Beavo J A, Bechtel P J, Krebs E G
Adv Cyclic Nucleotide Res. 1975;5:241-51.
A number of properties of homogeneous cyclic 3',5'-AMP (cAMP)-dependent protein kinase from rabbit skeletal muscle were determined. The enzyme is shown to be a tetramer consisting of one regulatory subunit dimer and two catalytic subunit monomers. Skeletal muscle protein kinase interacts with cAMP and MgATP as illustrated in the following equilibrium expression: R2C2 - (MgATP)2 + 2 cAMP in equilibrium R2 - (cAMP)2 + 2C + 2 MgATP. MgATP is shown to decrease the affinity of the enzyme for cAMP and to be necessary for recombination of the subunits. The concentration of the enzyme in tissue relative to that of cAMP is high enough to influence kinetic parameters of the activation process by cAMP. The cumulative effects of MgATP and high enzyme concentration are to increase the apparent activation constant for cAMP so that in vivo the enzyme would not be highly activated under basal conditions but would be greatly stimulated by increases in cAMP concentration. As a result, it is not necessary to invoke the concept of compartmentalization of cAMP to explain how it could regulate protein kinase activity in vivo. Finally, data are presented which indicate that a possible function of the heat-stable protein inhibitor of cAMP-dependent protein kinases may be to suppress the activity of protein kinase due to basal concentrations of cAMP. As such, the inhibitor could indirectly change the amount of cAMP needed to allow expression of protein kinase activity.
测定了来自兔骨骼肌的同源环化3',5'-腺苷一磷酸（cAMP）依赖性蛋白激酶的一些特性。该酶被证明是一种四聚体，由一个调节亚基二聚体和两个催化亚基单体组成。骨骼肌蛋白激酶与cAMP和MgATP相互作用，如下列平衡表达式所示：R2C2 - (MgATP)2 + 2 cAMP ⇌ R2 - (cAMP)2 + 2C + 2 MgATP。已表明MgATP会降低酶对cAMP的亲和力，并且是亚基重组所必需的。相对于cAMP而言，组织中该酶的浓度足够高，足以影响cAMP激活过程的动力学参数。MgATP和高酶浓度的累积效应是增加cAMP的表观激活常数，因此在体内，该酶在基础条件下不会被高度激活，但会因cAMP浓度升高而受到极大刺激。结果，无需援引cAMP分隔化的概念来解释其如何在体内调节蛋白激酶活性。最后，给出的数据表明，cAMP依赖性蛋白激酶的热稳定蛋白抑制剂的一个可能功能可能是抑制由于基础浓度的cAMP导致的蛋白激酶活性。因此，该抑制剂可能间接改变允许蛋白激酶活性表达所需的cAMP量。
