Amino acid dehydrogenases catalytic mechanism

Alcohol dehydrogenase contains two atoms of zinc. One atom participates in the catalytic mechanism, while the other serv es a structural role. The enzyme actually is a dimer, and is composed of two subuiuts. Several types of alcohol dehydrogenase occur in the body. The enzymes have closely related amino acid sequences, and thus consist of a family of related enzymes. 

These cross-linked amino acid residues appear to have no direct participation in the catalytic mechanism. They do play a structural role in determining the tertiary structure of the 7 suhunit. It is interesting to note that the TTQ dependent methylamine dehydrogenase does not have these thioether cross-linked residues, but does have six intra-subunit disulfide bonds between cysteine residues, which play a structural role in determining the tertiary structure of the TTQ bearing (3 subunit of that enzyme. The a subunit of QHNDH contains two r-type hemes. One heme c is solvent-accessible and the other is fully buried within the a subunit and located approximately 9 A from the tryptophylquinone moiety of CTQ on the 7 subunit. The a and 7 subunits sit on the surface of the / subunit that with the 7 subunit forms the enzyme active site. 

Table VIII lists the side chain hydrogen bonds within one subunit. Table IX gives the distances of all tyrosines and tryptophans within the molecule to the center of the nicotinamide in the red subunit and these are depicted in Fig. 14. Table IX also describes the environment of these residues. Amino acid residues that have been chemically modified will be further discussed in Section II,B. Tryptophan has not been modified in LDH, but Shallenberg had implicated it in the catalytic mechanism of many dehydrogenases (152). His data could not be confirmed (153,154). As can be inferred from Table IX, there is no tryptophan in the neighborhood of the active site.

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