Which component transfer four protons?

It is also called NADH-CoQ reductase or NADH dehydrogenase complex. It is tightly bound to the inner membrane of mitochondria. ii. It contains a flavoprotein (Fp), consisting of FMN as prosthetic group and an iron-sulfur protein (Fe-S). NADH is the donor of electrons, FMN accepts them and gets reduced to FMNH2 (Fig.19.8). Two electrons and one hydrogen ion are transferred from NADH to the flavin prosthetic group of the enzyme.NADH + H+ + FMN - FMNH2 + NAD+The electrons from FMNH2 are then transferred to Fe-S.The electrons are then transferred to coenzyme Q (ubiquinone) (CoQ). Overall function of this complex is to collect the pair of electrons from NADH and pass them to CoQ. There is a large negative free energy change; the energy released is 12 kcal/mol. This is utilized to drive 4 protons out of the mitochondria.