Transamination action mechanism

The Schiff base can undergo a variety of reactions in addition to transamination, shown in Fig. 6.4 for example, racemization of the amino acid via the a-deprotonated intermediate. Many of these reactions are catalyzed by metal ions and each has its equivalent nonmetallic enzyme reaction, each enzyme containing pyridoxal phosphate as a coenzyme. Many ideas of the mechanism of the action of these enzymes are based on the behavior of the model metal complexes. 

The role of pyridoxal in the transamination reaction has been extensively investigated by comparing the mechanism of action of nonenzymic transamination (see Fig. 4-28) in the presence of metal (iron) acting as a catalyst to the transamination catalyzed by the specific proteins. The mechanism of the nonenzymic transamination was deduced from a series of rather simple observations (1) when an amino acid is added to a dilute solution of pyridoxal or pyridoxine phosphate, the absorption maximum of the solution shifts from 345 mp to 430 mp. This change in absorption... 

One of the mechanisms which has been proposed to account for the oxidative deamination of amino acids invokes a transamination followed by a deamination. This mechanism appears to be capable of explaining the rapid and reversible deaminations whose character is not in accordance with the properties and action of the L-amino acid oxidases. A transamination to a-ketoglutaric acid would remove the amino group from an... 

The chemical transamination of pyridoxal to pyridoxamine, discovered by Snell,not only led to the discovery of the role of vitamin Be in enzymatic transamination but was suggested as a possible mechanism of action. (See Schlenk and Snell. )... 

If this were indeed the mechanism of action of pyridoxal phosphate, then one would think that either pyridoxal or pyridoxamine phosphate would be active in catalyzing the enzymatic transamination. In one case recently with a highly purified glutamic aspartic transaminase only pyridoxal phosphate was capable of reconstituting the system. 

Thus the mechanism of action of pyridoxal phosphate in transamination must be left open. There is as yet no clue to its mechanism of action in decarboxylation and in the tryptophanase reaction. 

There are apparently two distinct enzymic reactions which result in the anaerobic degradation of cysteine. The first is a direct desfilfhydration, the second is initiated by transamination with a-ketoglutarate to yield glutamate and 8-mercaptopyruvate. According to the original observations of Desnuelle and Fromageot, Smythe and others (71) the mechanism of cysteine desulfhydrase action is as follows ... 

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