Marshall M, Cohen P P
J Biol Chem. 1980 Aug 10;255(15):7296-300.
The essential sulfhydryl group of the ornithine transcarbamylases (ornithine carbamoyltransferase, 2.1.3.3) from bovine liver and Streptococcus faecalis reacts preferentially with 2-chloromercuri-4-nitrophenol. The spectra of this derivative between pH 4.4 AND 8.8 HAVE BEEN RESOLVED INto the spectrum of the nitrophenolate ion (III) and two species of phenol (I and II). The lambda max of I and II (both enzymes) and III (bovine) are red shifted from those of the comparable species in the same derivative of 2-mercaptoethanol. Deprotonation of a residue on the enzyme must be responsible for the transition from I to II. The pK values of the phenolic group are 7.1 (mercaptoethanol), 7.7 (bovine), and 8.8 (S. faecalis). The red shift in the lambda max of III and the modest increase in the pK of the phenolic group are consistent with a relatively hydrophobic environment for the nitrophenolate ion in the bovine enzyme. Since deprotonation of the residue in the bovine enzyme perturbs the pK of the phenolic group only slightly, its effect may be indirect. Interaction with a neighboring carboxyl group (pK 5.3) would account for the large increase in the pK of the phenolic group in the S. faecalis enzyme, which is not accompanied by an appreciable shift in the lambda max. Carbamyl-P increases the pK of the phenolic group in both enzymes, a result consistent with its binding site being close to the essential sulfhydryl group.
来自牛肝和粪肠球菌的鸟氨酸转氨甲酰酶（鸟氨酸氨甲酰基转移酶，2.1.3.3）的必需巯基优先与2-氯汞基-4-硝基苯酚反应。该衍生物在pH 4.4至8.8之间的光谱已被解析为硝基酚离子（III）以及两种酚类物质（I和II）的光谱。I和II（两种酶）以及III（牛肝酶）的最大吸收波长相较于2-巯基乙醇同一衍生物中的可比物质发生了红移。酶上一个残基的去质子化必定导致了从I到II的转变。酚基团的pK值分别为7.1（巯基乙醇）、7.7（牛肝酶）和8.8（粪肠球菌）。III的最大吸收波长的红移以及酚基团pK值的适度增加与牛肝酶中硝基酚离子所处的相对疏水环境相一致。由于牛肝酶中该残基的去质子化仅轻微扰动了酚基团的pK值，其影响可能是间接的。与相邻羧基（pK 5.3）的相互作用可以解释粪肠球菌酶中酚基团pK值的大幅增加，而此时最大吸收波长并未出现明显位移。氨甲酰磷酸会提高两种酶中酚基团的pK值，这一结果与其结合位点靠近必需巯基相符。
