Shift in pH-rate profile and enhanced discrimination between dicarboxylic and aromatic substrates in mitochondrial...

Tyr70 of chicken mitochondrial aspartate aminotransferase was replaced with a histidine residue by oligonucleotide-directed mutagenesis. Aspartate aminotransferase Y70H retained at pH 7.5 13% of the activity toward dicarboxylic amino acids, whereas the activity toward aromatic amino acids was only 0.6% of that of the wild-type enzyme, corresponding to a 22-fold increase in the ratio of the activities toward these two types of substrates. In comparison to that of the wild-type enzyme, the low-pH limb of the pH-activity profile of the mutant enzyme was shifted to higher pH values, very likely reflecting the titration curve of the newly introduced histidine residue with a pKa' of 6.3. Apparently, a positively charged residue at position 70 abolishes enzymic activity. The spectrophotometrically determined pKa' value of the internal aldimine formed between pyridoxal 5'-phosphate and Lys258 in the mutant enzyme was 6.0, similar to that in the wild-type enzyme. The rate constant of the dissociation of pyridoxamine 5'-phosphate from the mutant enzyme was increased only 3 times over that of the wild-type enzyme, in contrast to the 80-fold increase in Escherichia coli aspartate aminotransferase Y70F [Toney, M. D., & Kirsch, J. F. (1987) J. Biol. Chem. 262, 12403-12405], suggesting that His70 can replace Tyr70 in forming a hydrogen bond to the coenzyme.