Question

Complex II does not pump any protons. But it does contribute electrons. Describe the path of the electron(s) from succinate to the end of the ETC, showing how many protons that/those electron(s) are responsible for.

Answer 1

Complex II of the Electron Transport Chain (ETC) is actually an enzyme called Succinate Dhydrogenase. This complex is presen embedded in the inner mitochondrial membrane. This complex is also known as  Succinate-Coenzyme Q Reductase (SQR). This enzyme catalyses the reaction of Krebs Cycle which converts Succinate into fumarate with the production of FADH2 molecule. The FADH2 molecule donates its two electrons to complex II bypassing complex I. This bypassing leads to only 1.5 ATP synthesis per FADH2 during oxidative phosphorylation.

From this complex II, the two electrons are passed to a lipid soluble, mobile electron carrier called Ubiquinone or Coenzyme Q.

The reduced Q picks two protons from the matrix to give QH2 and being lipid soluble moves to complex III and donates electrons to cytochrome b of complex III, from cytochrome b the electrons are passed to cytocrome c1 through iron sulphur protein. Then the electrons are passed to cytocrome c and the two protons are pushed into the intermembrane space. From cytocrome c1, the electrons are passed to complex IV which is a copper containing complex. when the complex IV accepts two electrons it picks two protons from the matrix into the intermembrane space. In this complex again two protons are pushed into the inter membrane space. From complex IV the electrons are picked by the oxygen atom which also accepts two protons to give water.

Thus four protons are transferred from matrix into intermembrane space per FADH2 molecule.