Purification and characterization of an 8‐hydroxy‐5‐deazaflavin‐reducing hydrogenase from the archaebacterium Methanococcus voltae

Abstract

A methylviologen and 8-hydroxy-5-deazaflavin(F₄₂₀)-reducing hydrogenase was purified over 800-fold to near homogeneity from the archaebacterium Methanococcus voltae with 10 U mg⁻¹ F₄₂₀-reducing activity. It is the only hydrogenase in this organism. The enzyme showed K_m values of 16 μM for F₄₂₀ and 1.2 mM for methylviologen. A turnover number of 1050 min⁻¹ was calculated for the minimal active unit. The protein tends to aggregate. The molecular mass of the minimal active unit is 105 kDa. Larger molecules of 745 kDa were regularly observed. The enzyme was resolved into subunits with molecular masses of 55 kDa, 45 kDa, 37 kDa and 27 kDa by SDS/polyacrylamide gel electrophoresis. Reversible conversion of an anionic into an uncharged form was observed by DEAE-cellulose chromatography with concomitant changes in substrate specifities. The methylviologen-reducing activity was heat-resistant up to 65°C and was not affected by antiserum raised against the native enzyme, while F₄₂₀ reduction was inactivated by both treatments. Nickel and selenium contents were determined as 0.6–0.7 mol each, FAD content as 1 mol and iron as 4.5 mol/mol protein (105 kDa), respectively. Electron micrographs taken from the purified enzyme show ring-shaped molecules of 18 nm diameter, which represent the high-molecular-mass species of the enzyme.