Mitochondria and chloroplasts SBS922 Membrane Biochemistry John F. Allen School of Biological and Chemical Sciences, Queen Mary, University of London 1

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 ATP synthesis Energised State… 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)

Mitchell proposed that electron transfer directly produced an electrochemical gradient of protons across the coupling membrane that was subsequently used to drive ATP synthesis. The theory was subsequently adapted and expanded, principally by Mitchell and his colleague Jennifer Moyle to account for other membrane-linked energy dependent functions such as the active transport of solutes across the membrane. The chemiosmotic hypothesis is named because it is postulated to involve both a) chemical reactions, the transfer of chemical groups (electrons,protons and O 2 2- ) within the membrane b) osmotic reactions, the transport of a solute (protons) across the membrane

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 (intramembrane proton gradients were the basis of the localised proton hypothesis).

IIIIIIIVATPase Mitochondrial matrix Inter-membrane space H+H+ H+H+ H+H+ H+H+ NADHO2O2 ATP ADP H2OH2O NAD + succinatefumarate

ATPase Also known as “ATP Synthase” F-ATPases (c.f. V-ATPases; P-ATPases) - F 1 - F o ATPADP + P i

F1F1

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

FoFo

Rotation of the Fo-ATPase.Fo-ATPase as a proton-driven, rotary stepping motor, as proposed by Junge (1997).

The F 1 -F o ATPase