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Significant improvement to the catalytic properties of aspartate aminotransferase: role of hydrophobic and charged residues in the substrate binding pocket.

The substrate specificity of tyrosine aminotransferase (eTAT) from Escherichia coli has been tested by transferring the critically different residues Leu39, Glu141, and Arg293 into equivalent positions of aspartate aminotransferase (eAAT). These residues are not directly involved in the catalytic process. The single mutant eAAT V39L possesses greater values of kcat/KM not only for tyrosine but also for aspartate and glutamate. In contrast, the double mutant eAAT P141E,A293R and also the triple mutant eAAT V39L,P141E,A293R exhibit smaller changes of kcat/KM. The converse mutants of tyrosine aminotransferase, in which critical residues of eAAT (Val39) and of mitochondrial AAT (Ala39, Val37) were transferred into equivalent positions of eTAT, exhibited generally decreased values of kcat/KM for both dicarboxylic and aromatic substrates. On the basis of the known structures of eAAT and eAAT V39L as well as of a refined model of eTAT, these results indicate that the different substrate specificities of eAAT and eTAT are due to multiple side chain differences and minor rearrangements of the backbone. The generally improved catalytic efficiency of the mutant eAAT V39L appears to be due to an indirect effect, namely, the facilitated closure of the active site upon substrate binding.