Febs Letters Volume 107, Number 1-2./Volume 108, Number 1-2. [antikvár]

Volume 107, number 1 FEBS LETTERS November 1979 Hypothesis BIOLOGICAL ENERGY PRODUCTION IN THE APPARENT ABSENCE OF ELECTRON TRANSPORT AND SUBSTRATE LEVEL PHOSPHORYLATION Alexander J. B. ZEHNDER and Thomas D. BROCK Department of Bacteriology, University of Wisconsin, Madison, WI53706, USA Received 8 July 1979 1. Introduction and basic considerations The energy required for the synthesis of ATP is generally provided by exergonic redox processes. Energy is generated either by donating electrons (formally, hydrogen formation) or by accepting electrons (formally, hydrogen consumption). Two general mechanisms have been recognized for conservation of chemical energy in the high-energy phosphate bonds of ATP: substrate-level phosphorylation and electron-transport phosphorylation. In the former process, part of the energy released in a redox reaction is conserved in an 'energy-rich' bond of one of the products, and a transfer to ATP occurs by a kinase reaction. In electron-transport phosphorylation, energy is used to drive protons through a membrane, thus establishing a pH gradient and a membrane potential. According to the chemiosmotic theory [1] the resulting protonmotive force can be used to synthesize ATP. A few substrate-level phosphorylation reactions are known which do not involve redox reactions: several anaerobes have been shown to metabolize substrates by lysis rather than by redox [2]. The methanogenic bacteria are a unique group of organisms which have been the subject of considerable evolutionary interest [3]. Most of the methanogenic bacteria obtain energy from redox reactions involving the oxidation of hydrogen gas or formate, with carbon dioxide serving as the termmal election Abbreviations: Ap, protonmotive force, it is the amount of work done by a single proton going once around the circuit; AiJ, membrane potential; ApH, pH gradient. These are related by: Ap = Al// - (2.3 RT ¦ p-') ApH. Units: millivohs, mV. have their usual meaning acceptor. ATP is apparently synthesized by conventional electron-transport phosphorylation [2]. A redox carrier has been identified, factor 420, and a specific coenzyme has been shown to be the methyl carrier in the terminal reaction of methane formation, coenzyme M [4]. Another process of methanogenesis that has been recognized for many years but only recently demonstrated in pure culture is the anaerobic conversion of acetate to equal amounts of methane and carbon dioxide [5]. Chemically, acetate cleavage is a decarboxylation reaction. Studies with deuterium-labeled compounds have shown that the methyl group of acetate goes intact into methane, and the fourth proton is supplied by water [6]. CH3COO- + H2O = CH4 + HCO3" AG°' = -6.7 kcal .mor'(-28 kJ .mol"') (1) Such a decarboxylation, however, is not a process by which electrons are transferred from one molecule to another, and therefore an electron transport mechanism for energy conservation is not obvious. ATP formation by substrate-level phosphorylation is also unlikely to occur, since an enzymic mechanism for substrate-level phosphorylation is not obvious and the energy released from acetate cleavage is too low to form an 'energy-rich' compound. Nevertheless, two methane hdiCtma., Methanosarcinabarkeri [7] and the'acetate organism' [8] grow with acetate as exclusive energy source. The overall stoichiometry for growth and methane formation for the 'acetate organism' is as foUows: Elsevier !North-Holland Biomedical Press 1