Malthouse J P, George G N, Lowe D J, Bray R C
Biochem J. 1981 Dec 1;199(3):629-37. doi: 10.1042/bj1990629.
Different reduced forms of xanthine oxidase, labelled specifically in the cyanide-labile site with 33S, were prepared and examined by electron paramagnetic resonance. Coupling of this isotope to molybdenum(V) was quantified with the help of computer simulations and found to differ markedly from one reduced form to another. The xanthine Very Rapid signal shows strong, highly anisotropic, coupling with A(33S)av. 1.27 mT. For this signal, axes of the g- and A(33S)-tensors are rotated relative to one another. One axis of the A-tensor is in the plane of gxx ang gyy, but rotated by 40 degrees relative to the gxx axis, whereas the direction of weakest coupling to sulphur deviates by 10 degrees from the gzz axis. In contrast with this signal, only rather weaker coupling was observed in different types of Rapid signal [A(33S)av. 0.3--0.4 mT], and in the Inhibited signal coupling was weaker still [A(33S)av. 0.1--0.2 mT]. Clearly, there must be substantial differences in the structures of the molybdenum centre in the different signal-giving species, with the sulphur atom perhaps in an equatorial type of ligand position in the Very Rapid species but in a more axial one in the other species. Structures are discussed in relation to the mechanism of action of the enzyme and the nature of the proton-accepting group that participates in turnover.
制备了用³³S特异性标记在氰化物敏感位点的不同还原形式的黄嘌呤氧化酶，并通过电子顺磁共振进行检测。借助计算机模拟对该同位素与钼（V）的耦合进行了定量，发现不同还原形式之间存在显著差异。黄嘌呤的“非常快速”信号显示出与A(³³S)av. 1.27 mT的强且高度各向异性的耦合。对于该信号，g张量和A(³³S)张量轴相互旋转。A张量的一个轴在gxx和gyy平面内，但相对于gxx轴旋转了40度，而与硫的最弱耦合方向偏离gzz轴10度。与该信号相反，在不同类型的“快速”信号中仅观察到相当弱的耦合[A(³³S)av. 0.3 - 0.4 mT]），而在“受抑制”信号中耦合更弱[A(³³S)av. 0.1 - 0.2 mT]。显然，不同信号产生物种中钼中心的结构一定存在实质性差异，硫原子在“非常快速”物种中可能处于赤道型配体位置，而在其他物种中处于更轴向的位置。结合酶的作用机制以及参与周转的质子接受基团的性质对结构进行了讨论。
