Physiological Effect of XoxG(4) on Lanthanide-Dependent Methanotrophy

Area of Science:

• Biochemistry
• Microbiology

Background:

• Rare earth metals (lanthanides) are increasingly recognized for their roles beyond biological inertia, acting as enzyme cofactors and transcriptional regulators.
• Previous studies primarily linked lanthanide activity to alcohol dehydrogenases, leaving other metabolic pathways underexplored.

Purpose of the Study:

• To investigate the physiological roles of the novel cytochrome XoxG and its involvement in lanthanide-dependent metabolism.
• To compare the effects of mutations in xoxG and xoxF (a lanthanide-dependent methanol dehydrogenase) on enzyme activity and community function.
• To elucidate the function of XoxG as a potential electron acceptor from XoxF and its role in methane metabolism.

Main Methods:

• Comparative phenotypic characterization of wild-type and mutant strains of *Methylomonas* sp. strain LW13.
• Enzyme activity assays to assess lanthanide-dependent metabolism.
• Purification of XoxG and characterization as a cytochrome *c*.
• Demonstration of direct electron transfer between purified XoxF and XoxG.

Main Results:

• XoxG is identified as the second protein directly involved in lanthanide-dependent metabolism, acting as an electron acceptor from XoxF.
• Mutation in *xoxG* resulted in a distinct phenotype compared to *xoxF* mutants, suggesting a secondary role in methane metabolism.
• Purified XoxG confirmed as a cytochrome *c* and shown to be directly reduced by XoxF.
• The study highlights the complex interplay between calcium-dependent and lanthanide-dependent alcohol oxidation systems.

Conclusions:

• XoxG plays a crucial role in lanthanide-dependent methanotrophy, functioning both as an electron acceptor from XoxF and in an as-yet-unknown critical function for methane metabolism.
• Understanding these alternative oxidation systems is essential at the enzyme, community, and transcriptional levels.
• This research expands the knowledge of lanthanide-dependent biochemistry and identifies a novel redox protein involved in methanotrophy.