| Chem 431 |
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Fall 2001 |
| Lecture Notes:: 28 September |
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| PREVIOUS |
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): (A model of D-glyceraldehyde may be viewed by clicking on the button at this site.}
Note that the glyceraldehyde occurs in D and L forms. As we shall see the natural sugars are D. Look at the two families, aldoses and ketoses. The important aldoses (Figure 8.1, p 197) [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: {Models of glucose, linear and ring forms, may be viewed by clicking on the buttons at this site.}
which commonly occur in the cyclic pyranose form (as shown for glucose) [glucose model], and the six carbon ketohexose, fructose: (A model of fructose, in the ring form, may be viewed by clicking on the buttons at this site.}
which commonly occurs in a cyclic furanose form. The important ketoses include dihydroxyacetone, D-Xylulose, D-Ribulose, and D-Fructose (Figure 8.2, p 198) [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. (Figure 8.3, p 199) [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 8.5, p 201), 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 catalysed 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.
Can link sugars via acetal bonds, known as glycosidic bonds.
There are four common disaccharides [overhead 9.24, P]:
The first three are reducing sugars, that is they have "free" aldehyde groups, whereas fructose has both carbonyl groups tied up in the relatively stable glycosidic bond. Maltose and fructose are joined in a-glycosidic bonds. In general the a-glycosidic bond is easily cleaved (it is less stable chemically and organisms have enzymes to cleave it) whereas the b-glycosidic bond is very difficult to break down.
Thus for cellobiose, and more importantly, cellulose which is also linked by b-bonds, essentially only bacteria can digest this bond.
So animals can't digest (possible exception of snails). You may ask, What about Cows and things? Well they use bacteria. Cows for instance are basically walking fermentation tanks.
 Pathway Diagrams |
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