Note the relationships of the various cofactors to their vitamin precursors. Chapter 7 of your text has images of the various cofactors and vitamins, as well as example chemistries. You should keep it in mind to refer back too during our studies.
Nucleotide Functions: Most involve use of the nucleotide as a recognition molecule, e.g. ATP [Figure 7.4]
Nicotinamide Adenine Dinucleotide (NAD+) uses ADP (bolded) as a recognition "handle." (Note the two nitrogenous bases each attached to a ribose and linked through a phosphoric acid anhydride linkage: [Figure 7.8]
Similarly adenosine with a modified ribose (reduced to the alcohol - ribitol) is used in Flavin Adenine Dinucleotide = FAD (not truly a dinucleotide since ribitol instead of ribose!): [Figure 7.10B]
Coenzyme A has an ADP attached to an arm of pantothenic acid, which in turn is attached to beta-mercaptoethylamine. Acetyl groups can be carried on the sulfhydryl group: [Figure 7.12]
Three vitamins give cofactors with long "arms" which enable the cofactors to shift an attached substrate between adjacent active sites on a single enzyme.
Note attachment of biotin and lipoate to lysine side-chain to give 10 atom arms. [Figure 7.29]
Let's look at the two families, aldoses and ketoses. The important aldoses include the five carbon aldopentose, ribose [Figure 8.3]:
The six carbon aldohexoses, glucose, mannose, and galactose [Figure 8.3].
The ring is then sealed via a hemiacetal bond. [Figure 8.8] 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 8.12], 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.
Can link sugars via acetal bonds, known as glycosidic bonds. [Figure 8.19]
There are four common disaccharides [Figure 8.20]:
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).
© R. A. Paselk 2010;
Last modified 4 March 2013