N 5, N10-Methylenetetrahydromethanopterin reductase from Methanosarcina barkeri

N ⁵ N ¹⁰-Methylenetetrahydromethanopterin reductase was purified 13-fold to apparent homogeneity from methanol grown Methanosarcina barkeri . The colourless enzyme was found to be composed of four identical subunits of apparent molecular mass 36 kDa. It catalysed the reduction of methylenetetrahydromethanopterin ( K _m=15 μM) to methyltetrahydromethanopterin with reduced coenzyme F₄₂₀ ( K _m=12 μM) at a specific rate ( V _max) of 2200 μmol min⁻¹· mg protein⁻¹ ( K _cat=1320 s⁻¹). With respect to coenzyme specificity, molecular properties and catalytic mechanism the enzyme was found to be similar to CH₂=H₄MPT reductase of Methanobacterium thermoautotrophicum which phylogenetically is only distantly related to M. barkeri .