The effect of Ca²⁺ on kinetic parameters of wild-type and mutated form of muscle FBPase.

<p><b>A)</b> Activation of the Tyr⁵⁷Trp muscle FBPase mutant by Mg²⁺ in the presence of various concentrations of calcium. <b>B)</b> Calcium-induced increase in apparent dissociation constant for Mg²⁺ (<i>K_a</i>^app_Mg2+) does not affect the value of dissociation constant for Ca²⁺ (<i>K_i</i>_Ca2+). Hill constant (<i>n</i>) is given for the activation by Mg²⁺. The plot shows that the increase in <i>K_a</i>^app_Mg2+ is a linear function of Ca²⁺ concentration. The average value of <i>K_i</i> for Ca²⁺ calculated from the plot (<i>K_i</i>_Ca2+) equals to 21.65 µM. <b>C)</b> The mechanism that produce competition between magnesium and calcium ions. From this, the equation describing the competitive inhibition is: , where <i>K_a</i>^app_Mg2+ is the apparent activator’s (Mg²⁺) dissociation constant and <i>K_a</i>_Mg2+ is the dissociation constant for Mg²⁺ as determined in the absence of Ca²⁺.