Abstract
Abstract By digital computer simulation, it was shown that a sequential mechanism for the 2,3-diphosphoglyceric acid-dependent (DPGA-dependent) muscle phosphoglyceric acid mutase reaction cannot be disproved by kinetic analysis of the available velocity data, contrary to the suggestion of Grisolia and Cleland (Biochemistry, 7, 1115 (1968)). At the high concentrations of DPGA and 3-phosphoglyceric acid (3PGA) used in the experimental velocity assay, the rates of product formation calculated from the sequential and the ping-pong mechanisms were found to be very similar; possible experiments to distinguish them are suggested. A sequential mechanism model (supported by other types of experimental data) was fitted to the velocity data over the range 0.5 µM to 6 mM DPGA and 1 to 40 mM 3PGA. Calculated kinetic parameter values are presented. Competitive substrate inhibitions required 2 molecules of 3PGA bound per molecule of apoenzyme, and 1 of DPGA bound per phosphoglyceric acid mutase-DPGA complex. At high 3PGA and DPGA the enzyme protein is modified into a form with an increased turnover number as a function of 3PGA and DPGA concentrations. Binding constants depend on the ligand concentrations, in accord with experiment.
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