Calpastatin simultaneously binds four calpains with different kinetic constants

Calpastatin is the endogenous, specific protein inhibitor of the calcium-dependent protease, calpain. Using an active site knock-out m-calpain mutant we have studied the enzyme’s calcium-dependent binding to calpastatin by surface plasmon resonance without the complication of proteolysis. Calpastatin was capable of simultaneously binding four molecules of calpain. Its four inhibitory domains (CAST1, 2, 3, and 4) were individually expressed in Escherichia coli and the kinetics of their interaction with calpain was separately compared. Their Kd values ranged from picomolar to nanomolar in the order CAST1 > 4 > 3 > 2. They have similar kon values but the koff values ranged over three orders of magnitude and can account for the differences in affinity.