In vitro enzyme-mediated conversion of methane and methanol toward value-added chemicals.

Methane (CH4) is recognized not only as a greenhouse gas but also as a promising feedstock for carbon-based chemicals. Biological methane conversion has gained attention for operating under mild conditions. Efforts have focused on enhancing methane-oxidizing enzymes, including methane monooxygenases and cytochrome P450, for both in vivo and in vitro applications. Methanol dehydrogenase is traditionally used for methanol conversion, whereas recombinant alcohol oxidase has emerged as an attractive alternative due to cofactor-independent oxidation with molecular oxygen, reduced cofactor demand, and favorable thermodynamics. Formaldehyde, produced by methanol oxidation, serves as a versatile C1 intermediate, enabling condensation reactions to form higher-value compounds. This review summarizes recent enzymatic advances, multi-step methanol upgrading pathways, and design principles, and discusses current challenges and future directions for sustainable methane valorization.