Gorbunoff M J, Fosmire G, Timasheff S N
Biochemistry. 1978 Sep 19;17(19):4055-65. doi: 10.1021/bi00612a029.
The mechanism of the acid dimerization of alpha-chymotrypsin in solution was reexamined using a number of chemical derivatives. Blocking of the carboxyl of Tyr-146, while that of ASP-64 remained free, eliminated completely the ability of alpha-chymotrypsin to dimerize, as did methylation of His-57. O-Acetylation of Tyr-146 reduced the dimerization constant to that of gamma-chymotrypsin, and deacetylation of the other accessible tyrosines did not affect the dimerization. It is concluded that the mechanism proposed by Aune and Timasheff [Aune, K.C., and Timasheff, S.N.(1971) Biochemistry 10, 1609-1617] for the solution dimerization which involves the electrostatic interaction between the His-57 imidazolium ring and the terminal carboxyl of Tyr-146 is still most consistent with all the experimental observations. The interactions in dilute solution may differ somewhat from those observed in crystals. In particular, the two intermolecular bridges formed by sulfate ions in crystals cannot be present in solution.
利用多种化学衍生物重新研究了溶液中α-胰凝乳蛋白酶的酸二聚化机制。当ASP-64的羧基保持游离时，阻断Tyr-146的羧基会完全消除α-胰凝乳蛋白酶的二聚化能力，His-57甲基化时也是如此。Tyr-146的O-乙酰化将二聚化常数降低至γ-胰凝乳蛋白酶的水平，而其他可及酪氨酸的脱乙酰化并不影响二聚化。结论是，Aune和Timasheff [Aune, K.C., and Timasheff, S.N.(1971) Biochemistry 10, 1609 - 1617] 提出的溶液二聚化机制，即His-57咪唑环与Tyr-146末端羧基之间的静电相互作用，仍然与所有实验观察结果最为一致。稀溶液中的相互作用可能与晶体中观察到的有所不同。特别是，晶体中由硫酸根离子形成的两个分子间桥在溶液中不可能存在。
