During cellular respiration, the majority of oxygen consumed is transformed into water through the electron transport chain (ETC). A key player in this process is cytochrome c, which is part of complex III and is notable for its ability to accept only one electron at a time. This limitation is significant because it necessitates the Q cycle, a mechanism that facilitates the transfer of electrons between different components of the ETC.
When intact mitochondria are treated with antimycin, a compound that inhibits electron transfer between cytochromes b and c, it is essential to understand the implications for the oxidation states of various molecules involved. In this scenario, cytochrome A will be found in an oxidized state because the blockage prevents the transfer of electrons from cytochrome b to cytochrome c, thereby halting the flow of electrons through the chain.
Furthermore, it is crucial to recognize that reduced quinones are not formed at complex III or cytochrome c. Instead, quinones are oxidized at complex III, where ubiquinone (also known as coenzyme Q) donates electrons. In contrast, reduced quinones are generated at complex I and complex II, where NADH and succinate, respectively, contribute electrons to the ETC. Additionally, fatty acid oxidation produces FADH2, which also donates electrons to ubiquinone, while the oxidation of glycerol 3-phosphate facilitates the entry of electrons into the mitochondria.
Regarding the proton motive force (PMF), it is a common misconception that its generation requires succinate. In reality, succinate is just one of several substrates that can feed electrons into the ETC via succinate dehydrogenase (complex II). Other pathways include the transfer of electrons from NADH through complex I, as well as contributions from the malate-aspartate shuttle, which transports cytosolic NADH into the mitochondria. This diversity of entry points into the electron transport chain underscores the complexity and efficiency of cellular respiration.
