Energy coupling: an introduction

Structural basis for the network of functional cooperativities in cytochrome c(3) from Desulfovibrio gigas: solution structures of the oxidised and reduced states

Cytochrome c(3) is a 14 kDa tetrahaem protein that plays a central role in the bioenergetic metabolism of Desulfovibrio spp. This involves an energy transduction mechanism made possible by a complex network of functional cooperativities between redox and redox/protolytic centres (the redox-Bohr effect), which enables cytochrome c(3) to work as a proton activator. The three-dimensional structures of the oxidised and reduced Desulfovibrio gigas cytochrome c(3) in solution were solved using 2D (1)H-NMR data. The reduced protein structures were calculated using INDYANA, an extended version of DYANA that allows automatic calibration of NOE data. The oxidised protein structure, which includes four paramagnetic centres, was solved using the program PARADYANA, which also includes the structural paramagnetic parameters. In this case, initial structures were used to correct the upper and lower volume restraints for paramagnetic leakage, and angle restraints derived from (13)C Fermi contact shifts of haem moiety substituents were used for the axial histidine ligands. Despite the reduction of the NOE intensities by paramagnetic relaxation, the final family of structures is of similar precision and accuracy to that obtained for the reduced form. Comparison of the two structures shows that, although the global folds of the two families of structures are similar, significant localised differences occur upon change of redox state, some of which could not be detected by comparison with the X-ray structure of the oxidised state: (1) there is a redox-linked concerted rearrangement of Lys80 and Lys90 that results in the stabilisation of haem moieties II and III when both molecules are oxidised or both are reduced, in agreement with the previously measured positive redox cooperativity between these two haem moieties. This cooperativity regulates electron transfer, enabling a two-electron step adapted to the function of cytochromes c(3) as the coupling partner of hydrogenase; and (2) the movement of haem I propionate 13 towards the interior of the protein upon reduction explains the positive redox-Bohr effect, establishing the structural basis for the redox-linked proton activation mechanism necessary for energy conservation, driving ATP synthesis.

NMR studies of cooperativity in the tetrahaem cytochrome c3 from Desulfovibrio vulgaris

The thermodynamic properties of the Desulfovibrio vulgaris (Hildenborough) tetrahaem cytochrome c3 (Dvc3) are rationalised by a model which involves both homotropic (e-/e-) and heterotropic (e-/H+) cooperativity. The paramagnetic shifts of a methyl group from each haem of the Dvc3 have been determined in each stage of oxidation at several pH values by means of two-dimensional exchange NMR. The thermodynamic parameters are obtained by fitting the model to the NMR data and to redox titrations followed by visible spectroscopy. They show significant positive cooperativity between two of the haems whereas the remaining interactions appear to be largely electrostatic in origin. These parameters imply that the protein undergoes a proton-assisted two-electron transfer which can be used for energy transduction. Comparison with the crystal structure together with measurement of the kinetics of proton exchange suggest that the pH dependence is mediated by a charged residue(s) readily acessible to the solvent and close to haem I.

Photosynthetic phosphorylation

A brief history of the discovery of photosynthetic phosphorylation by chloroplasts and bacterial chromatophores is presented. Arnon early introduced the terminology of 'Cyclic' and 'Non-cyclic photophosphorylation' and 'Cyclic' and 'Non-Cyclic electron transport' to the processes observed in illuminated chloroplasts. He made major contributions to the elucidation of these processes and stressed their great biological significance. Investigations of the electron transport components of chromatophores have led to the isolation, purification and crystallization of bacterial reaction centers. The development of three-dimensional molecular structures, and the characterization of their electron transfer components have provided a great deal of information about the early reactions of bacterial photosynthesis. The electron transfer schemes presented clearly support the 'cyclic' nature of light-induced electron transfer. Recent developments in the understanding of ATP synthesis in oxidative phosphorylation by mitochondria and in photophosphorylation by chloroplasts and bacterial chromatophores are discussed.