Final answer:
Complex III of the ETC transfers electrons from QH₂ to Cyt c while pumping protons into the intermembrane space. This action contributes to the proton gradient required for ATP synthesis. Complex II also transfers electrons but does not pump protons, producing fewer ATP molecules.
Step-by-step explanation:
Mechanism of Complex III in the Electron Transport Chain
The electron transport chain (ETC) encompasses a series of complexes that facilitate electron transfer and the generation of an electrochemical gradient used for ATP synthesis. Complex III, also known as cytochrome bc1 or cytochrome oxidoreductase, is a pivotal component in this process. This complex is composed of cytochrome proteins that contain heme groups, which are integral for electron transport
Complex III functions by accepting electrons from coenzyme-Q (QH₂) and passing them to cytochrome c (Cyt c) in a process that simultaneously pumps protons from the matrix to the intermembrane space. This transfer of electrons occurs in two steps due to QH₂ carrying two electrons and Cyt c only being able to accept one at a time. Initially, QH₂ gives one electron to Cyt c and the other to a Q molecule in the quinone form, thereby becoming a semiubiquinone (Q*¯). In the subsequent step, another QH₂ hands off one electron to a second Cyt c and the other to Q*¯, transforming it back into Q, which then repeats the cycle, allowing for the transport of two electrons from Q to two Cyt c molecules. During this process, four protons are also pumped into the intermembrane space, contributing to the proton gradient essential for ATP production.
While Complex II also feeds electrons into the chain via FADH₂ oxidation, it does not contribute to proton pumping across the inner membrane, resulting in fewer ATP molecules synthesized from FADH₂-derived electrons when compared to those derived from NADH via Complex I and III.