The F₄₂₀-Reducing Hydrogenase of Methanospirillum hungatei Srain GPl

Abstract

The F₄₂₀-reducing hydrogenase (H₂ase) of Methanospirillum hungatei was isolated from spheroplast lysates by sedimentation, followed by either sucrose gradients or nickel-affinity chromatography, and gel filtration. Most of the enzyme was free of the cytoplasmic membrane, although isolated membranes retained ca. 15% of the F₄₂₀-reducing H₂ase activity. The brown H₂ase protein had an absorption spectrum characteristic of a nonheme iron protein. In electron micrographs it was a coin-shaped, multisubunit protein of 15.9 nm diameter with a central depression on one surface. During chromatography on phenyl Sepharose the H₂ase exhibited hydrophobic properties. Labeling with the isoprenoid precursor, [¹⁴C]-mevalonate, and lipid analysis of the H₂ase CHCl₃/MeOH extracts, established that lipid was associated with the enzyme. This association appears to result from membrane contamination of the hydrophobic enzyme. The holoenzyme was about 720 kDa and contained 6–7 Ni²⁺ atoms. H₂-dependent reduction of F₄₂₀ activity was readily, but transiently, reactivated by anaerobic conditions following exposure of the enzyme to air. Mg²⁺ or Ca²⁺ were stimulatory. The holoenzyme was composed of α-subunits of 51 kDa, and 30–31 kDa β and γ subunits of nearly identical mass. The N-terminal amino acid sequences of the first 20–25 residues were very similar in β and γ subunits. Comparisons made to sequences known for other H₂ases, established that the M. hungatei H₂ase was quite different. Antibody raised against the purified hydrogenase of strain GP1 gave negative reactions with extracts of nine other methanogens, and a reaction of identity with M. hungatei strain JF1 and Methanosarcina barkeri strain MS.