COMPARISON OF MOLYBDENUM CENTERS OF NATIVE AND DESULFO XANTHINE-OXIDASE - NATURE OF CYANIDE-LABILE SULFUR ATOM AND NATURE OF PROTON-ACCEPTING GROUP
COMPARISON OF MOLYBDENUM CENTERS OF NATIVE AND DESULFO XANTHINE-OXIDASE - NATURE OF CYANIDE-LABILE SULFUR ATOM AND NATURE OF PROTON-ACCEPTING GROUP
复制标题
DOI：
10.1042/bj1750887
复制
发表时间：
1978-01-01
影响因子：
4.1
通讯作者：
BRAY, RC
中科院分区：
文献类型：
--
作者：
GUTTERIDGE, S;TANNER, SJ;BRAY, RC
黄嘌呤氧化酶的非功能形式被称为脱硫酶，通过各种方法与功能酶进行比较，以获得关于中心结构和催化反应机理的信息。与功能性脱硫酶一样，脱硫酶也有一个与阴离子结合的部位，推测是作为钼的配体。证据表明，在脱硫酶的钼(V)EPR信号中，与功能酶的信号一样，除了强耦合的质子外，还有一个弱耦合的质子与金属作用。在用2H2O稀释的酶样品上，用EPR测量了质子与钼的强偶联交换速率。脱硫酶的交换速率常数为0.40 S~(-1)，pH为8.2，温度为12度。C，功能酶为85 S~(-1)。所报道的酶形式之间的绝大多数差异与含有(酶).sbd.Mo.bd.S基团的功能酶一致，以脱硫型被(酶).sbd.Mo.bd.O取代。这些基团的质子化，其pk值分别约为8和10，将得到(酶).sbd.Mo.sbd.SH和(酶).sbd.Mo.sbd.OH，这些是EPR观察到的形式。因此，对于在催化反应中Mo被还原时从底物转移的质子，功能酶中的接受基团被认为是Mo.bd.S。
The non-functional form of xanthine oxidase known as the desulfo enzyme was compared with the functional enzyme in various ways, to obtain information on the structure of the center and the mechanism of the catalytic reaction. The desulfo enzyme, like the functional one, possesses a site for the binding of anions, presumably as ligands of Mo. Evidence is presented that in the Mo(V) EPR signal from the desulfo enzyme, as in that from the functional enzyme, a weakly coupled proton, in addition to a strongly coupled proton, interacts with the metal. Measurements were carried out by EPR on the rate at which the proton strongly coupled to Mo exchanged, on diluting enzyme samples with 2H2O. For the desulfo enzyme the exchange rate constant was 0.40 s-1, at pH 8.2 and 12.degree. C, and for the functional enzyme it was 85 s-1. The great majority of reported differences between the enzyme forms are consistent with functional enzyme containing an (Enzyme).sbd.Mo.dbd.S grouping, replaced in the desulfo form by (Enzyme).sbd.Mo.dbd.O. Protonation of these groups, with pK values of about 8 and 10, respectively, would give (Enzyme).sbd.Mo.sbd.SH and (Enzyme).sbd.Mo.sbd.OH, these being the forms observed by EPR. The accepting group in the functional enzyme, for the proton tranferred from the substrate while Mo is reduced in the catalytic reaction, is thus taken to be Mo.dbd.S.