Figure 6.

The possible effect of ammonia on the catalytic cycle of cytochrome bd-I. Enzyme catalytic intermediates O¹ (b₅₅₈²⁺ b₅₉₅³⁺ d³⁺–OH), A¹ (b₅₅₈³⁺ b₅₉₅³⁺ d²⁺–O₂), A³ (b₅₅₈²⁺ b₅₉₅²⁺ d²⁺–O₂), P (b₅₅₈²⁺ b₅₉₅³⁺ d*⁴⁺=O² where d*⁴⁺=O² is a ferryl porphyrin π-cation radical [47,49]), and F (b₅₅₈³⁺ b₅₉₅³⁺ d⁴⁺=O^2-) are shown. At alkaline pH, A¹ is probably converted into the fully oxidized form O. O is not involved in the catalytic cycle [72]. NH₃ possibly promotes the formation of P from O, thereby leading to the acceleration of the oxidase activity. It is also possible that NH₃ reacts with O¹ producing F. In the latter two reactions, NH₃ serves as a two-electron donor being oxidized to NH₂OH. The reaction of NH₃ with O¹ is not shown for the sake of simplicity.