<P> In Complex IV (cytochrome c oxidase; EC 1.9. 3.1), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome c and transferred to molecular oxygen (O), producing two molecules of water . At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient . The activity of cytochrome c oxidase is inhibited by cyanide, carbon monoxide, azide, hydrogen sulphide (H S). </P> <P> According to the chemiosmotic coupling hypothesis, proposed by Nobel Prize in Chemistry winner Peter D. Mitchell, the electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane . The efflux of protons from the mitochondrial matrix creates an electrochemical gradient (proton gradient). This gradient is used by the F F ATP synthase complex to make ATP via oxidative phosphorylation . ATP synthase is sometimes described as Complex V of the electron transport chain . The F component of ATP synthase acts as an ion channel that provides for a proton flux back into the mitochondrial matrix . It is composed of a, b and c subunits . Protons in the inter-membranous space of mitochondria first enters the ATP synthase complex through a subunit channel . Then protons move to the c subunits . The number of c subunits it has determines how many protons it will require to make the F turn one full revolution . For example, in humans, there are 8 c subunits, thus 8 protons are required . After c subunits, protons finally enters matrix using a subunit channel that opens into the mitochondrial matrix . This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD and Q). The free energy is used to drive ATP synthesis, catalyzed by the F component of the complex . Coupling with oxidative phosphorylation is a key step for ATP production . However, in specific cases, uncoupling the two processes may be biologically useful . The uncoupling protein, thermogenin--present in the inner mitochondrial membrane of brown adipose tissue--provides for an alternative flow of protons back to the inner mitochondrial matrix . Thyroxine is also a natural uncoupler . This alternative flow results in thermogenesis rather than ATP production . Synthetic uncouplers (e.g., 2, 4 - dinitrophenol, 2, 4 - dinitrocresol, CCCP) also exist, and can be lethal at high doses . </P> <P> In the mitochondrial electron transport chain electrons move from an electron donor (NADH or QH) to a terminal electron acceptor (O) via a series of redox reactions . These reactions are coupled to the creation of a proton gradient across the mitochondrial inner membrane . There are three proton pumps: I, III, and IV . The resulting transmembrane proton gradient is used to make ATP via ATP synthase . </P> <P> The reactions catalyzed by Complex I and Complex III work roughly at equilibrium . This means that these reactions are readily reversible, by increasing the concentration of the products relative to the concentration of the reactants (for example, by increasing the proton gradient). ATP synthase is also readily reversible . Thus ATP can be used to build a proton gradient, which in turn can be used to make NADH . This process of reverse electron transport is important in many prokaryotic electron transport chains . </P>

Where do the electrons in nadh come from