Malthouse J P, Bray R C
Biochem J. 1983 Oct 1;215(1):101-6. doi: 10.1042/bj2150101.
The effect of pH variation on the exchangeability with deuterium of protons strongly coupled to Mo(V) in the active and desulpho forms of xanthine oxidase was studied by e.p.r. and rapid freezing, in extension of the work of Gutteridge, Tanner & Bray [Biochem. J. (1978) 175, 887-897]. Above neutrality, exchange rates increased with increasing pH. Detailed studies were made on the desulpho enzyme under a variety of conditions, and exchange rate constants at 22 degrees C ranged from 0.16s -1 at pH 6.6 to 1.6s -1 at pH 11.3. The mechanism of proton exchange in the enzyme is discussed. The interpretation by the above workers that the strongly coupled proton of the active enzyme is on sulphur and that of the desulpho enzyme is on oxygen remains valid (and is in agreement with other work), as do their proposals for the structures of the protonated and deprotonated species. However, pK values cannot be calculated from the exchange data. It is likely that the relatively low rates of exchange observed are due to the difference of structure between the protonated and the deprotonated forms. In the case of the desulpho enzyme, an exchange mechanism, which involves the proton exchanging both as such and along with oxygen in the form of a hydroxyl ion, is discussed.
通过电子顺磁共振（e.p.r.）和快速冷冻技术，在Gutteridge、Tanner和Bray [《生物化学杂志》（1978年）175卷，887 - 897页] 工作的基础上，研究了pH变化对黄嘌呤氧化酶活性形式和脱硫形式中与钼（V）强耦合质子的氘交换能力的影响。在中性以上，交换速率随pH升高而增加。在各种条件下对脱硫酶进行了详细研究，22℃时的交换速率常数范围从pH 6.6时的0.16s⁻¹到pH 11.3时的1.6s⁻¹。讨论了酶中质子交换的机制。上述研究人员关于活性酶的强耦合质子在硫上以及脱硫酶的强耦合质子在氧上的解释仍然有效（并且与其他工作一致），他们对质子化和去质子化物种结构的提议也是如此。然而，无法从交换数据计算出pK值。观察到的相对较低的交换速率可能是由于质子化形式和去质子化形式之间的结构差异。对于脱硫酶，讨论了一种交换机制，该机制涉及质子本身以及以氢氧根离子形式与氧一起进行交换。
