Activation of human erythrocyte Ca2+-dependent Mg2+-activated ATPase by calmodulin and calcium: quantitative analysis.

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Abstract
The effect of Ca2+ and calmodulin on (CaM) on the activation of Ca2+-dependent Mg2+-activated ATPase (Ca2+, Mg2+-ATPase; ATP phosphohydrolase, EC 3.6.1.3) was carried out because of the finding that CaM dependence of the activation varies with the concentration of free Ca2+, similarly to brain phosphodiesterase and adenylate cyclase. The study was carried out in the absence of chelating agents because they strongly interfere in the enzyme kinetics. CaM.cntdot.Ca3 and CaM.cntdot.Ca4 together are the biochemically active species in vitro. These species bind in a noncooperative way to the CaM-binding site of the enzyme with a Kd of 6 .times. 10-10 or 1.1 .times. 10-8 M, depending on whether Ca2+ saturates the substrate binding site of the enzyme. The binding of CaM.cntdot.Ca3 to the enzyme lowers the Kd of the enzyme for Ca2+ at the substrate binding site from 51.5 to 2.8 .mu.M. CaM does not induce pronounced positive cooperativity in the binding of Ca2+ to the enzyme. Such a cooperativity is seen only when the enzyme is incompletely saturated with the activator, but it disappears in the presence of saturating concentrations of CaM.cntdot.Ca3. The rate equation proposed accurately predicts the extent of enzyme activation over a wide range of Ca2+ and CaM concentrations. In healthy erythrocytes the concentrations of Ca2+ and CaM are such that the Ca pump works with a minimal dissipation of energy, but a small increase in the intracellular Ca2+ concentration leads to a strong amplification of the pumping activity.

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