The effect of pH on the cooperative behavior of aspartate transcarbamylase from Escherichia coli.

Saturation curves of activity versus concentration were determined for aspartate transcarbamylase from Escherichia coli (EC 2.1.3.2) for the substrate L-aspartate at saturating carbamyl phosphate (4.8 mM) in buffered solution at pH values from 6.0 to 12.0. Hill coefficients were obtained from the sigmoidal curves. At pH values from 7.8 to 9.1, where substrate inhibition causes difficulties in the Hill approximation, our kinetic scheme includes substrate inhibition and residual activity in the abortive enzyme-substrate complex. The plot of Hill coefficient versus pH has pKalpha values of 7.4 and 9.8 at the half-maximum positions of the curve which has a plateau from pH 8.1 to 9.1. These pKalpha values may be associated with functional groups involved in the allosteric transition which activates the enzyme. A plot of [S]0.5 versus pH shows a pKalpha of 8.5, which may belong to a residue either at or near the aspartate binding site. At 50 mM aspartate concentration the pH-rate profile shows maxima at pH values of 8.8 and 10.0 (cf. Weitzman, P.D.J., and Wilson, I.B.(1966)J. Biol. Chem. 2418 5481-5488, who used 100 mM aspartate). However, when the pH-dependent substrate inhibition is included, the calculated Vmax--H curve is bell-shaped like that of the isolated catalytic subunit.