Davydov A, Davydov R, Gräslund A, Lipscomb J D, Andersson K K
Department of Biophysics, Arrhenius Laboratories, Stockholm University, S-106 91 Stockholm, Sweden.
J Biol Chem. 1997 Mar 14;272(11):7022-6. doi: 10.1074/jbc.272.11.7022.
The soluble form of methane monooxygenase (MMO) consists of three components: reductase, hydroxylase (MMOH), and "B" (MMOB). Resting MMOH contains a diferric bis-mu-hydroxodinuclear iron "diamond core" cluster which is the site of oxygen activation chemistry after reduction. Here it is shown that gamma-irradiation of MMOH at 77 K results in reduction of the diiron cluster to an EPR active Fe(II). Fe(III) mixed valence state. At this temperature, the conformation of the enzyme remains essentially unchanged during reduction, so the EPR-spectrum becomes a probe of the conformation of the diferric state. The gamma-irradiated MMOH exhibits EPR spectra that differ in lineshape and saturation properties from those of the mixed valence MMOH generated by chemical reduction in solution; annealing the gamma-irradiated sample at 230 K yields the spectra of the chemically reduced sample. This demonstrates that the conformation of diferric MMOH in the vicinity of the diiron cluster changes during reduction to the mixed valence state. The analogous experiment for the MMOB.MMOH complex gives a new mixed valence species following gamma-irradiation that differs from all previously reported mixed valence species. Thus, binding of MMOB also causes a change in the conformation of diferric MMOH. It is hypothesized that the structural changes observed for the first time here may involve conversion of the dihydroxo-bridged diamond core structure to one with more readily dissociable bridging oxygen ligands to facilitate reaction with O2 following cluster reduction.
可溶性甲烷单加氧酶(MMO)由三个组分组成:还原酶、羟化酶(MMOH)和“B”组分(MMOB)。静止状态的MMOH含有一个双铁双-μ-羟基双核铁“菱形核心”簇,该簇是还原后氧活化化学反应的位点。本文表明,在77 K下对MMOH进行γ辐照会导致双铁簇还原为具有顺磁共振(EPR)活性的Fe(II).Fe(III)混合价态。在此温度下,酶的构象在还原过程中基本保持不变,因此EPR谱成为双铁态构象的探针。γ辐照的MMOH所呈现的EPR谱在谱线形状和饱和特性方面与溶液中化学还原产生的混合价MMOH不同;在230 K下对γ辐照样品进行退火处理可得到化学还原样品的谱图。这表明在还原为混合价态的过程中,双铁簇附近的双铁MMOH的构象发生了变化。对MMOB.MMOH复合物进行的类似实验在γ辐照后产生了一种新的混合价态物种,该物种不同于所有先前报道的混合价态物种。因此,MMOB的结合也会导致双铁MMOH的构象发生变化。据推测,此处首次观察到的结构变化可能涉及二羟基桥联菱形核心结构转变为具有更易解离的桥联氧配体的结构,以便在簇还原后促进与O2的反应。