title: The magnetic and electronic properties of Methanobacterium thermoautotrophicum (strain delta H) methyl coenzyme M reductase and its nickel tetrapyrrole cofactor F430. A low temperature magnetic circular dichroism study.
abstract: Variable temperature magnetic circular dichroism (MCD) spectroscopy has been used to characterize the magnetic and electronic properties of the Ni(II) tetrapyrrole, F430, which is the cofactor of the S-methyl coenzyme M methylreductase enzyme from Methanobacterium thermoautotrophicum (strain delta H). 4-Coordinate forms are found to be diamagnetic (S = 0 ground state), whereas 6-coordinate forms are paramagnetic (S = 1 ground state). MCD studies, together with parallel low temperature UV-visible absorption and resonance Raman investigations, show that the equilibrium distribution of 4-coordinate square-planar and 6-coordinate bis-aquo forms of the native isomer of F430 in aqueous solution is affected by both temperature and the presence of glycerol. In the presence of 50% glycerol, the 12,13-diepimer of F430 is shown to be partially 6-coordinate in frozen solution at low temperature. Low temperature MCD magnetization data allow the determination of the axial zero-field splitting (D) of the S = 1 ground state of bis-ligand complexes of F430. The value of D is sensitive to the nature of the Ni(II) axial ligands: bis-aquo F430, D = +9 +/- 1 cm-1; bis-imidazole F430, D = -8 +/- 2 cm-1. Measurement of D = +10 +/- 1 cm-1 for F430 in the methylreductase holoenzyme argues strongly against histidine imidazole coordination to Ni(II) in the enzyme. The possible existence of alcoholic or phenolic oxygen-containing ligands (serine, threonine, tyrosine, water) to Ni(II) in the enzyme-bound cofactor is discussed.
authors.affiliation: School of Chemical Sciences, University of Georgia, Athens 30602.
authors.forename: C L
authors.initials: CL
authors.lastname: Hamilton
authors.forename: R A
authors.initials: RA
authors.lastname: Scott
authors.forename: M K
authors.initials: MK
authors.lastname: Johnson
mesh_terms: D002942:Circular Dichroism; D055590:Electromagnetic Phenomena; D019605:Euryarchaeota / Q000201:enzymology*; D005990:Glycerol / Q000494:pharmacology; D008665:Metalloporphyrins; D008667:Metalloproteins; D009532:Nickel; D010088:Oxidoreductases; D011487:Protein Conformation; D013057:Spectrum Analysis; D013059:Spectrum Analysis, Raman; D013696:Temperature
chemical_list: D008665:Metalloporphyrins; D008667:Metalloproteins; C023540:factor F430; D009532:Nickel; D010088:Oxidoreductases; C021572:methyl coenzyme M reductase; D005990:Glycerol