Paracoccus denitrificans aromatic amino acid aminotransferase: a model enzyme for the study of dual substrate recognition mechanism

The gene for aromatic amino acid aminotransferase (ArAT) from Paracoccus denitrificans was cloned, sequenced, and overexpressed in Escherichia coli cells. The sequence differed from that reported previously [Takagi, T., Tanigachi, T., Yamamoto, Y., and Shibatani, T. (1991) Biotechnol. Appl. Biochem. 13, 112-119]. The enzyme (pdArAT) was purified to homogeneity, and characterized. It was similar to aspartate aminotransferase (AspAT) and ArAT of E. coli (ecArAT) in many respects, including gross protein structure and spectroscopic properties. pdArAT showed activities toward both dicarboxylic and aromatic substrates, and analysis of the binding of substrate analogs and quasisubstrates to the enzyme showed that both dicarboxylic and aromatic substrates take a similar orientation in the active site of pdArAT; these properties are essentially identical with those of ecArAT. As in the case of ecArAT, neutral amino acids with larger side chains are better substrates for pdArAT, suggesting that hydrophobic interaction between the substrate and the enzyme is important for the recognition of substrates with neutral side chains. pdArAT catalyzed transamination of phenylalanine and tyrosine far more efficiently (10(2)-fold in terms of kappa(cat)/Km) than those of straight-chain aliphatic amino acids with similar side-chain surface area, whereas ecArAT did not show significant preference for aromatic amino acids over aliphatic amino acid B. This shows that the substrate-side-chain-binding pocket of pdArAT, as compared with the pocket of ecArAT, is well suited in shape for interaction with the phenyl and hydroxyphenyl rings of substrates. Thus, pdArAT is an ideal enzyme among ArATs for the study of the high-specificity recognition of two different kinds of substrates, the one having a carboxylic side chain and the other having an aromatic side chain.