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The regulatory control of β-receptor dependent adenylate cyclase

The characteristics of the β-receptor in turkey erythrocyte adenylate cyclase were studied using both kinetics of enzyme activation and direct binding measurement of the β-agonists and antagonists to the β-receptor. The regulatory ligands Gpp(NH)p and Ca2+ do not have any direct effect on the β-receptor, but modulate the enzyme activity through the interaction with specific regulatory sites. 
 
 
 
It was found that the role of the catecholamine hormone is to facilitate the activation of the enzyme by the guanyl nucleotide. The regulatory guanyl nucleotide binds to its allosteric site in the absence of hormone, but the activation of the enzyme is slow in the absence of hormone. This role of the hormone can be described by the scheme: 
 
 
 
 
 
 
Where R is the receptor, E the enzyme, G the guanyl nucleotide, H the hormone, and E′ the activated form of the enzyme. The binding steps are fast and reversible but the conversion of the inactive enzyme E to its active form occurs with a k∼1.0 min−1 In the absence of the β-agonist (l-catecholamine) at the β-receptor and at physiological free Mg2+ concentrations, the activation of the enzyme is insignificant. Thus the presence of a guanyl nucleotide at the allosteric site is obligatory but not sufficient to induce the conversion of the inactive enzyme to its active form. At high (nonphysiological) Mg2+ concentration the conversion of E to E′ occurs slowly in the absence of hormone probably by another pathway. 
 
 
 
There are two classes of Gpp(NH)p regulatory sites: tight sites and loose sites, both of which can be identified kinetically. We have also identified the tight sites by direct binding studies using 3H-Gpp(NH)p. It is not clear, however, whether these are two distinct classes of sites or whether their existence reflects the presence of negative cooperativity among the guanyl nucleotide regulatory sites. 
 
 
 
Calcium was found to be a negative allosteric inhibitor of adenylate cyclase. The inhibitory effect of Ca2+ is exerted on the nonactivated enzyme as well as on the Gpp(NH)p preactivated enzyme.