Effect of pH on Csk kinetics with ATP as the variable substrate. (A) Double reciprocal plot of Csk activity using ATP as the variable substrate at pH 6, 6.1 and 6.2. (B) Plots of relative kcat and Km for ATP as a function of pH. The phosphate-accepting substrate was polyE4Y at 1 mg ml-1. The kcat (9.1 min-1) and Km (200 μM) at pH 6 are taken as 1 and all other values are relative to those.