Purified methyl-coenzyme-M reductase is activated when the enzyme-bound coenzyme F430 is reduced to the nickel(I) oxidation state by titanium(III) citrate.

The nickel porphinoid, coenzyme F430, is the prosthetic group of methyl-coenzyme M reductase. The active form of the enzyme exhibits Ni-EPR signals designated as MCR-red1 and MCR-red2. The inactive form of the enzyme is either EPR silent or it exhibits a distinct Ni-EPR signal designated MCR-ox1. Evidence is presented here that the MCR-ox1 form of the enzyme can be converted in vitro to the MCR-red1 form by reduction with titanium(III) citrate at pH 9. During conversion, the specific activity increases with increasing MCR-red1 spin concentration from 2 U/mg to approximately 100 U/mg at spin concentrations higher than 80%. The reduced methyl-coenzyme-M reductase shows an ultraviolet/visible spectrum characteristic for coenzyme F430 in the Ni(I) oxidation state, with maxima at 386 nm and at 750 nm. The results indicate that methyl-coenzyme-M reductase is activated when the enzyme-bound coenzyme F430 is reduced to the Ni(I) oxidation state. The experiments were performed with purified methyl-coenzyme-M reductase isoenzyme I of Methanobacterium thermoautotrophicum (strain Marburg).