期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1981
卷号：78
期号：2
页码：871-874
DOI：10.1073/pnas.78.2.871
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Kinetic studies on the activation of cyclic nucleotide phosphodiesterase (3',5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17 ) as a function of calmodulin and Ca2+ concentrations have been carried out. A general approach to analyzing the mechanism of activation, which takes into consideration the various interactions among phosphodiesterase and calmodulin liganded with Ca2+ to differing degrees, is presented. The method is applicable to other calmodulin-regulated enzyme systems. Our kinetic analysis reveals that all four Ca2+ must be bound to calmodulin for the protein to form an activated complex with phosphodiesterase. The mechanistic and regulatory advantages of having four Ca2+ sites on calmodulin can be briefly stated as follows. (i) With the enzyme--calmodulin--Ca4(2+) complex as the dominant active species, the activation of phosphodiesterase as a function of Ca2+ concentration is highly cooperative. This phenomenon serves as an effective on/off switch for phosphodiesterase activation. (ii) At normal cellular levels of Ca2+ (less than 0.1 microM), phosphodiesterase and calmodulin do not form a complex. Thus, the distribution of calmodulin among its various target enzymes is reshuffled for each Ca2+ surge. (iii) The affinity between the enzyme and the fully liganded calmodulin (0.1-1 mM) is 10(4)-10(5) times better than that in the absence of Ca2+ (greater than or equal to 10 microM). The tremendous increase in affinity can be achieved rather easily through a 10- to 20-fold increase in the affinity of Ca2+ for the enzyme-calmodulin complex in each of the four binding steps.
