Stellwagen E, Tucker M M, Robinson J B
Adv Cyclic Nucleotide Protein Phosphorylation Res. 1984;16:49-54.
In summary, our studies suggest that unproteolyzed PDE, illustrated as a dimer of identical 63,000-D polypeptide chains in Fig. 5, can exist in two equilibrium conformations. In the absence of calmodulin, the predominant conformational form has a nonfunctional catalytic site, whereas the minor conformational form has a fully functional site. Preferential complexation of calmodulin with the minor conformation shifts the population to that form resulting in a marked activation of catalysis. Alternatively, cleavage of a sizable terminal fragment, termed the calmodulin binding domain, facilitates irreversible acquisition of the functional conformation in the absence of calmodulin activation. By contrast, proteolytic cleavage at the opposite terminus does not significantly alter the conformational equilibrium. This model suggests that a single in vivo cytoplasmic cyclic nucleotide PDE can accommodate many of the variations in the polypeptide chain sizes and in the fold activation observed for enzyme preparations in vitro. The model also places this form of cyclic nucleotide PDE activity in vivo under absolute control by intracellular Ca2+ concentration.
总之，我们的研究表明，如图5所示，未被蛋白酶水解的磷酸二酯酶（PDE）以相同的63,000-D多肽链二聚体形式存在，可处于两种平衡构象。在没有钙调蛋白的情况下，主要的构象形式具有无功能的催化位点，而次要的构象形式具有完全功能的位点。钙调蛋白与次要构象的优先结合使群体向该形式转变，导致催化作用显著激活。另外，一个相当大的末端片段（称为钙调蛋白结合结构域）的切割，有助于在没有钙调蛋白激活的情况下不可逆地获得功能构象。相比之下，在相反末端的蛋白水解切割不会显著改变构象平衡。该模型表明，单一的体内细胞质环核苷酸磷酸二酯酶可以适应体外酶制剂中观察到的多肽链大小和折叠激活的许多变化。该模型还将这种形式的环核苷酸磷酸二酯酶活性置于体内受细胞内Ca2+浓度的绝对控制之下。
