Mitochondria: the cell's powerhouse

Mitochondria are organelles found in eukaryotic cells that play an important role in the production of ATP, the universal energy currency used in all cells. Most mitochondrial proteins are transported from the cytosol into mitochondria through specialized protein translocator complexes. Interactions between these complexes bring the outer and inner mitochondrial membranes into close proximity. The successful targeting of many proteins that reside in the intermembrane space requires translocator complexes that cross both the outer and inner membranes. After entering the inner membrane translocator, many of these proteins do not completely cross the membrane, but instead are released into the membrane and diffuse laterally. They then remain embedded in the inner membrane or are cleaved, releasing a portion into the intermembrane space. Very few resident proteins are simply transported across the outer membrane translocator complex. Matrix proteins do not transit through the intermembrane space, but are directly transported from the cytoplasm across the interacting outer and inner membrane translocators. The protein-rich matrix contains enzymes necessary for cellular respiration, a process during which carbon fuel molecules are oxidized and reduced electron carriers are produced. Invaginations of the inner membrane, called cristae, contain four large protein complexes that harvest electrons from these carriers. Complex II accepts pairs of electrons from succinate and transfers them through a series of redox centers to coenzyme Q. The lipid-soluble coenzyme Q is reduced by electrons harvested from complexes I and II, and then diffuses through the inner membrane, transferring its electrons to complex III. In complex III, electrons are transferred through two distinct series of redox centers, which allow them to cross the inner membrane one at a time. Finally, electrons are accepted by cytochrome c, which carries them to complex IV, where they are transferred through another series of redox centers to their final acceptor, a molecule of oxygen, that combines with hydrogen ions to form water. Electron transport in complexes II, III, and IV is coupled with the pumping of protons from the matrix to the intermembrane space. The resulting electrochemical gradient across the inner membrane is called the proton motive force. Protons flow back into the matrix through a component of the ATP synthase called F0. The membrane-embedded ring structure of F0 binds protons in the intermembrane space and releases them on the other side of the inner membrane into the matrix. Proton flow drives the rotation of the ring structure, which in turn leads to the rotation of the F0 central shaft. The rotating shaft sequentially contacts the three catalytic subunits of the ATP synthase F1 complex, altering the subunit's affinity for ATP and ADP, and catalyzing the synthesis and subsequent release of ATP. Most of the ATP synthesized in the mitochondrial matrix is consumed outside of the mitochondria, but the inner membrane is impermeable to ATP and ADP. Therefore, an ATP-ADP carrier is responsible for the coordinated import of ADP and the export of ATP across the inner membrane. In cells, mitochondria are distributed near sites where ATP and other mitochondrial metabolites are in high demand. Mitochondria move alongside skeletal structures such as microtubules and can undergo dynamic changes in shape, including both fusion with other mitochondria and division. Perturbations in the dynamic behavior of mitochondria and alterations in mitochondrial membrane permeability are associated with the early stages of programmed cell death.