Mutational analysis of active site residues in pig heart aconitase

A cDNA encoding mature porcine heart aconitase was over-expressed in Escherichia coli under the control of a phage T7 promoter. Recombinant aconitase purified from E. coli was identical to the enzyme from pig and beef heart in size, [3Fe-4S] and [4Fe-4S] cluster structure and enzymatic activity. Nine amino acid residues in close proximity to the Fe-S cluster and bound substrate (Lauble, H., Kennedy, M.C., Beinert, H., and Stout, C.D. (1992) Biochemistry, in press) were replaced by site-directed mutagenesis. Fe-S cluster environment as indicated by the EPR spectrum, tight binding of substrate, and enzymatic activity were compared for the mutant and wild type enzymes. Significant perturbations were detected for all of the mutant enzymes. Replacements for Asp100, His101, Asp165, Arg580, and Ser642 result in a 10(3)-10(5)-fold drop in activity, which suggests that these residues are involved in critical aspects of the reaction. Arg580 appears to be a key residue for substrate binding, as shown by a 30-fold increased Km and loss of tight substrate binding. Results of mutagenesis support the interpretation of the x-ray model, namely that Asp100 and His101 form an ion pair for elimination of the substrate hydroxyl and Ser642 may function as a general base for proton abstraction from citrate or isocitrate in the dehydration step and protonation of cis-aconitate in the hydration step. Asp165 appears to play a critical role in the interaction of Fea with substrate.