SPECTROSCOPIC CHARACTERIZATION OF THE ALTERNATE FORM OF S-METHYLCOENZYME M-REDUCTASE FROM METHANOBACTERIUM-THERMOAUTOTROPHICUM (STRAIN-DELTA-H)

Two forms (MR1 and MR2) of S-methylcoenzyme M reductase were purified from Methanobacterium thermoautotrophicum (strain DELTA-H) as recently described (Rospert, S., Linder, D., Ellerman, J. and Thauer, R.K. (1990) Eur. J. Biochem. 194, 871-877). MR2 was at least 50-fold more active than MR1, independent of assay conditions. The two forms are spectroscopically similar, but not identical, by UV-visible, magnetic circular dichroism and resonance Raman spectroscopies. MR, exhibited an EPR signal corresponding to 20% of the enzyme-bound nickel. Strong EPR signals similar to those previously assigned to Ni(I)F430 bound to methylreductase in Methanobacterium thermoautotrophicum (strain Marburg) (Albracht, S.P.J., Ankel-Fuchs, D., Bocher, R., Ellerman, J.. Moll, J., Van der Zwann, J.W. and Thauer, R.K. (1988) Biochim. Biophys. Acta 955, 86-102) were observed in MR2-rich, log-phase, as well as in MR1-rich, slow-growing bacteria. Log-phase cells had dramatically different EPR spectra depending on whether they were removed from the fermenter (under gas flow) before or after cooling to 10-degrees-C. EPR spectra of slow-growing cells were insensitive to harvesting conditions. The possible biological significance of the alternate form of methylreductase is discussed.