Howarth O W, Lian L Y
Biochemistry. 1984 Jul 17;23(15):3515-21. doi: 10.1021/bi00310a020.
Assignments have been made for 11 methyl, one Gln-C gamma, one Thr-C beta, and all six Tyr-C zeta carbon resonances of ribonuclease A. These partially serve to delineate the binding sites for Cu2+, Mn2+, phosphate, cytidine and its 2'-, 3'-, and 5'-phosphates (Cyd and Cyd-2'-P, -3'-P, and -5'-P), and one or a few urea molecules at low concentration. Evidence is presented for a conformational change, and hence flexibility, in the active site region around the optimum pD for enzymic activity and another such change at around the optimum temperature. The binding of cytidine-containing ligands is shown to have extensive conformational consequences for methyl groups but less for hydrophobic aromatic residues, implying that the former make a special contribution to molecular flexibility. The cytosine ring in Cyd-2'-P, -3'-P, and -5'-P is found to be close but far from parallel to the ring of Phe-120. In contrast to previous claims, ribonuclease A is shown not to unfold even partially before denaturation. On denaturation, it passes to a new but structured state.
已对核糖核酸酶A的11个甲基、1个谷氨酰胺 - Cγ、1个苏氨酸 - Cβ以及所有6个酪氨酸 - Cζ碳共振峰进行了归属。这些归属部分用于描绘铜离子（Cu2+）、锰离子（Mn2+）、磷酸根、胞苷及其2'-、3'-和5'-磷酸酯（Cyd和Cyd - 2'-P、-3'-P和-5'-P）以及低浓度下一个或几个尿素分子的结合位点。有证据表明，在酶活性的最佳pD附近的活性位点区域存在构象变化，因此具有灵活性，并且在最佳温度附近也存在另一种这样的变化。含胞苷配体的结合对甲基具有广泛的构象影响，但对疏水芳香族残基的影响较小，这意味着前者对分子灵活性有特殊贡献。发现Cyd - 2'-P、-3'-P和-5'-P中的胞嘧啶环与苯丙氨酸 - 120的环接近但不平行。与之前的说法相反，核糖核酸酶A在变性前并未显示出哪怕部分展开的情况。变性时，它转变为一种新的但有结构的状态。
