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SBS-922 Membrane Proteins Mitochondria and respiratory chains John F. Allen School of Biological and Chemical Sciences, Queen Mary, University of London QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. The chloroplast homologue of respiratory complex III: The cytochrome b6f complex Chloroplast stroma Light NADP+ Light FNR Fd n-side Cyt b6f PS II PQ p-side PS I 2H2O O2 + 4H+ 2H+ PC, Cyt c6 Chloroplast lumen Peter Mitchell c. 1943 (adapted from Mitchell 1981). A young Peter Mitchell in the Department of Biochemistry at Cambridge. Left to right are Joan Keilin, Jim Danielli, Peter Mitchell, Mary Danielli. The ideas of David Keilin on the cytochromes and Jim Danielli on the lipid bilayer were seminal in the development of MitchellХs views on chemiosmosis and vectorial metabolism. Crofts, A. R. (2004) The Q-cycle, - a personal perspective. Photosynth. Res. MitchellХsproton pumping loops (Mitchell 1961, 1966). Crofts, A. R. (2004) The Q-cycle, - a personal perspective. Photosynth. Res. MitchellХsoriginal Q-cyc le (Mitchell 1975a). The Modified Q-cycle. The experiments from the Crofts lab in the early 1980’s provided severe constraints that limited the types of plausible Q-cycle model. This version is essentially the same as that proposed by Crofts et al 1982, and reviewed by Crofts (1986) Crofts, A. R. (2004) The Qcycle, - a personal perspective. Photosynth. Res. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. http://jfa.bio.qmul.ac.uk/lectures Why do mitochondria and chloroplasts have genome? Typical prokaryotic (left) and eukaryotic (right) cells. W. Ford Doolittle Nature 392, 15-16, 1998 The endosymbiont hypothesis for the origin of mitochondria. W. Ford Doolittle Nature 392, 15-16, 1998 Problem Why Do Mitochondria and Chloroplasts Have Their Own Genetic Systems? Why do mitochondria and chloroplasts require their own separate genetic systems when other organelles that share the same cytoplasm, such as peroxisomes and lysosomes, do not? …. The reason for such a costly arrangement is not clear, and the hope that the nucleotide sequences of mitochondrial and chloroplast genomes would provide the answer has proved unfounded. We cannot think of compelling reasons why the proteins made in mitochondria and chloroplasts should be made there rather than in the cytosol. Molecular Biology of the Cell © 1994 Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson Molecular Biology of the Cell, 3rd edn. Garland Publishing Why Do Mitochondria and Chloroplasts Have Their Own Genetic Systems? Proposed solutions (hypotheses) There is no reason. “That’s just how it is”. (Anon) The “Lock-in” hypothesis. (Bogorad, 1975). In order for core components of multisubunit complexes to be synthesised, de novo, in the correct compartment. The evolutionary process of transfer of genes from organelle to nucleus is still incomplete. E.g. Herrmann and Westhoff, 2001: The partite plant genome is not in a phylogenetic equilibrium. All available data suggest that the ultimate aim of genome restructuring in the plant cell, as in the eukaryotic cell in general, is the elimination of genome compartmentation while retaining physiological compartmentation. The frozen accident. The evolutionary process of gene transfer was underway when something happened that stopped it. E.g. von Heijne, 1986. It’s all a question of hydrophobicity. The five-helix rule. (Anon) Some proteins (with co-factors) cannot be imported. (Anon) Co-location for Redox Regulation - CORR (Allen 1993, 2003 et seq.) Bioenergetic organelle Endosymbiont Bacterium Proposed solution (hypothesis) Why Mitochondria and Chloroplasts Have Their Own Genetic Systems Co-location for Redox Regulation - CORR Vectorial electron and proton transfer exerts regulatory control over expression of genes encoding proteins directly involved in, or affecting, redox poise. This regulatory coupling requires co-location of such genes with their gene products; is indispensable; and operated continuously throughout the transition from prokaryote to eukaryotic organelle. Organelles “make their own decisions” on the basis of environmental changes affecting redox state. Allen, J. F. (1993) J. Theor. Biol. 165, 609-631 Allen, J. F. (2003) Phil. Trans. R. Soc. B458, 19-38 Prediction Explanation of previous knowledge Distribution of genes for components of oxidative phosphorylation between mitochondria and the cell nucleus Redox regulation Redox regulation Inter-membrane space I II III IV ATPase Mitochondrial matrix Inter-membrane space I II H+ III IV H+ ATPase H+ H+ NADH O2 NAD+ succinate fumarate H2O ADP ATP Mitochondrial matrix Redox regulation The end. Fin. Really. Thank you for listening.