The electron transport system (ETS) is a series of protein complexes embedded in the inner mitochondrial membrane, crucial for cellular respiration. Complex III, also known as cytochrome c reductase, plays a vital role in this system. It facilitates the transfer of electrons from ubiquinol (UQH2) to cytochrome c while simultaneously pumping protons from the mitochondrial matrix to the intermembrane space. This process, known as the Q cycle, is a key step in generating the proton gradient necessary for ATP synthesis. Complex III itself is composed of cytochrome b and cytochrome c1 complexes, incorporating a Rieske center (a 2Fe-2S cluster) and heme prosthetic groups.
Electrons arrive at Complex III via UQH2, originating from Complex I or Complex II of the ETS. UQH2 is a two-electron carrier, but cytochromes can only handle one electron at a time. To manage this electron flow, Complex III employs a sophisticated mechanism called the Q cycle. This cycle ensures efficient electron transfer and proton translocation. Let’s delve into the Electron Transport System Steps within Complex III, specifically the Q cycle.
The Q cycle initiates when the first UQH2 molecule binds to the Rieske center within Complex III. During this binding, UQH2 undergoes oxidation to ubiquinone radical (UQH•), releasing one electron. This electron is then transferred to cytochrome c1. Subsequently, UQH• is further oxidized to ubiquinone (UQ), donating its second electron to cytochrome b.
Cytochrome c1, now reduced, carries its electron to cytochrome c, a mobile electron carrier. Cytochrome c then transports this electron to Complex IV, the final protein complex in the electron transport chain. In parallel, cytochrome b, after accepting an electron, transfers it to ubiquinone (UQ) located on the opposite side of the complex. This reduction of UQ regenerates ubiquinol radical (UQH•).
Finally, this UQH• is further reduced back to UQH2 when it accepts another electron from a second cytochrome b molecule, which in turn has been reduced by a second UQH2 molecule entering Complex III. In essence, for every two molecules of UQH2 that enter Complex III, four electrons traverse the Q cycle. Two of these electrons are passed to cytochrome c and subsequently onward to Complex IV, while the other two are utilized to regenerate one molecule of UQH2.
Concurrent with electron transport, proton pumping occurs. For each electron donated to cytochrome c1 and for each electron transferred from cytochrome b, one proton is moved across the inner mitochondrial membrane into the intermembrane space. Consequently, in a single Q cycle, a total of four protons are pumped across the membrane, contributing significantly to the electrochemical proton gradient. This gradient is the driving force for ATP synthase, which ultimately produces ATP, the cell’s energy currency. Understanding these electron transport system steps within Complex III and the Q cycle is crucial to appreciating the overall process of cellular respiration and energy production.
