In contrast to many unicellular organisms including yeast, which displayed evolved mechanisms allowing them to grow and survive in the absence of oxygen, animals fully rely on cell respiration that takes place in the mitochondria. The oxidative phosphorylation (OxPhos) machinery is a key functional unit, located on the inner mitochondrial membrane that combines electron transport with cell respiration and ATP synthesis.
The energy given to the electrons of the reduced coenzyme NADH and to succinate by the TCA cycle is transferred in small steps in the inner membrane of the mitochondrion through a chain of five protein complexes.
This series of coupled reactions is often referred to as oxidative phosphorylation. OXPHOS consists of the electron transport chain (ETC), which comprises NADH-dehydrogenase (complex I), succinate dehydrogenase (complex II), ubiquinone (Coenzyme Q10 (CoQ10)), bc1 complex (complex III), cytochrome c (Cyt c), and cytochrome c oxidase (COX; complex IV).
– Complex I (NADH-coenzyme Q oxidoreductase, NADH dehydrogenase). The reduced coenzyme NADH binds to Complex I and accomplishes the reduction of Coenzyme Q10 (CoQ10). Electrons are transferred through Complex I using FMN (flavin mononucleotide) and a series of Fe-S clusters. The process accomplishes the pumping of four protons across the inner mitochondrial membrane to the intermembrane space.
– Complex II (Succinate-Q oxidoreductase). This complex forms a second entry point into the electron transport chain using the succinate produced in the TCA cycle.
– Complex III (Q-cytochrome c oxidoreductase). This complex oxidizes ubiquinol and reduces two molecules of cytochrome-c. Four hydrogens are pumped across the membrane to the intermembrane space.
– Complex IV (Cytochrome c oxidase). This final ETC complex finally transfer electrons to oxygen and pumps two protons across the membrane.
Thus, a total of 10 protons are released in the intermembrane space for one NADH that has entered into the electron transfer chain.
The movement of electrons through complexes I-IV causes protons (hydrogen atoms) to be pumped out of the intramitochondrial matrix into intermembrane space. As a result, a net negative charge (from the electrons) builds up in the matrix space while a net positive charge (from the proton pumping) builds up in the intermembrane space. This differential electrical charge establishes an electrochemical gradient.
ATP synthase (complex V) functions like a molecular nanomotor. It is composed of two rotary motors, each powered by a different fuel. The first one, termed F0, is likely to be an electric motor embedded in the membrane between mitochondrial matrix and intermembrane space. It is powered by the flow of hydrogen ions across the membrane down the gradient. As the protons flow through the motor, they turn a circular rotor. This rotor is connected to the second motor, termed F1. The F1 motor behaves as a chemical motor, powered by ATP. The two motors are connected together by a stator, shown on the right, so that when F0 turns, F1 turns too.
The synthesis of ATP requires several steps, including the binding of ADP and phosphate, the formation of the new phosphate-phosphate bond, and release of ATP. As the axle turns, it forces the motor into three different conformations that assist these difficult steps.