Intermembrane space
The intermembrane space is the space occurring between or involving two or more membranes. In cell biology, it is most commonly described as the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. It also refers to the space between the inner and outer nuclear membranes of the nuclear envelope, but is often called the perinuclear space. The IMS of mitochondria plays a crucial role in coordinating a variety of cellular activities, such as regulation of respiration and metabolic functions. Unlike the IMS of the mitochondria, the IMS of the chloroplast does not seem to have any obvious function.
Intermembrane space of mitochondria
Mitochondria are surrounded by two membranes; the inner and outer mitochondrial membranes. These two membranes allow the formation of two aqueous compartments, which are the intermembrane space and the matrix. Channel proteins called porins in the outer membrane allow free diffusion of ions and small proteins about 5000 daltons or less into the IMS. This makes the IMS chemically equivalent to the cytosol regarding the small molecules it contains. By contrast, specific transport proteins are required to transport ions and other small molecules across the inner mitochondrial membrane into the matrix due to its impermeability. The IMS also contains many enzymes that use the ATP moving out of the matrix to phosphorylate other nucleotides and proteins that initiate apoptosis.Translocation
Most of proteins destined for the mitochondrial matrix are synthesized as precursors in the cytosol and are imported into the mitochondria by the translocase of the outer membrane and the translocase of the inner membrane. The IMS is involved in the mitochondrial protein translocation. The precursor proteins called small TIM chaperones which are hexameric complexes are located in the IMS and they bind hydrophobic precursor proteins and delivery the precursors to the TIM.Oxidative phosphorylation
The pyruvate generated by glycolysis and the fatty acids produced by breakdown of fats enter the mitochondrial IMS through the porins in the outer mitochondrial membrane. Then they are transported across the inner mitochondrial membrane into the matrix and converted into the acetyl CoA to enter the citric acid cycle.The respiratory chain in the inner mitochondrial membrane carries out oxidative phosphorylation. Three enzyme complexes are responsible for the electron transport: NADH-ubiquinone oxidoreductase complex, ubiquinone-cytochrome c oxidoreductase complex, and cytochrome c oxidase. The protons are pumped from the mitochondrial matrix to the IMS by these respiratory complexes. As a result, an electrochemical gradient is generated, which is combined by forces due to a H+ gradient and a voltage gradient. The pH in the IMS is about 0.7 unit lower than the one in the matrix and the membrane potential of the IMS side becomes more positively charged than the matrix side. This electrochemical gradient from the IMS to the matrix is used to drive the synthesis of ATP in the mitochondria.