FIGURE 12-20 Heme and iron-sulfur prosthetic groups in the electron-transport chain. (a) Heme portion of cytochromes b_L and b_H, which are components of CoQH₂–cytochrome c reductase (complex III). The same porphyrin ring (yellow) is present in all hemes. The chemical substituents attached to the porphyrin ring differ in the other cytochromes in the electron-transport chain. All hemes accept and release one electron at a time. (b) Dimeric iron-sulfur cluster (Fe-S). Each Fe atom is bonded to four S atoms: two are inorganic sulfur, and two are in cysteine side chains of the associated protein. All Fe-S clusters accept and release one electron at a time.