Recent sequencing experiments have identified α-His246 as the phosphorylation site of Escherichia coli succinyl-CoA synthetase [Buck, D., Spencer, M. E., & Guest, J. R. (1985) Biochemistry 24, 6245-6252]. We have replaced α-His246 with an asparagine residue using site-directed mutagenesis techniques. The resulting mutant enzyme (designated H246N) exhibited no enzyme activity, as expected, but was found as a structurally intact, stable tetramer. Small differences in the net charge of H246N and wild-type enzymes were first detected on native polyacrylamide gels. These charge differences were resolved by using native isoelectric focusing gels to further separate the wild-type enzyme into diphosphorylated, monophosphorylated, and unphosphorylated species. The enzyme species were found to be interconvertible upon incubation with the appropriate enzyme substrate(s). Sample mixtures containing increasing molar ratios of H246N (αH246Nβ)2 to wild-type enzyme (αβ)2 were unfolded and then refolded. The refolded enzyme mixtures were analyzed for enzymatic activity and separated on native isoelectric focusing gels. The hybrid enzyme (αβαH246Nβ) retained a significant amount of enzyme activity and also exhibited substrate synergism (stimulation of succinate ⃡ succinyl-CoA exchange in the presence of ATP). Substrate synergism with this enzyme has been interpreted as evidence for interaction between active sites in such a way that only a single phosphoryl group is covalently attached to the enzyme at a given time [Wolodko, W. T., Brownie, E. R., O’Connor, M. D., & Bridger, W. A. (1983) J. Biol. Chem. 258, 14116-14119]. On the contrary, we conclude that tetrameric succinyl-CoA synthetase from E. coli is comprised of two independently active dimer molecules associated together to form a “dimer of dimers” that displays substrate synergism within each dimer and not necessarily between dimers. Taken together, the observations described in this work would appear to argue against the hypothesis that E. coli succinyl-CoA synthetase is a half of sites reactive protein in which the active sites function in an alternating manner [Bild, G. S., Janson, C. A., & Boyer, P. D. (1980) J. Biol. Chem. 255, 8109-8115].
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