The B protein acts as a regulator of electron flow through the soluble mmo complex, switching the enzyme from an oxidase to a hydroxylase in the presence of the substrate. The soluble methane monooxygenase (sMMO) consists of four components A/MMOH (composed of alpha/MmoX, beta/MmoY and gamma/MmoZ), B/MMOB (MmoB), C/MMOR (MmoC) and D/MMOD (MmoD). Belongs to the TmoD/XamoD family.