Acetolactate decarboxylase, /3-keto acid decarboxylation

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... 

Important evidence that a-acetolactate and a-aceto-a-hydroxybutyrate are precursors of valine and isoleucine, respectively, was obtained by Wagner et cd. 164). These investigators isolated and identified acetyl-methylcarbinol and acetylethylcarbinol from a mutant strain of Neurospora blocked in the biosynthesis of valine and isoleucine. The two carbinol compounds were obtained as pteridine derivatives. They would be formed by decarboxylation of acetolactate and aceto-a-hydroxybutyrate. That the latter compounds are the probable sources of the carbinols is indicated by the fact that Neurospora possesses an active decarboxylase which catalyzes the conversion of acetolactate and aceto-a-hydroxybutyrate to their respective carbinol. Furthermore, since the aceto compounds are 3-keto acids, they would be expected also to decarboxylate spontaneously. 

Pyruvate decarboxylase is able to catalyze two different reactions the nonoxidative decarboxylation of a-keto acids to the corresponding aldehydes [10,15-17] and a car-boxyligase side reaction leading to the formation of hydroxy ketones [18,19]. An understanding of why the last reaction is catalyzed by pyruvate decarboxylase, the physiological role of which is to decarboxylate pyruvate to acetaldehyde, was revealed by the discovery that pyruvate decarboxylase is homologous with acetolactate synthase [20], the enzyme catalyzing an acyloin condensation in the first step of isoleucine-valine biosynthesis. 

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