Complex IV - Cytochrome c Oxidase

Step 1

Two Cyt c donate an electron (each) to separate Cu-containing peptide units of the CuA dimer (CuA/CuA)

Step 1a

One electron from CuA reduces a Heme a which reduces Heme a3 which reduces CuB

Step 1b

The other electron from CuA reduces a Heme a which reduces Heme a3- however, since CuB can only accept one electron, the 2nd electron stays bound to Heme a3, producing 1 reduced Heme a3 and 1 reduced CuB peptide

Step 2

The reduced Heme a3 - CuB peptide can now bind and reduce O2 forming a "peroxide bridge

Step 3

Two more Cyt c donate an electron (each) to 1 CuA then to heme a then to the Heme a3-CuB peroxide bridge. The addition of 2 electrons to peroxide bridge cleaves the bridge. Subsequent protonation forms Heme a3(-OH) and CuB(-OH)

How many net protons are released by complex IV

2 protons