The molecular mechanism of oxidative phosphorylation.

Current scientific interest in the molecular mechanism of respiratory chain oxidative phosphorylation is witnessed by the multitude of attractive working hypotheses which have appeared in the literature during the last eight years. But in addition to discussions of electron transfer" 2 and possible "energy-rich" intermediates,3-" a satisfactory mechanism must also account for the following wellestablished facts: (1) the tight coupling between electron transport and phosphorylation; (2) the observed P:0 ratio; (3) the crossover points of Chance and Williams; (4) the coupling factors; (5) the action of uncouplers; (6) the inhibitors of electron transport and phosphorylating oxidation; (7) the data on phosphohistidine; (8) the reversal of electron transport by adenosine 5'-triphosphate (ATP). Unfortunately, none of the molecular mechanisms proposed to date seems to account for all these facts. The only exception appears to be the chemi-osmotic hypothesis of Mitchell'2 which, because of its inherent complexity, is rather difficult to prove or disprove. While the observed phosphorylation of adenosine 5'diphosphate (ADP) in pH-jump experiments is consistent with Mitchell's theory, it is also understandable in terms of the conventional theories involving "energyrich" intermediates. For example, if electron transport in mitochondria produces local pH-changes, then a sudden pH-change could also induce electron transport along the respiratory chain with concomitant phosphorylation through the conventional "energy-rich" intermediates. Chance and Mela found that the initial rate of hydrogen ion formation is much slower than the rate of electron transport in submitochondrial particles.'3 In this paper, a new attempt is made to formulate a molecular mechanism of oxidative phosphorylation consistent with most of the known facts of the mitochondrial system. The mechanism is suggested by experimental results on model oxidative phosphorylation systems obtained in our laboratory during the last five years, encouraged by the work of Boyer's group on phosphohistidine," and prompted by the possibly naive hope that all biological phenomena are understandable in simple chemical terms. It has previously been shown that aerobic oxidation of ferroheme in solutions of N,N-dimethylacetamide (DMAC) containing imidazole, the imidazolium salts of inorganic orthophosphate (Pi), and adenosine 5'-phosphate (AMP) or ADP produces ATP.14 If the nucleotides are omitted and the Pi is labeled with p32, a radioactive "energy-rich" intermediate product can be detected which has hydrolytic, paper-chromatographic, and electrophoretic behavior indistinguishable from that of synthetic 1-phosphoimidazole.'6 The principal mechanism of this model reaction may be summarized as follows. When the dimethyl ester of ferroheme is oxidized by molecular oxygen in pure, anhydrous DMAC solution, the radical ion 2is produced,