SBS 922 Membrane Biochemistry Mitochondria and chloroplasts John

SBS 922 Membrane Biochemistry Mitochondria and chloroplasts John F. Allen School of Biological and Chemical Sciences, Queen Mary, University of London 1

http: //jfa. bio. qmul. ac. uk/lectures/

The chemiosmotic theory

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. Electron transport Energised State… ATP synthesis 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.

Inter-membrane space I II III H+ IV H+ ATPase H+ H+ NADH O 2 NAD+ succinate fumarate H 2 O ADP ATP Mitochondrial matrix

ATPase ATP + H 2 O ADP + Pi Also known as “ATP Synthase” F-ATPases (c. f. V-ATPases; P-ATPases) - F 1 - Fo

F 1

F 1

Figure from: ALLEN, J F (2002) Photosynthesis of ATP - Electrons, Proton Pumps, Rotors, and Poise. Cell 110, 273– 276

24 http: //www. res. titech. ac. jp/~seibutu/

25 http: //www. res. titech. ac. jp/~seibutu/

F 1 Fo

Rotation of the Fo-ATPase as a proton-driven, rotary stepping motor, as proposed by Junge (1997). http: //jfa. bio. qmul. ac. uk/~john/webstar/ltm/06/3 ATP. html