiple hydroxyl groups. The smallest sugars are thus the three carbon trioses, glyceraldehyde (aldotriose) and dihydroxyacetone (ketotriose), text Figure 7-1a.

Note that sugars occur in both D and L forms (text Figure 7-2b). As we shall see the natural sugars are generally D. Let's look at the two families, aldoses and ketoses. The important aldoses (text Figure 7-3a-1,2) include the five carbon aldopentose, ribose:
which commonly occurs in the cyclic furanose form.The six carbon aldohexoses, glucose, mannose, and galactose. (text Figure 7-3a-3)
 
which commonly occur in the cyclic pyranose form (as shown for glucose) (text Figure 7-7), and the six carbon ketohexose, fructose. (text Figure 7-3b-2)
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.

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 steriochemically 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.

DISACCHARIDES

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.

Pathway Diagrams

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Last modified 16 November 2007