Redesign of the substrate specificity ofescherichia coliaspartate aminotransferase to that ofescherichia colityrosine aminotransferase by homology modeling and site-directed mutagenesis

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Abstract
Although several high-resolution X-ray crystallographic structures have been determined for Escherichia coli aspartate aminotransferase (eAATase), efforts to crystallize E. coli tyrosine aminotransferase (eTATase) have been unsuccessful. Sequence alignment analyses of eTATase and eAATase show 43% sequence identity and 72% sequence similarity, allowing for conservative substitutions. The high similarity of the two sequences indicates that both enzymes must have similar secondary and tertiary structures. Six active site residues of eAATase were targeted by homology modeling as being important for aromatic amino acid reactivity with eTATase. Two of these positions (Thr 109 and Asn 297) are invariant in all known aspartate aminotransferase enzymes, but differ in eTATase (Ser 109 and Ser 297). The other four positions (Val 39, Lys 41, Thr 47, and Asn 69) line the active site pocket of eAATase and are replaced by amino acids with more hydrophobic side chains in eTATase (Leu 39, Tyr 41, Ile 47, and Leu 69). These six positions in eAATase were mutated by site-directed mutagenesis to the corresponding amino acids found in eTATase in an attempt to redesign the substrate specificity of eAATase to that of eTATase. Five combinations of the individual mutations were obtained from mutagenesis reactions. The redesigned eAATase mutant containing all six mutations (Hex) displays second-order rate constants for the transamination of aspartate and phenylalanine that are within an order of magnitude of those observed for eTATase. Thus, the reactivity of eAATase with phenylalanine was increased by over three orders of magnitude without sacrificing the high transamination activity with aspartate observed for both enzymes. Examination of the dissociation constants of the dicarboxylate inhibitor maleate and the aromatic inhibitor hydrocinnamate with the mutant constructs demonstrates that the T109S and N297S mutations are specific determinants for high-affinity association of nonpolar ligands, whereas the other four mutations have the general effect of decreasing the dissociation constants for both dicarboxylate and nonpolar ligands. The latter four changes presumably exert their general effect by stabilizing the closed conformation of the enzyme that is observed in X-ray crystal structures of eAATase complexed with dicarboxylate ligands.

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