Question

Outline the basic constitution (ie types of metal site present) of cytochrome c oxidase

Answer 1

The complex is a large integral membrane protein composed of several metal prosthetic sites and 14 ^[1] protein subunits in mammals. In mammals, eleven subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two hemes, a cytochrome a and cytochrome a₃, and two copper centers, the Cu_A and Cu_B centers.^[2] In fact, the cytochrome a₃ and Cu_B form a binuclear center that is the site of oxygen reduction. Cytochrome c, which is reduced by the preceding component of the respiratory chain (cytochrome bc1 complex, complex III), docks near the Cu_A binuclear center and passes an electron to it, being oxidized back to cytochrome c containing Fe³⁺. The reduced Cu_Abinuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a₃-Cu_B binuclear center. The two metal ions in this binuclear center are 4.5 Å apart and coordinate a hydroxide ion in the fully oxidized state.

Crystallographic studies of cytochrome c oxidase show an unusual post-translational modification, linking C6 of Tyr(244) and the ε-N of His(240) (bovine enzyme numbering). It plays a vital role in enabling the cytochrome a₃- Cu_Bbinuclear center to accept four electrons in reducing molecular oxygen to water. The mechanism of reduction was formerly thought to involve a peroxide intermediate, which was believed to lead to superoxide production. However, the currently accepted mechanism involves a rapid four-electron reduction involving immediate oxygen-oxygen bond cleavage, avoiding any intermediate likely to form superoxide.

Cytochrome oxidase is one of a superfamily of proteins which act as the terminal enzymes of respiratory chains. The two main classes are cytochrome c oxidases, and quinol oxidases. The common features are:

• There are two catalytic subunits, I and II
• Subunit I contains two heme centers. The first of these (heme a in cytochrome oxidaes) acts as an electron input device to the second. The second heme (heme a₃ in cytochrome oxidase) is part of a binuclear center, with a Cu (Cu_B in cytochrome oxidase) as the other metal. The binuclear center is the site of oxygen reduction.
• Subunit II processes the electron donation. In cytochrome c oxidases, the subunit contains a Cu center (Cu_A) with 2 Cu atoms, which is thought to be the immediate electron acceptor from cytochrome c. A possible electron transfer pathway from this center to heme a has been identified in the structure (see below). In quinol oxidases, this subunit processes the quinol substrate.
• The enzymes are vectorially arranged so as to transfer charge across the membrane as electrons are passed to O₂. The electrogenic process is contributed by vectorial electron transfer, and the uptake of H⁺on reduction of O₂.