SBS922 Membrane Biochemistry Mitochondria and chloroplasts John F. Allen School of Biological and Chemical Sciences, Queen Mary, University of London 1
The membrane energised state We have seen how observed characteristics of oxidative phosphorylation led to conclusion that there was a membrane energised state linking electron transfer in mitochondria to ATP synthesis and other membrane-linked energy-dependent functions such as active transport of solutes. Energised State… ATP synthesis Electron transport What is the “energised state”….?
CHEMIOSMOTIC HYPOTHESIS Proposed by Peter Mitchell 1961 and further elaborated 1966 Competing hypotheses 1) Chemical hypothesis (i.e. like substrate-level phosphorylation) 2) Conformational hypothesis 3) Localised proton hypothesis (variation of chemiosmotic)
Transmembrane Proton gradient Energy transduction occurs via a proton circuit which circulates through the insulating coupling membrane and the two adjacent bulk phases (the matrix and the cytosol/ intermembrane space in mitochondria). Since each of the two bulk phases is in equilibrium, energy storage is transmembrane rather than intramembrane.
H+ H+ Inter-membrane space I II III IV ATPase H+ H+ NADH O2 H2O ADP NAD+ succinate fumarate ATP Mitochondrial matrix
ATPase ATP + H2O ADP + Pi Also known as “ATP Synthase” F-ATPases (c.f. V-ATPases; P-ATPases) - F1 - Fo
Figure from: ALLEN, J F (2002) Photosynthesis of ATP - Electrons, Proton Pumps, Rotors, and Poise. Cell 110, 273–276
Rotation of the Fo-ATPase.Fo-ATPase as a proton-driven, rotary stepping motor, as proposed by Junge (1997). http://jfa.bio.qmul.ac.uk/~john/webstar/ltm/06/3ATP.html
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