Terminal oxidase is the component of the respiratory chain that catalyzes the reduction of oxygen to water. SoxM forms the functional core of the enzyme complex. Catalytic subunit of the enzyme. Electrons originating in a quinol are transferred to the bimetallic center of SoxM formed by a heme and copper B. 2 a quinol + O2 = 2 a quinone + 2 H2O Binds 1 copper B ion per subunit. Binds 2 heme groups per subunit. Energy metabolism; oxidative phosphorylation. Forms a complex with at least SoxC and a 30 kDa Rieske Fe-S protein, but neither with SoxA nor SoxB. SoxM is probably a precursor form of subunits I and III. In the N-terminal section; belongs to the heme-copper respiratory oxidase family. In the C-terminal section; belongs to the cytochrome c oxidase subunit 3 family.