Mole, transaminases

Although the utility of transaminases has been widely examined, one such limitation is the fact that the equilibrium constant for the reaction is near unity. Therefore, a shift in this equilibrium is necessary for the reaction to be synthetically useful. A number of approaches to shift the equilibrium can be found in the literature.53 124135 Another method to shift the equilibrium is a modification of that previously described. Aspartate, when used as the amino donor, is converted into oxaloacetate (32) (Scheme 19.21). Because 32 is unstable, it decomposes to pyruvate (33) and thus favors product formation. However, because pyruvate is itself an a-keto acid, it must be removed, or it will serve as a substrate and be transaminated into alanine, which could potentially cause downstream processing problems. This is accomplished by including the alsS gene encoding for the enzyme acetolactate synthase (E.C. 4.1.3.18), which condenses two moles of pyruvate to form (S)-aceto-lactate (34). The (S)-acetolactate undergoes decarboxylation either spontaneously or by the enzyme acetolactate decarboxylase (E.C. 4.1.1.5) to the final by-product, UU-acetoin (35), which is meta-bolically inert. This process, for example, can be used for the production of both l- and d-2-aminobutyrate (36 and 37, respectively) (Scheme 19.21).8132 136 137... 

The Stickland reaction (47) has received much attention as a possible route to chiral acetate due to the availability of chiral glycine (48). In the Stickland reaction two moles of glycine and one mole of d-alanine are converted quantitatively into three moles of acetate, three moles of ammonia, and one mole of C02 by the organism Clostridium sticklandii. The presence of amino acid transaminase in the intact organisms leads to extensive hydrogen exchange although in the purified enzyme the replacement of NH2 by H occurs stereospecifically with inversion (49, 50). Unfortunately, the rates of conversion with the purified enzyme are too low to be synthetically useful. 

Obviously, the elucidation of the enzymic mechanism required the preliminary purification of at least one of the transaminases. An 85-90% pure glutamic aspartic transaminase was obtained and found to contain 2 moles of pyridoxal phosphate per mole of enzyme. But pyridoxal is not the active coenzyme. Gunsalus, Bellamy, and Umbreit discovered that the addition of pyridoxal to a culture medium of a strain of Streptococcus faecalis grown on a pyri-doxal-deficient medium has little effect on the ability of the bacteria to decarboxylate tyrosine. When the culture was supplemented with pyridoxal and adenosine triphosphate, or with phosphorylated derivatives of pyridoxal, the tyrosine decarboxylation activity was greatly enhanced. It was later established that... 

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