Bray R C, Vänngård T
Biochem J. 1969 Oct;114(4):725-34. doi: 10.1042/bj1140725.
Further electron-paramagnetic-resonance studies relating to the role of molybdenum in the enzymic mechanisms of xanthine oxidase were carried out. The classification of the various molybdenum signals obtained on reducing the enzyme is briefly discussed. The group of ;Rapidly appearing' signals, which are obtained with all substrates within the turnover time and which show interaction with exchangeable protons, were studied in detail. Signals with salicylaldehyde, purine and xanthine in H(2)O and in 95% D(2)O were examined at 9 and 35GHz and interpreted with the help of computer simulation. Molybdenum atoms in a number of different chemical environments are involved, each substrate giving rise to two superimposed spectra with slightly different parameters; g values and proton splittings were determined. The spectrum with salicylaldehyde is believed to represent the reduced enzyme alone not in the form of a complex with substrate and its two constituents are believed to represent the two molybdenum atoms bonded slightly differently within the enzyme molecule. With purine and xanthine the spectra are thought to represent complexes of reduced enzyme with substrate molecules. With xanthine one signal-giving species shows coupling to two equivalent protons, whereas in all the other species observed two non-equivalent protons are involved. The origin of the protons is discussed in the light of the direct hydrogen-transfer mechanism implicated earlier for the enzyme. It is concluded that the proton derived from the substrate is located at least 3å from the molybdenum atom with which it interacts.
开展了与钼在黄嘌呤氧化酶酶促机制中的作用相关的进一步电子顺磁共振研究。简要讨论了还原酶时获得的各种钼信号的分类。详细研究了“快速出现”的信号组，这些信号在周转时间内与所有底物均可获得，并且显示出与可交换质子的相互作用。在9GHz和35GHz下检测了水杨醛、嘌呤和黄嘌呤在H₂O和95%D₂O中的信号，并借助计算机模拟进行了解释。涉及多种不同化学环境中的钼原子，每种底物产生两个参数略有不同的叠加光谱；确定了g值和质子分裂。水杨醛的光谱被认为仅代表还原酶，而不是与底物形成的复合物形式，其两个成分被认为代表酶分子内键合略有不同的两个钼原子。对于嘌呤和黄嘌呤，光谱被认为代表还原酶与底物分子的复合物。对于黄嘌呤，一种产生信号的物种显示与两个等效质子偶合，而在观察到的所有其他物种中涉及两个不等效质子。根据先前涉及该酶的直接氢转移机制讨论了质子的来源。得出的结论是，来自底物的质子与其相互作用的钼原子至少相距3埃。
