Oxidative phosphorylation coupling factor

Adolfson, R., and Moudrianokis, E.N. (1976) Molecular polymorphism and mechanisms of activation and deactivation of the hydrolytic function of the coupling factor of oxidative phosphorylation. Biochemistry 15, 4164—4170. 

The simple regulatory mechanism which ensures that ATP synthesis is automatically coordinated with ATP consumption is known as respiratory control. It is based on the fact that the different parts of the oxidative phosphorylation process are coupled via shared coenzymes and other factors (left). 

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. 

ADP phosphorylation is tightly coupled to electron transport. Shutting down one shuts down the other. It is well known that if ADP phosphorylation is inhibited by such compounds as oligomycin, electron transport also ceases. If the proton gradient is broken by a proton ionophore, however, such as 2,4-dinitrophenol, electron transport resumes at a rapid pace and no phosphorylation takes place. Such proton ionophores are also termed "uncouplers" of electron transport and ADP phosphorylation. Under normal conditions, the factors limiting ATP production are the pH gradient across the inner mitochondrial membrane and the cellular ADP/ATP ratio. An increase in the proton gradient shuts down phosphorylation and electron transport, whereas an increase in the ADP/ATP ratio stimulates both. Stimulation of oxidative phosphorylation by increases in cellular ADP concentration is termed respiratory control. 

Oxidative phosphorylation occurs in the mitochondria of all animal and plant tissues, and is a coupled process between the oxidation of substrates and production of ATP. As the TCA cycle runs, hydrogen ions (or electrons) are carried by the two carrier molecules NAD or FAD to the electron transport pumps. Energy released by the electron transfer processes pumps the protons to the intermembrane region, where they accumulate in a high enough concentration to phosphorylate the ADP to ATP. The overall process is called oxidative phosphorylation. The cristae have the major coupling factors F, (a hydrophilic protein) and F0 (a hydrophobic lipoprotein complex). F, and F0 together comprise the ATPase (also called ATP synthase) complex activated by Mg2+. F0 forms a proton translocation pathway and Fj... 

Two major ATP synthesizing reactions in living organisms are oxidative phosphorylation and photophosphorylation. Both reactions take place in H -ATPase (FqF,), which is driven by an electrochemical potential difference of protons across the biomembrane, as predicted by Mitchell [1]. In Racket s laboratory, ATPases related to oxidative phosphorylation were prepared, but their relationship to Mitchell s chemiosmotic hypothesis [1] was not described [2], Later, an insoluble ATPase (H -ATPase) was shown to translocate protons across the membrane when it was reconstituted into liposomes [3], H -ATPase was shown to be composed of a catalytic moiety called F, (coupling factor 1) [4], and a membrane moiety called Fq [5], which confers inhibitor sensitivity to F,. F was shown to be a proton channel, which translocates down an electrochemical potential gradient across the membrane when Fg is reconstituted into liposomes (Fig. 5.1) [6]. Thus, -ATPase was called FqFj or ATP synthetase. 

Studies with beef-heart submitochondrial particles initiated in Green s laboratory in the mid-1950s resulted in the demonstration of ubiquinone and of non-heme iron proteins as components of the electron-transport system, and the separation, characterisation and reconstitution of the four oxidoreductase complexes of the respiratory chain. In 1960 Racker and his associates succeeded in isolating an ATPase from submitochondrial particles and demonstrated that this ATPase, called F, could serve as a coupling factor capable of restoring oxidative phosphorylation to F,-depleted particles. These preparations subsequently played an important role in elucidating the role of the membrane in energy transduction between electron transport and ATP synthesis. 

Like most bacteria, the Microtox strain has many metabolic pathways which function in respiration, oxidative phosphorylation, osmotic stabilization, and transport of chemicals and nutrients into and out of the cell, and which are located within or near the cytoplasmic membrane. The luciferase pathway [9], which functions as a shunt for electrons directly to oxygen at the level of reduced flavin mono-nucleotide, is also located within the cell membrane complex. This, coupled with lack of membrane-aided compartmentalization of internal functions, gives many target sites at or near the cytoplasmic membrane. These factors all contribute to a rapid response of the organisms to a broad spectrum of toxic substances. 

Electron Transport Chain Oxidative Phosphorylation 46 Sites of Phosphorylation Intermediates and Coupling Mechanisms Coupling Factors Other Reactions... 

Among the many systems Racker used, one is a particulate system prepared by exposing beef mitochondria to sonic oscillation. The system can oxidize succinate and NADH but cannot couple the oxidation of the substrate to the phosphorylation of ADP. Coupling can be achieved by adding to the system two soluble proteins referred to by Racker as factor 1 (FJ and factor 4 (F4). F is a protein with ATPase activity obtained by sonic disintegration of mitochondria. F4 is prepared by alkaline extraction of the mitochondria. Fi, which has been extensively purified, is a decamer with a molecular weight of 284,000, each monomer weighing 26,000. 

Other coupling factors have been described and sometimes partially purified [145, 146]. The exact intracellular role of these coupling factors is not known. Although it is clear that they restore oxidative phosphorylation in submitochondrial particles, it is not certain whether this effect is specific or direct. It is indeed distressing that albumin stimulates oxidative phosphorylation, probably by binding natural uncouplers and Co A restores oxidative phosphorylation in uncoupled submitochondrial fractions by facilitating the removal of free fatty acids, which act as uncouplers. 

Therefore, much more work is needed before a clear understanding of the role of coupling factors will be available. Yet, one thing is certain, such investigations have permitted the systematic reconstruction of oxidative phosphorylating sites, a fact undreamed of a decade ago. 

These calculations must be accepted with caution. In fact they imply values which may in some cases only be approached rather than attained. For example the P/O ratio in the course of oxidative phosphorylations. Moreover they assume a perfect coupling between phosphorylations and oxido-reductions. In practice numerous factors bring about the uncoupling of these two processes, examples are dinitrophenol and thyroxine. There are good reasons to believe that even in a given species the eiEciency of coupling varies from one individual to another. 

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