Bacteria transaminases

Transamination (equation 12) requires two sets of reactions firstly, (34) is protonated at C-4 of PLP to yield a ketimine, the hydrolysis of which yields a carbohyl compound and PMP. The reverse reaction of PMP with a different carbonyl compound to that produced in the forward reaction accounts for the stoichiometry of equation (12). Transamination reactions mediated by PLP-dependent enzymes are widespread both in bacteria and in mammals. Over 50 different transaminases are known (B-73MI11000). 

L-Amino transaminase activities are ubiquitous in Nature since they are involved in the biosynthesis of most natural amino acids. On the other hand, D-amino transaminase activities have been identified in bacteria, mostly in the Bacillus strains and are involved in the production of D-amino acids for the peptidoglycan layer of the cell wall. The mechanism is weU established, with PLP shuttHng... 

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... 

Certain D-amino acids can be formed in bacteria by processes involving a racemase and D-amino acid transaminases. Amino acid activating systems have been found for D-alanine in Staph, aureus and a number of other bac-teria 34 is thus possible that free D-amino acids are sometimes incorporated as such into peptide antibiotics. On the other hand, the D-valine fragment of the penicillins arises from changes in an intermediate peptide that contains an L-valine residue. L-Valine in the culture fluid appears to be the precursor of the D-valine residue found in some of the actinomycins, but this relationship does not reveal the stage at which the inversion of configuration occurs. Differences in the permeability of cells or intracellular structures to l- and D-amino acids are liable to complicate the interpretation of experiments in this field. 

Reported equilibriiun constants for purified glutamic-oxalacetic transaminase preparations have been found to range between 3-7.8 ( 9 , 98, 34 , 345). Reported glutamic-pyruvic transaminase equilibrium values are of the order of 1-2.09 ( 9 , 98, 34 , 343, 344, 340). The pH optimum for glutamic-oxalacetic transaminase from animal tissues has been reported as 7.5 ( 9 , 34 ) and approximately 8 ( 94) from plant seedlings, 8.6 (310, 347) from wheat germ, 8 ( 98) from green plants, 6.9 (31 ) from bacteria, 5.8-S.8 (311, 316, 348) from yeast, 7.8 (349) and from N. crassa, 9 (350). 

A role for a-keto-c-aminopimelic acid on the pathway of lysine syntheds is indicated by the discovery of a transaminase that acts on both diaminopimelic acid and on lysine 194). Cell suspendons of acetone-dried bacteria at pH 5 were able to transaminate all three stereoisomers of diaminopimelic acid and also d- and L-lysine in the presence of pyridoxal phosphate, with pyruvate, oxalacetate, and a-ketoglutarate serving as amino group acceptors. 

Interactions with enantiomeric cycloserines. The antibiotic D-cycloserlne (if-aminoisoxazolidone-3) irreversibly inhibits,in bacteria certain enzymes metabolizing D-amino acids. In eucarlotes, the L-enantiomer (and DL-cycloserine) are the potent inactivators of transaminases and some other PLP-enzymes acting on structurally analogous L-amino acids. As demonstrated earlier by our colleagues H.Karpeisky, R.Khomutov, E. Severin et al.,27-29 the following steps appear to be in-... 

A drawback of using lactate dehydrogenase as a biocatalyst to remove pyruvate from the reaction equilibrium is the need for the NADH cofactor. Another possibility to eliminate the coproduct is the application of a p5uruvate decarboxylase (Scheme 29.6b). A cofactor is not required, and the resulting products of pyruvate decarboxylation, acetaldehyde, and CO are highly volatile, shifting the equilibrium toward the product [68]. Several pyruvate decarboxylases from yeast and bacteria are commercially available and are active at the same pH value as the transaminase required for the asymmetric synthesis of chiral amines. 

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