Fig. 4: Catalytic intermediates of the binuclear centre of complex IV.
The binuclear centre is shown as a rectangle, consisting of haem a3 (Fe) the nearby CuB (Cu) and the conserved tyrosine (tyr-OH, tyrosine; tyr-O−, tyrosinate; tyr-O*, neutral tyrosine radical) covalently linked to one of the three histidine ligands of CuB. The three histidine ligands of CuB and the proximal histidine of haem a3 are not shown for clarity. The name of each intermediate state is found outside the right corner of each rectangle. The red arrows specify reactions that each involve electron transfer to the site from the low spin haem a, net transfer of one substrate proton from the N-side of the membrane to complete oxygen- reduction chemistry, and uptake (from the N-side) and release (to the P-side) of another proton (i.e. proton pumping). Hence, the red arrows indicate partial redox reactions that are reversible at high ΔP. The blue arrow depicts the irreversible partial reaction step where the O–O bond of O2 is broken in a reaction not linked to energy transduction. Note that the classical “ferrous-oxy” intermediate A26 is more accurate described as a ferric-superoxide state of haem a35.
