The control of adenylate cyclase by calcium in turkey erythrocyte ghosts

The adenylate cyclase of turkey erythrocytes is inhibited by low concentrations of calcium. Calcium binds to the enzyme system so tightly that the enzyme can compete with ethylene glycol bis (β aminoethyl ether) N,N' tetraacetic acid (EGTA) for the metal. The calcium binding site is shown to be distinct from the magnesium binding sites required for activity. Thus Ca²⁺ functions as a negative allosteric effector. Calcium decreases dramatically the V(max) of the catecholamine stimulated activity without affecting the affinity for the hormone or for the substrate ATP. The cooperativity in the response toward Mg²⁺ dependence (Hill coefficient, n(H) = 3) is also unaffected by Ca²⁺ whereas the S0.5 (concentration yielding one half V(max)) for Mg²⁺ is affected only slightly. The Ca²⁺ effect is cooperative (n(H) = 2) and therefore brought about by a cluster of Ca²⁺ binding sites. Mn²⁺ can substitute for Mg²⁺ as the enzyme activator but the Mn²⁺ activated enzyme is no longer inhibited by Ca²⁺. The possible physiological significance of the Ca²⁺ effect is discussed.