Characterization of a novel cytochrome c GJ as the electron acceptor of XoxF-MDH in the thermoacidophilic methanotroph Methylacidiphilum fumariolicum SolV

Abstract

Methanotrophs play a prominent role in the global carbon cycle, by oxidizing the potent greenhouse gas methane to CO 2 . Methane is first converted into methanol by methane monooxygenase. This methanol is subsequently oxidized by either a calcium-dependent MxaF-type or a lanthanide-dependent XoxF-type methanol dehydrogenase (MDH). Electrons from methanol oxidation are shuttled to a cytochrome redox partner, termed cytochrome c L . Here, the cytochrome c L homolog from the thermoacidophilic methanotroph Methylacidiphilum fumariolicum SolV was characterized. SolV cytochrome c GJ is a fusion of a XoxG cytochrome and a periplasmic binding protein XoxJ. Here we show that XoxGJ functions as the direct electron acceptor of its corresponding XoxF-type MDH and can sustain methanol turnover, when a secondary cytochrome is present as final electron acceptor. SolV cytochrome c GJ (XoxGJ) further displays a unique, red-shifted absorbance spectrum, with a Soret and Q bands at 440, 553 and 595 nm in the reduced state, respectively. VTVH-MCD spectroscopy revealed the presence of a low spin iron heme and the data further shows that the heme group exhibits minimal ruffling. The midpoint potential E m,pH7 of +240 mV is similar to other cytochrome c L type proteins but remarkably, the midpoint potential of cytochrome c GJ was not influenced by lowering the pH. Cytochrome c GJ represents the first example of a cytochrome from a strictly lanthanide-dependent methylotrophic microorganism.