This paper presents a reformulation of the submarine alkaline hydrothermal theory for the emergence of life in response to recent experimental findings. The theory views life, like other self-organizing systems in the Universe, as an inevitable outcome of particular disequilibria. In this case, the disequilibria were two: (1) in redox potential, between hydrogen plus methane with the circuit-completing electron acceptors such as nitrite, nitrate, ferric iron, and carbon dioxide, and (2) in pH gradient between an acidulous external ocean and an alkaline hydrothermal fluid. Both CO2 and CH4 were equally the ultimate sources of organic carbon, and the metal sulfides and oxyhydroxides acted as protoenzymatic catalysts. The realization, now 50 years old, that membrane-spanning gradients, rather than organic intermediates, play a vital role in life's operations calls into question the idea of "prebiotic chemistry." It informs our own suggestion that experimentation should look to the kind of nanoengines that must have been the precursors to molecular motors-such as pyrophosphate synthetase and the like driven by these gradients-that make life work. It is these putative free energy or disequilibria converters, presumably constructed from minerals comprising the earliest inorganic membranes, that, as obstacles to vectorial ionic flows, present themselves as the candidates for future experiments. Key Words: Methanotrophy-Origin of life. Astrobiology 14, 308-343. The fixation of inorganic carbon into organic material (autotrophy) is a prerequisite for life and sets the starting point of biological evolution. (Fuchs, 2011 ) Further significant progress with the tightly membrane-bound H(+)-PPase family should lead to an increased insight into basic requirements for the biological transport of protons through membranes and its coupling to phosphorylation. (Baltscheffsky et al., 1999 ).

Iron-sulfur clusters are versatile electron transfer cofactors, ubiquitous in metalloenzymes such as hydrogenases. In the oxygentolerant Hydrogenase I from Aquifex aeolicus such electron "wires" form a relay to a diheme cytb, an integral part of a respiration pathway for the reduction of O 2 to water. Amino acid sequence comparison with oxygen-sensitive hydrogenases showed conserved binding motifs for three iron-sulfur clusters, the nature and properties of which were unknown so far. Electron paramagnetic resonance spectra exhibited complex signals that disclose interesting features and spin-coupling patterns; by redox titrations three iron-sulfur clusters were identified in their usual redox states, a ½3Fe4S and two ½4Fe4S , but also a unique high-potential (HP) state was found. On the basis of 57 Fe Mössbauer spectroscopy we attribute this HP form to a superoxidized state of the [4Fe4S] center proximal to the [NiFe] site. The unique environment of this cluster, characterized by a surplus cysteine coordination, is able to tune the redox potentials and make it compliant with the ½4Fe4S 3þ state. It is actually the first example of a biological [4Fe4S] center that physiologically switches between 3þ, 2þ, and 1þ oxidation states within a very small potential range. We suggest that the (1 þ ∕2þ) redox couple serves the classical electron transfer reaction, whereas the superoxidation step is associated with a redox switch against oxidative stress.electrochemistry | EPR | iron-sulfur centers | O2-sensitivity H ydrogenases are metalloproteins occurring in the metabolic pathway of a wide variety of microbial organisms and catalyze the reversible oxidation of dihydrogen: H 2 ⇌ 2H þ þ 2e − (1). The growing interest in alternative sources of energy has focused scientific research on understanding and engineering these enzymes for future applications (2). One of the major limitations of hydrogenases, however, is their sensitivity towards oxygen. Recently, the discovery of hydrogenases that retain catalytic activity in oxygenic environments has potentially opened new applications as "green" vanguard catalysts, in particular as electrocatalysts on electrodes for biofuel cells (3,4).Aquifex aeolicus is a hyperthermophilic Knallgas bacterium with optimum growth temperature of 85°C (5). This microorganism harbors three distinct [NiFe] hydrogenases, among which Hase I is located in the aerobic respiration pathway and attached to the membrane via a diheme cytb (6). Hase I consists of two subunits; the large subunit contains the hetero-bimetallic nickel-iron site and the small subunit the electron transfer cofactors, namely iron-sulfur clusters (6). Based on spectroelectrochemical studies, this enzyme exhibits enhanced thermostability and tolerance for inhibitors (e.g., O 2 and CO) (4, 7).Although the structures of O 2 -sensitive hydrogenases are well characterized (8, 9), such information is still lacking for O 2 -tolerant enzymes. The molecular mechanism and structural determinants for this increased oxygen tolerance remain to ...

The mechanism of the electron transfer from the soluble protein plastocyanin to the multiprotein complex of photosystem I from spinach has been studied in detail. The two kinetic components of P700+ reduction by plastocyanin after a laser flash, showing a constant half-life of 11 microseconds and a variable half-life of the second-order reaction, respectively, are used to monitor the electron transfer from bound and soluble plastocyanin. The effect of increasing concentration of reduced plastocyanin on both of these kinetic components and the competition by oxidized plastocyanin is used to estimate the individual dissociation constants of the complex between the proteins in each of its oxidized and reduced state. The dissociation constant of oxidized plastocyanin is about six times larger than that of 7 microM found for reduced plastocyanin and purified PSI. Consistent with this result the midpoint redox potential of plastocyanin bound to photosystem I either in equilibrium with soluble plastocyanin or after cross-linking to photosystem I is found to be 50-60 mV higher than that of soluble plastocyanin. It is concluded that the driving force of the intracomplex electron transfer is decreased in favor of an optimized turnover of photosystem I. Double-flash excitation shows that oxidized plastocyanin has to leave the complex after the electron transfer before a new reduced plastocyanin molecule can bind to photosystem I. This release of oxidized plastocyanin with a half-life of about 60 microseconds limits the turnover of photosystem I. All data are consistently described by a model including the formation of a complex at a single binding site of photosystem I. Differences in the rate and binding constants are discussed with respect to the structure and the electrostatic and hydrophobic interactions stabilizing the complex as well as their modification by the membrane environment in situ.

We created a Qo pocket mutant by site-directed mutagenesis of the chloroplast petD gene in Chlamydomonas reinhardtii. We mutated the conserved PEWY sequence in the EF loop of subunit IV into PWYE. The pwye mutant did not grow in phototrophic conditions although it assembled wild-type levels of cytochrome b 6 f complexes. We demonstrated a complete block in electron transfer through the cytochrome b 6 f complex and a loss of plastoquinol binding at Qo. The accumulation of cytochrome b 6 f complexes lacking affinity for plastoquinol enabled us to investigate the role of plastoquinol binding at Qo in the activation of the light-harvesting complex II (LHCII) kinase during state transitions. We detected no fluorescence quenching at room temperature in state II conditions relative to that in state I. The quantum yield spectrum of photosystem I charge separation in the two state conditions displayed a trough in the absorption region of the major chlorophyll a/b proteins, demonstrating that the cells remained locked in state I. 33 P i labeling of the phosphoproteins in vivo demonstrated that the antenna proteins remained poorly phosphorylated in both state conditions. Thus, the absence of state transitions in the pwye mutant demonstrates directly that plastoquinol binding in the Qo pocket is required for LHCII kinase activation.