Question

Describe the two mechanisms by which protons are thought to be translocated across the inner mitochondrial membrane by electron transport system protein complexes.

Short answer

Answer: The two main mechanisms responsible for proton translocation across the inner mitochondrial membrane are direct proton pumping by Complexes I, III, and IV, and the Q (ubiquinone) cycle via coenzyme Q. Both mechanisms contribute to the generation of an electrochemical gradient, ultimately leading to ATP synthesis.

Step-by-step solution

Introduction to the electron transport chain and its role in proton translocation

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane, responsible for the transfer of electrons from donors to acceptors, ultimately leading to the production of ATP through oxidative phosphorylation. The energy released during this process is used to pump protons (H+) from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient. This gradient ultimately drives ATP synthesis by ATP synthase.

Describe the first mechanism - Direct proton pumping

The first mechanism by which proton translocation occurs across the inner mitochondrial membrane is direct proton pumping. In this process, some of the protein complexes of the ETC (Complexes I, III, and IV) have the ability to directly pump protons from the matrix to the intermembrane space as electrons flow through them. When electrons move through these complexes, it releases energy that is directly used to transport protons across the membrane. This direct proton pumping by the complexes contributes to the electrochemical gradient, which powers ATP synthesis.

Describe the second mechanism - Q (Ubiquinone) cycle

The second mechanism involved in proton translocation is the Q cycle, also known as the ubiquinone cycle. In this mechanism, protons are translocated via the coenzyme Q (ubiquinone), which acts as a mobile electron carrier that connects Complexes I and II to Complex III (cytochrome bc1 complex) in the ETC. As electrons are transferred from Complex I or II to coenzyme Q, it becomes reduced and picks up protons from the mitochondrial matrix. This reduced form of coenzyme Q (ubiquinol) then moves to Complex III, where it releases the protons into the intermembrane space and transfers the electrons to cytochrome c. This process also helps create the electrochemical gradient needed for ATP synthesis. In conclusion, the translocation of protons across the inner mitochondrial membrane in the electron transport system involves two primary mechanisms: direct proton pumping by Complexes I, III, and IV, and the Q (ubiquinone) cycle via coenzyme Q. Both mechanisms contribute to the generation of an electrochemical gradient, ultimately leading to ATP synthesis.