Dey Mishtu, Kunz Ryan C, Van Heuvelen Katherine M, Craft Jennifer L, Horng Yih-Chern, Tang Qun, Bocian David F, George Simon J, Brunold Thomas C, Ragsdale Stephen W
Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588, USA.
Biochemistry. 2006 Oct 3;45(39):11915-33. doi: 10.1021/bi0613269.
Methyl-coenzyme M reductase (MCR) catalyzes the final step in methane biosynthesis by methanogenic archaea and contains a redox-active nickel tetrahydrocorphin, coenzyme F430, at its active site. Spectroscopic and computational methods have been used to study a novel form of the coenzyme, called F330, which is obtained by reducing F430 with sodium borohydride (NaBH4). F330 exhibits a prominent absorption peak at 330 nm, which is blue shifted by 100 nm relative to F430. Mass spectrometric studies demonstrate that the tetrapyrrole ring in F330 has undergone reduction, on the basis of the incorporation of protium (or deuterium), upon treatment of F430 with NaBH4 (or NaBD4). One- and two-dimensional NMR studies show that the site of reduction is the exocyclic ketone group of the tetrahydrocorphin. Resonance Raman studies indicate that elimination of this pi-bond increases the overall pi-bond order in the conjugative framework. X-ray absorption, magnetic circular dichroism, and computational results show that F330 contains low-spin Ni(II). Thus, conversion of F430 to F330 reduces the hydrocorphin ring but not the metal. Conversely, reduction of F430 with Ti(III) citrate to generate F380 (corresponding to the active MCR(red1) state) reduces the Ni(II) to Ni(I) but does not reduce the tetrapyrrole ring system, which is consistent with other studies [Piskorski, R., and Jaun, B. (2003) J. Am. Chem. Soc. 125, 13120-13125; Craft, J. L., et al. (2004) J. Biol. Inorg. Chem. 9, 77-89]. The distinct origins of the absorption band shifts associated with the formation of F330 and F380 are discussed within the framework of our computational results. These studies on the nature of the product(s) of reduction of F430 are of interest in the context of the mechanism of methane formation by MCR and in relation to the chemistry of hydroporphinoid systems in general. The spectroscopic and time-dependent DFT calculations add important insight into the electronic structure of the nickel hydrocorphinate in its Ni(II) and Ni(I) valence states.
甲基辅酶M还原酶（MCR）催化产甲烷古菌甲烷生物合成的最后一步，其活性位点含有一种具有氧化还原活性的镍四氢卟吩，即辅酶F430。光谱学和计算方法已被用于研究一种新型辅酶形式，称为F330，它是通过用硼氢化钠（NaBH4）还原F430得到的。F330在330 nm处有一个突出的吸收峰，相对于F430发生了100 nm的蓝移。质谱研究表明，在用NaBH4（或NaBD4）处理F430后，基于质子（或氘）的掺入，F330中的四吡咯环发生了还原。一维和二维核磁共振研究表明，还原位点是四氢卟吩的环外酮基。共振拉曼研究表明，消除这个π键会增加共轭框架中的整体π键序。X射线吸收、磁圆二色性和计算结果表明，F330含有低自旋Ni(II)。因此，F430向F330的转化会还原氢化卟吩环，但不会还原金属。相反，用柠檬酸钛（III）还原F430以生成F380（对应于活性MCR(red1)状态）会将Ni(II)还原为Ni(I)，但不会还原四吡咯环系统，这与其他研究一致[皮斯科尔斯基，R.，和扬，B.（2003年）《美国化学会志》125，13120 - 13125；克拉夫特，J.L.等人（2004年）《生物无机化学杂志》9，77 - 89]。在我们的计算结果框架内讨论了与F330和F380形成相关的吸收带位移的不同起源。这些关于F430还原产物性质的研究，在MCR形成甲烷的机制以及一般氢化卟吩系统化学的背景下具有重要意义。光谱学和含时密度泛函理论计算为镍氢化卟啉在其Ni(II)和Ni(I)价态下的电子结构提供了重要见解。
