Role of arginine-292 in the substrate specificity of aspartate aminotransferase as examined by site-directed mutagenesis

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Abstract
X-ray crystallographic data have implicated Arg-292 as the residue responsible for the preferred side-chain substrate specificity of aspartate aminotransferase. It forms a salt bridge with the .beta. or .gamma. carboxylate group of the substrate [Kirsch, J. F., Eichele, G., Ford, G. C., Vincent, M. G., Jansonius, J. N., Gehring, H., and Christen, P. (1984) J. Mol. Biol. 174, 497-525]. In order to test this proposal and, in addition, to attempt to reverse the substrate charge specificity of this enzyme, Arg-292 has been converted to Asp-292 by site-directed mutagenesis. The activity (kcat/KM) of the mutant enzyme, R292D, toward the natural anionic substrates L-aspartate, L-glutamate, and .alpha.-ketoglutarate is depressed by over 5 orders of magnitude, whereas the activity toward the keto acid pyruvate and a number of aromatic and other neutral amino acids is reduced by only 2-9-fold. These results confirm the proposal that Arg-292 is critical for the rapid turnover of substrates bearing anionic side chains and show further that, apart from the desired alteration, no major perturbations of the remainder of the molecule have been made. The activity of R292D toward the cationic amino acids L-arginine, L-lysine, and L-ornithine is increased by 9-16-fold over that of wild type and the ratio (Kcat/KM)cationic/(kcat/KM)anionic is in the range 2-40-fold for R292D, whereas this ratio has a range of [(0.3-6) .times. 10-6]-fold for wild type. Thus, the mutation has produced an inversion of the substrate charge specificity. Possible explanations for the less-than-expected reactivity of R292D with arginine, based in part on the preliminary X-ray crystallographic data of D. Ringe and D. Smith, are discussed.