Spectroscopic and computational studies of reduction of the metalversus the tetrapyrrole ring of coenzyme F-430 from methyl-coenzyme Mreductase
Methyl-coenzyme M reductase (MCR) catalyzes the final stepin methane biosynthesis by methanogenic archaea and contains aredox-active nickel tetrahydrocorphin, coenzyme F430, at its active site.Spectroscopic and computational methods have been used to study a novelform of the coenzyme, called F330, which is obtained by reducing F430with sodium borohydride (NaBH4). F330 exhibits a prominent absorptionpeak at 330 nm, which is blue shifted by 100 nm relative to F430. Massspectrometric studies demonstrate that the tetrapyrrole ring in F330 hasundergone reduction, on the basis of the incorporation of protium (ordeuterium), upon treatment of F430 with NaBH4 (or NaBD4). One- andtwo-dimensional NMR studies show that the site of reduction is theexocyclic ketone group of the tetrahydrocorphin. Resonance Raman studiesindicate that elimination of this pibond increases the overall pi-bondorder in the conjugative framework. X-ray absorption, magnetic circulardichroism, and computational results show that F330 contains low-spinNi(II). Thus, conversion of F430 to F330 reduces the hydrocorphin ringbut not the metal. Conversely, reduction of F430 with Ti(III) citrate togenerate F380 (corresponding to the active MCRred1 state) reduces theNi(II) to Ni(I) but does not reduce the tetrapyrrole ring system, whichis 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 bandshifts associated with the formation of F330 and F380 are discussedwithin the framework of our computational results. These studies on thenature of the product(s) of reduction of F430 are of interest in thecontext of the mechanism of methane formation by MCR and in relation tothe chemistry of hydroporphinoid systems in general. The spectroscopicand time-dependent DFT calculations add important insight into theelectronic structure of the nickel hydrocorphinate in its Ni(II) andNi(I) valencestates.
- Research Organization:
- COLLABORATION - University of Nebraska,Lincoln
- DOE Contract Number:
- AC02-05CH11231
- OSTI ID:
- 910341
- Report Number(s):
- LBNL--63244
- Journal Information:
- Biochemistry, Journal Name: Biochemistry Journal Issue: 39 Vol. 45; ISSN 0006-2960; ISSN BICHAW
- Country of Publication:
- United States
- Language:
- English
Similar Records
Structural consequences of nickel versus macrocycle reductions in F430 models: EXAFS studies of a Ni(I) anion and Ni(II). pi. anion radicals
Structural Insight into Methyl-Coenzyme M Reductase Chemistry Using Coenzyme B Analogues