A gate in a metabolic pathway

A purified enzyme converts S to P. Initial rates follow v = Vmax[S]/(Km + [S]); enzyme amount is identical throughout. Rates (micromol/min) at [S] = 1, 2, 4 and 8 mmol/L are respectively: control 20, 30, 40, 48; with compound X 10, 17.1, 26.7, 36.9; with compound Y 10, 15, 20, 24. Treat these data as exact apart from rounding. X and Y do not interfere with the assay. A second enzyme rapidly removes P, so product accumulation does not affect initial rates.