ACE Biochemistry

Overview of the Electron Transport Chain

The NADH and FADH2 formed in glycolysis, fatty acid oxidation, and the TCA cycle can be used to reduce molecular oxygen to water in oxidative phosphorylation. The flow of electrons from NADH and FADH2 to oxygen through protein complexes leads to the pumping of protons out of the mitochondrial matrix, creating a pH gradient resulting in a proton-motive force. ATP is synthesized from ADP and Pi when protons flow back into the matrix through ATP synthase.

Three electron driven proton pumps form the pH gradient.
          (1) NADH-Q oxidoreductase
          (2) Q-cytochrome c oxidoreductase
          (3) cytochrome c oxidase

Mitochondria

          Mitochondria contain the respiratory assembly, the enzymes of the TCA cycle, and the enzymes of fatty acid oxidation.
          There are two compartments in the mitochrondria, due to its double membrane structure: (1) the intermembrane space between the outer and inner membranes and (2) the matrix which is surrounded by the inner membrane.
          The TCA cycle and Fatty Acid oxidation take place in the matrix, where as, oxidative phosphorylation takes place in the inner membrane
          The outer membrane of mitochodria is permeable to most small molecules, as it contains mitochondrial porin. However, the inner membrane is nearly impermeable to almost all ions and polar molecules. Metabolites, such as ATP, pyruvate, and citrate, are shuttled into the matrix via transporters.

NADH

NADH, being a strong reducing agent, is poised to donate electrons and has a negative reduction potential, whereas oxygen, a strong oxidizing agent is ready to accept electrons an has a positive reduction potential. The energy released from reduction by NADH is initially used to generate a proton gradient that is then used for the synthesis of ATP and the transport of metabolites across the mitochondrial membrane.

Complexes of the Respiratory Chain

Electrons are transferred from NADH to oxygen through three protein complexes, and electron flow through these complexes is what leads to the transport of protons across the inner mitrochondrial membrane. A fourth protein complex (succinate Q-reductase), contains the enzyme succinate dehydrogenase (recall this enzyme produces FADH2 in the TCA cycle when succinate is converted to fumarate). Note: Succinate Q-reductase does not move protons across the inner membrane.

Each of these complexes is referred to as:
Complex I: NADH-Q oxidoreductase
Complex II: Succinate-Q reductase
Complex III: Q-cytochrome c oxidoreductase
Complex IV: cytochrome c oxidase

Special electron carriers transport the electrons from one complex to the next. Coenzyme Q (ubiquinone) is reduced to transport electrons from complex I to complex III. Cytochrome c, a small soluble protein, transports electrons from complex III to complex IV. Complex IV (cytochrome c oxidase) is the final component of the chain, and catalyzes the reduction of O2. Electrons from FADH2 are transferred to coenzyme Q and then to complex III.

NADH-Q Oxidoreductase (Complex I)

The electrons of NADH enter the chain at complex I. Complex I has two prosthetic groups: FMN and Iron-Sulfur clusters. NADH binds to complex I, transfers its electrons to FMN. These electrons flow through some Fe-S clusters, and then to coenzyme Q. The flow of these electrons pumps 4 protons into the intermembrane space, and in accepting 2 electrons, coenzyme Q takes up 2 protons (QH2), which heads to the interior of the membrane.

Coenzyme Q

FADH2 does not leave the succinate dehydrogenase complex, instead, its electrons are transferred to Fe-S centers and then to coenzyme Q for entry into the ETC. Succinate-Q reductase (complex II), does not transport protons, therefore, less ATP is formed from the oxidation of FADH2 than from NADH. Glycerol phosphate dehydrogenase and fatty acyl CoA dehyrdrogenase, also transfer their electrons from FADH2 to coenzyme Q. These enzymes are responsible for the oxidation of glycerol and fats.

Q-Cytochrome c Oxidoreductase (Complex III)

The second proton pump, Q-cytochrome c oxidoreductase (complex III), functions to transfer electrons from coenzyme Q to cytochrome c. Cytochrome c is a water soluble protein. The flow of a pair of electrons through this pump leads to the transport of 2 protons across the inner membrane. In addition to the cytochromes, complex III also has an Fe-S center, known as the Rieske center.

A cytochrome is an electron-transferring protein that contains a heme prosthetic group, with the iron alternating between ferrous (+2) and ferric (+3).

Cytochrome c Oxidase (Complex IV)

Complex IV is the last of the proton pumping enzymes, and catalyzes the transfer of electrons from the reduced cytochrome c to oxygen, the final electron acceptor. Complex IV has 2 heme A groups and 3 copper ions. Complex IV uses 4 protons and transports and additional 4 protons across the inner membrane for a total of 8 protons removed from the matrix.

Proton Gradient and ATP Synthase