D-amino add oxidase

The natural amino acids are mainly a-amino acids, in contrast to (3-amino acids such as p-alanine and taurine. Most a-amino acids have four different substituents at C-2 (Ca). The a atom therefore represents a chiral center—I e., there are two different enantiomers (L- and D-amino acids see p. 8). Among the proteinogenic amino acids, only glycine is not chiral (R = H). In nature, it is almost exclusively L-amino acids that are found. D-Amino acids occur in bacteria—e. g., in murein (see p.40)—and in peptide antibiotics. In animal metabolism, D-Amino acids would disturb the enzymatic reactions of L-amino acids and they are therefore broken down in the liver by the enzyme D-amino add oxidase. 

The preparation of D-amino acids with the above three-enzyme system requires enzymes with opposite stereochemical selectivity and a suitable amino add as a donor. While D-amino add oxidase is an enzyme, the function of which in Nature is mainly related to the ehmination of D-amino adds, L-amino acid oxidases are usually found in aggressive animals (snakes). Bacterial L-amino acid oxidases often show a specific activity that is too low for preparative purposes [33]. Moreover, D-amino transferases are less common than the L-specific ones and require more expensive D-amino adds as amino donors. 

Thus, using L-amino add oxidase from P. myxcfaciens and various amine-borane complexes or D-amino acid oxidase from porcine kidney and sodium cyanoboro-hydride, the preparation of several natural and non-natural enantiopure D- and L-amino adds was achieved, respectively [51]. In a more recent report, several P- and y-substituted a-amino adds were deracemized using D-amino add oxidase from Trigonopsis variahilis and sodium cyanoborohydride or sodium borohydride [52] (Scheme 13.20). 

Caligiuri, A., D Arrigo, P., Rosini, E Tessaro, D., Molla, G., Servi, S. and Polle-gioni, L. (2006) Enzymatic conversion of unnatural amino adds by yeast D-amino add oxidase. Advanced Synthesis Catalysis, 348, 2183-2190. 

On the mechanism of D-amino add oxidase. Structure/linear free energy correlations and deuterium kinetic isotope effects using substituted phenylglydnes,... 

Pilone M S, Pollegioni L (2002) D-amino add oxidase as an industrial biocatalyst. Biocatal Biotransform 20 145-159... 

J. Fang, T. Sawa, T. Akaike, H. Maeda, Tumor-targeted delivery of polyethylene glycol-conjugated d-amino add Oxidase for antitumor therapy via enzymatic generation of hydrogen peroxide. Cancer Res. 62 (2002) 3138-3143. 

The enzymes catalysing this reaction are the L-amino add oxidases and the D-amino add oxidases. The role of the latter in metabolism has not yet been elucidated, forthenaturally occurring amino acids are generallyof the L-series. 

This classification indicates that the L- and D-amino add oxidases have broad substrate specificities, whereas the other enzymes have a narrower specificity. For the purpose of simplicity the above clasdfication will be used. The amino acid oxidase systems may be subclassified according to the nature of the hydrogen acceptor under two categories, aerobic and anaerobic. The former have been more commonly referred to as amino acid oxidases, whereas the latter are frequently referred to as dehydrogenases. 

Although D-amino adds can no longer be considered unnatural, thoy are, from a quantitative standpoint, neverthdess, of limited distribution in nature. This fact makes the explanation of the widespread occurrence of hi y active o-amino acid oxidases somewhat difficult. The capriciousness of nature—or of the enzymologist—is clearly revealed in the fact that a great deal is known about the properties and mechanism of action of D-amino add oxidases in the animal but very little about the role of D-amino acids in metabolism. 

Norvaline is strongly ketogenic (79). The L-form is attacked by L-amino acid oxidase (77) and the D-form by D-amino add oxidase (1S9). Its susceptibility to transamination has not been reported. Hassan and Greenberg (1S6) demonstrated that it is readily oxidized to COt in the intact animal. More of this amino add is excreted unchanged in the urine than is leudne. 

PoUegioni, L. and Molla, G. (2011) New biotech applications from evolved D-amino add oxidases. Trends Biotechnol., 29, 276-283. 

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