Orchestration of diverse components in soluble methane monooxygenase for methane hydroxylation

Methane (CH₄) has a higher heat capacity (104.9 kcal/mol) than carbon dioxide (CO₂), and this has inspired research aimed at reducing methane levels to retard global warming. Hydroxylation under ambient conditions through methanotrophs can provide crucial information for understanding the harsh C–H activation of methane. Soluble methane monooxygenase (sMMO) belongs to the bacterial multi-component monooxygenase superfamily and requires hydroxylase (MMOH), regulatory (MMOB), and reductase (MMOR) components. Recent structural and biophysical studies have demonstrated that these components accelerate and retard methane hydroxylation in MMOH through protein-protein interactions. Complex structures of sMMO, including MMOH-MMOB and MMOH-MMOD, illustrate how these regulatory and inhibitory components orchestrate the di-iron active sites located within the four-helix bundles of MMOH, specifically at the docking surface known as the canyon region. In addition, recent biophysical studies have demonstrated the role of MmoR, a σ⁵⁴-dependent transcriptional regulator, in regulating sMMO expression. This perspective article introduces remarkable discoveries in recent reports on sMMO components that are crucial for understanding sMMO expression and activities. Our findings provide insight into how sMMO components interact with MMOH to control methane hydroxylation, shedding light on the mechanisms governing sMMO expression and the interactions between activating enzymes and promoters.