Oxidative phosphorylation high energy intermediates

By the mid-1950s, therefore, it had become clear that oxidation in the tricarboxylic acid cycle yielded ATP. The steps had also been identified in the electron transport chain where this apparently took place. Most biochemists expected oxidative phosphorylation would occur analogously to substrate level phosphorylation, a view that was tenaciously and acrimoniously defended. Most hypotheses entailed the formation of some high-energy intermediate X Y which, in the presence of ADP and P( would release X and Y and yield ATP. A formulation of the chemical coupling hypothesis was introduced by Slater in 1953,... 

How is the oxidation of NADH coupled to the phosphorylation of ADP It was first suggested that electron transfer leads to the formation of a covalent high-energy intermediate that serves as a high phosphoryl transfer potential compound or to the formation of an activated protein conformation, which then drives ATP synthesis. The search for such intermediates for several decades proved fruitless. 

ATPase activity and exchange reactions, such as P,-ATP exchange, are partial reactions of oxidative and photosynthetic phosphorylation. These reactions have been described in detail and have been considered to consist of a series of reversible chemical reactions forming high energy intermediates (X-Y and X-P) [104-106]. In... 

Early proposals concerning the mechanism of the energy-linked transhydrogenase reaction were based on the chemical hypothesis of oxidative phosphorylation [82] and visualized the involvement of high-energy intermediates of the type 1 X, NADH I, NADP I, etc. [29,46]. These proposals, however, just as the chemical hypothesis as a whole, had to be abandoned because of lack of experimental evidence. 

These conjectures were soon supported by experimental evidence obtained with mitochondrial preparations, first by Chance and Hollunger and later by many others, " who demonstrated that reversed electron transfer could involve not only flavin-coupled systems but also NAD-, NADP, and cytochrome-linked reactions. It was subsequently realized that ATP itself might not be directly involved in the energy transfer, and the notion of a high-energy intermediate of oxidative phosphorylation was invoked... 

The search for the second messenger for mediation of insulin action has up to now been as unsuccessful as the quest for high-energy intermediates of oxidative phosphorylation. A not unreasonable possibility, therefore, is that the second messenger does not exist as a separate chemical entity and that insulin action is mediated by electronic processes involving macromolecular semiconduction. Support for this view comes from the finding that the most rapid observed action of insulin is a hyperpolarization of the muscle cell plasma... 

Throughout this discussion of oxidative phosphorylation, we have assumed that the coupling mechanisms involve the formation of high-energy intermediates. This chemical hypothesis is not accepted by all the chemiosmotic hypothesis of oxidative phosphorylation was proposed by Mitchell in 1961, and in 1966 Boyer [148] proposed a new hypothesis involving conformation coupling. 

If valid, the chemiosmotic theory indicates that the ultimate step of one of the most essential reactions of live coupling of oxidation and phosphorylation is not simply molecular, but is also vectorial because of the special orientation of the molecules. The separate distribution of the products generates a physical force capable of driving the phosphorylation of ATP, and there is no need to postulate the existence of high-energy intermediates. In conclusion, it seems likely that an adequate and comprehensive interpretation of the mechanism of coupling may well require cooperation of biochemists and physicists. 

A pathway for fatty acid activation, involving a reaction with nonphosphorylated high-energy intermediates rather than the formation of acetyl-CoA derivatives has also been postulated. The supporting evidence includes the observations that (1) blocking the electron transport chain with cyanide or uncoupling oxidative phosphorylation with dinitrophenol interferes with the fatty acid oxidation (2) oligomycin, which blocks the biosynthesis of ATP but does not affect the formation... 

The confirmation of one of the predictions of the so-called chemical hypothesis of oxidative phosphorylation inspired the active search for the postulated high-energy ( ) intermediates. Indeed, it was surprising how easy it appeared to be to prove their existence, but they seemed to run out through your fingers when you tried to grab them and isolate them. One by one claims were made, but were either withdrawn or shown by others to rest on artifacts. Not without justification, new claims were greeted with suspicion and you would now probably have to submit crystals of such an intermediate with your paper if you hoped to have it accepted for publication. 

The energetics of the translocation process and the role of GTP therein have fascinated investigators for many years. Brot (1977) and Spirin (1978) have reviewed this process and the proposed means by which translocation is carried out. Spirin has suggested that, while the ribosome binds peptidyl-tRNA on the acceptor site (i.e., prior to translocation), the whole system is akin to a high-energy intermediate in which eventual translocation is thermodynamically ensured. This provocative hypothesis merits careful reading in view of its similarity to the role of the membrane in the chemiosmotic hypothesis for oxidative phosphorylation (Chapter 9). 

A High-Energy Chemical Intermediate Coupling Oxidation and Phosphorylation Proved Elusive... 

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