Mitochondrial coupling factor

Baird, B.A., and Hammes, G.G. (1977) Chemical cross-linking studies of beef heart mitochondrial coupling factor 1./. Biol. Chem. 252, 4743-4748. 

Fig. 11. A pictorial representation of the mitochondrial ATP synthesizing coupling factor interacting with the mitochondrial membrane. F, contains five polypeptide chains a, (3, 7, 8, and e and is readily solubilized. The stalk is probably made up of three polypeptide chains, 8, OSCP, and Ffi, which interact with a small group of hydrophobic polypeptides, CF0, embedded in the membrane.

A number of cases are known in which the properties of an enzyme are markedly altered by interaction with a membrane. Of course, in some cases the normal function of an enzyme is destroyed when it is removed from the membrane. For example, the mitochondrial coupling factor cannot synthesize ATP when removed from the membrane, since coupling to a proton gradient is required. The portion of the coupling factor that is easily solubilized (F,) is an ATPase. The steady-state kinetic properties of this solubilized ATPase are appreciably changed when it is reconstituted with mitochondrial membranes The turnover numbers and pH dependencies are different the solubilized enzyme is strongly inhibited by ADP, whereas the reconstituted enzyme is not and the reconstituted enzyme is inhibited by oligomycin, whereas the solubilized enzyme is not. 

The question of whether an enzyme is membrane bound or membrane associated is to some extent a matter of semantics. However, it is certainly true that some proteins are readily dissociated from membranes whereas others require quite drastic conditions before they can be dissociated from the membrane. As limiting cases, the former can be designated as membrane associated and the latter as membrane bound. Enzymes that are generally considered membrane bound are firmly embedded in the membrane structure. For example, the mitochondrial coupling factor is strongly coupled to the bilayer structure by hydrophobic polypeptides. The Na+-K+ ATPases that have been purified have a small patch of associated phospholipids when the enzyme is delipidated, enzymatic activity is lost. In fact, membrane-bound enzymes appear to be... 

The conformation of membrane-bound enzymes is undoubtedly restricted by the membrane. However, the mechanism of action of these enzymes appears to be similar to that of soluble enzymes, so that the presence of clefts and conformational flexibility is to be expected. The mitochondrial coupling factor apparently contains both the ATP synthesizing enzyme and a proton channel conformational changes undoubtedly play a role in the function of this system. A large movement of polypeptide chains has been proposed in the functioning of this system (and for other membrane-bound enzymes), but no convincing experimental evidence is available to support such a hypothesis. 

Anotiier characteristic of die inner mitochondrial membrane is the presence of projections on the inside surface, which faces the mitochondrial matrix. See Fig. 18-14. These spherical 85-kDa particles, discovered by Fernandez Moran in 1962 and attached to die membrane tiirough a "stalk", display ATP-hydrolyzing (ATPase) activity. The latter was a clue that the knobs might participate in the synthesis of ATP during oxidative phosphorylation. In fact, tiiey are now recognized as a complex of proteins called coupling factor 1 (F ) or ATP synthase. 

Like mitochondria, chloroplasts (when illuminated) pump protons across their membranes (Fig. 23-18). However, while mitochondria pump protons to the outside, the protons accumulate on the inside of the thylakoids. The ATP synthase heads of coupling factor CEj are found on the outside of the thylakoids, facing the stromal matrix, while those of F, lie on the insides of mitochondrial membranes. However, the same mechanism of ATP formation is used in both chloroplasts and mitochondria (Chapter 18). 

ATP synthase is located in the inner mitochondrial membrane. It consists of two major components, F, ATPase [seen as spheres under the electron microscope and with a subunit structure of (aP ySe] attached to component F0 (coupling factor 0) which is a proton channel spanning this membrane. Hence, ATP synthase is also known as F0F, ATPase. In mitochondria, this complete complex uses the energy released by electron transport to drive ATP synthesis but, in isolation, F ATPase hydrolyzes ATP. During ATP hydrolysis, and presumably also during ATP synthesis, subunit y of F, ATPase rotates relative to (aP)3 and is the smallest rotatory engine known in nature. 

This oxidation transfers four electrons to the Manganese Center, a complex metalloprotein, which then donates the electrons through an intermediate to oxidized P680. The protons derived from water are transported into the thylakoid lumen. The protons pumped into the thylakoid lumen by PSII are used to make ATP through the action of coupling factor, in a mechanism similar to that of mitochondrial ATP synthesis. 

OH, NH2, SH groups glutamine synthetase plasmalemma ATPase coupling factor 1 mitochondrial membrane aspartate carbamoyl-transferase... 

It is interesting to note that Volk et al. are able to differentiate between the two enzyme activities by the lack of fluoride inhibition of PPase II, which is in contradiction with the results in [20]. Volk et al. also obtained, though careful differentiation of mitochondrial membranes, a localization of the PPases. They came to the conclusion that PPase I is localized in the mitochondrial matrix. PPase II is situated in the inner mitochondrial membrane, a localization in line with its proposed role as a coupling factor [71]. 

M-snbnnit - medinm (molecular-weight) subunit MBS - 2-(N-morpholino)-ethanesulfonic acid MF / MFjj - mitochondrial coupling factor l/o MGDG - monogalactosyldiacylglycerol... 

What is the coupiing factor in oxidative phosphor-yiation A complex protein oligomer is the coupling factor that links oxidation and phosphorylarion. The complete protein spans the inner mitochondrial membrane and projects into the matrix as well. The portion of the protein that spans the membrane is called Fq it consists of three different kinds of polypeptide chains (a, b, and c). 

It was at this stage that I once again made a decision which changed the course of our work. The complexity of the mitochondrial proton pump with its multiple coupling factors seemed a formidable barrier to an understanding of its mechanism. Two other pumps appeared to be simpler and more approachable. The Ca++ pump of sarcoplasmic reticulum and the Na+K+ pump of the plasma membrane. About 4 years ago we started working on these systems and I have recently formulated a molecular mechanism of the operation of the Ca++ pump. ... 

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