His⁶⁸ and His¹⁴¹ Are Critical Contributors to the Intersubunit Catalytic Site of Adenylosuccinate Lyase of <i>Bacillus subtilis</i>

Abstract

Mutant adenylosuccinate lyases of Bacillus subtilis were prepared by site-directed mutagenesis with replacements for His 141, previously identified by affinity labeling as being in the active site [Lee, T. T., Worby, C., Dixon, J. E., and Colman, R. F. (1997) J. Biol. Chem. 272, 458−465]. Four substitutions (A, L, E, Q) yield mutant enzyme with no detectable catalytic activity, while the H141R mutant is about 10 - 5 as active as the wild-type enzyme. Kinetic studies show, for the H141R enzyme, a K m that is only 3 times that of the wild-type enzyme. Minimal activity was also observed for mutant enzymes with replacements for His 68 [Lee, T. T., Worby, C., Bao, Z.-Q., Dixon, J. E., and Colman, R. F. (1998) Biochemistry 37, 8481−8489]. Measurement of the reversible binding of radioactive adenylosuccinate by inactive mutant enzymes with substitutions at either position 68 or 141 shows that their affinities for substrate are decreased by only 10−40-fold. These results suggest that His 141, like His 68, plays an important role in catalysis, but not in substrate binding. Evidence is consistent with the hypothesis that His 141 and His 68 function, respectively, as the catalytic base and acid. Circular dichroism spectroscopy and gel filtration chromatography conducted on wild-type and all His 141 and His 68 mutants reveal that none of the mutant enzymes exhibits major structural changes and that all the enzymes are tetramers. Mixing inactive His 141 with inactive His 68 mutant enzymes leads to striking increases in catalytic activity. This complementation of mutant enzymes indicates that His 141 and His 68 come from different subunits to form the active site. A tetrameric structure of adenylosuccinate lyase was constructed by homology modeling based on the known structures in the fumarase superfamily, including argininosuccinate lyase, δ-crystallin, fumarase, and aspartase. The model suggests that each active site is constituted by residues from three subunits, and that His 141 and His 68 come from two different subunits.