9.B.450.  The Alkane 1-Monooxygenase (A1MO) Family 

These enzymes, probably of 6 TMSs, catalyze the hydroxylation of n-alkanes and fatty acids in the presence of a NADH-rubredoxin reductase and rubredoxin (Ruettinger et al. 1977). The reaction catalyzed is: 2 H⁺ + O₂ + octane + 2 reduced [rubredoxin] = H₂O + octan-1-ol + 2 oxidized [rubredoxin]. Alkane monooxygenase (AlkB) is a widely occurring integral membrane metalloenzyme that catalyzes the initial step in the functionalization of recalcitrant alkanes with high terminal selectivity (Chai et al. 2023). AlkB enables diverse microorganisms to use alkanes as their sole carbon and energy source. Chai et al. 2023 presented the 48.6-kDa cryo-electron microscopy structure of a natural fusion from Fontimonas thermophila between AlkB and its electron donor AlkG at 2.76 Å resolution. The AlkB portion contains six TMSs with an alkane entry tunnel within its transmembrane domain. A dodecane substrate is oriented by hydrophobic tunnel-lining residues to present a terminal C-H bond toward a diiron active site. AlkG, an [Fe-4S] rubredoxin, docks via electrostatic interactions and sequentially transfers electrons to the diiron center. The archetypal structural complex presented reveals the basis for terminal C-H selectivity and functionalization within this broadly distributed evolutionary class of enzymes (Chai et al. 2023).