The allosterically regulated enzyme ATCase binds aspartic acid as a substrate and acylates the a -amino group. Succinate acts as a competitive inhibitor of ATCase because it binds the active site but can't be acylated. The dependence of v0 on aspartic acid for ATCase is shown in panel (a) of the accompanying figure. Panel (b) shows the effect of increasing succinate on v0 when Asp is held at a low concentration (see thick vertical arrow in panel (a)). In panel (b), v0 is not zero when succinate = 0 (see thin horizontal arrow). Explain the shape of the curve in panel (b). Why does v0 increase initially, before decreasing at higher succinate?

COO CH HC NH3 COO Coo CH CH2 COO Asp Succinate Aspl [Succinate] [Asp] in experiment b