Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2007

Lecture Notes: 26 February

© R. Paselk 2006
 
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CARBOHYDRATES, cont.

  Note that sugars occur in both D and L forms. As we shall see the natural sugars are generally D. Let's look at the two families, aldoses and ketoses. The important aldoses (Figure 8.3, p 234) [overhead 9.4 P] include the five carbon aldopentose, ribose:
which commonly occurs in the cyclic furanose form.The six carbon aldohexoses, glucose, mannose, and galactose.
 
which commonly occur in the cyclic pyranose form (as shown for glucose) [glucose model], and the six carbon ketohexose, fructose.
which commonly occurs in a cyclic furanose form. The important ketoses include dihydroxyacetone, D-Xylulose, D-Ribulose, and D-Fructose [overhead 9.7 P]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. [overhead 9.10 P] 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 (Figure 8.11, p 239), 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.

An important reaction is the Lobry-de-Bruyn-van Ekenstein Transformation. This base catalyzed reaction sequence interconverts three of the major hexoses, and will be used later in understanding some isomerase enzyme mechanisms. The mechanism is symmetrical. You should finish the second half on your own.

 

DISACCHARIDES

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

There are four common disaccharides (Fig 8.20, p 244) [overhead 9.24, P]:

The first three are reducing sugars, that is they have "free" aldehyde groups, whereas sucrose has both carbonyl groups tied up in the relatively stable glycosidic bond. Maltose and fructose 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.

Thus for cellobiose, and more importantly, cellulose which is also linked by beta-bonds, essentially only bacteria can digest this bond.

So animals can't digest cellulose! You may ask, What about Cows and things? Well they use bacteria. Cows for instance are basically walking fermentation tanks.

Cool biological examples of cellulose use by animals: Desert Iguana consume feces to maintain culture; Rabbits eat and reprocess first pass feces (soft) to take advantage of fermentation; Multiple stomachs in Ruminants; Ultimate symbiosis in some termites: protozoans in gut have bacteria in gut, and use spirochetes as "cilia" (rowers).

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


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Last modified 27 February 2006