Complex IV is also known as cytochrome oxidase and contains cytochromes a and a₃ as shown in Figure 8.19. The cytochromes are made up of two heme groups bound to different regions of the same large protein, which are therefore functionally and spectrally different. In addition, complex IV contains two copper ions, CuA and CuB, which are helpful in transferring the electrons to O₂. This complex is also involved in carrying the four reduced electrons to O₂ without generating the reduced intermediates such as hydrogen peroxide or hydroxy free radicals. The flow of electrons in complex IV causes a net movement of protons from the matrix to the intermembrane space. Complex IV functions as a proton pump and contributes to the proton motive force.

Figure 8.19 Pathway of Electrons through Complex IV

Summary of the flow of electrons and protons through the four complexes of respiratory chain is shown in Figure 8.20.

Figure 8.20 Flow of Electrons and Protons through the Four Complexes of Respiratory Chain