TABLE 1.

Parameters for the interaction of PMCA with calcium, vanadate, and ATP determined from the specific incorporation of [¹²⁵I]TID-PC/16 under equilibrium conditions (Equations 2 and 3)

E represents the enzyme in the absence of calcium and is supposed to be an equilibrium mixture of the conformational states E₁ and E₂ ([E] = [E₁] + [E₂]), which is shifted toward E₂. E* represents a state formed in the presence of vanadate and calcium whose conformation is different from that of E₁ and E₂. The parameters PC₀ and PC_∞ are the specific incorporation of [¹²⁵I]TID-PC/16 in the absence of reactants or when their concentration tends to infinity, respectively. K_x is the apparent dissociation constant of the complex of PMCA formed through the corresponding equilibrium reaction on the left. Data for reaction (i), which were included for comparison, were taken from Ref. 22.