Succinate dehydrogenase, which catalyzes succinate oxidation to fumarate, is the only enzyme in the tricarboxylic acids cycle that is not present in a solubel form in the mytochondrial matrix: it is an integral membrane protein. Its
substrate transfers two electrons to a
. One by one, those electrons are then transfered by
which donates two electrons to a lipid-soluble molecule, ubiquinone (Q), converting it to ubiquinol (QH2). Since the succinate/fumarate redox potential is very close to that of the Q/QH2 pair, the free energy change in this step is not enough to pump electrons from the matrix to the intermembrane space.