Hartmut Michel

Cytochrome c Oxidase and the Major Sodium Ion / Proton Exchanger from Escherichia coli: A Comparison of Two Proton Translocating Membrane Proteins


Abstract

In biology, membranes are barriers for the transport of ions and polar substances. They are even electric insulators. These properties have allowed nature to use mitochondrial and bacterial membranes for energy transduction via electric voltages (potentials) and ion gradients. Cytochrome c oxidase is an enzyme which transfers electrons from cytochrome c onto oxygen and consumes protons to form water as a product. This reaction creates an electric voltage and a pH difference, because cytochrome c delivers its electrons from the outer surface of the membrane whereas the protons originate form the inner surface of the mitochondria or bacteria. In addition, the enzyme translocates (“pumps”) four protons from the inner to the outer surface per reaction cycle enhancing the both electric voltage and pH difference. This so-called “electrochemical proton gradient” drives protons back via the ATP-synthase leading to the synthesis of the universal biological energy carrier adenosine-5’-triphosphate (“ATP”) from adenosine-5’-diphosphate (“ADP”) and inorganic phosphate. The reaction catalysed by the cytochrome c oxidase is understood insufficiently and the subject of controversial discussions. The author’s view, based on X-ray structures of the enzyme, will be presented. Long pathways for proton transfer reactions do exist in the enzyme.

Electric potentials and ion gradients across biological membranes are also used for the active transport of other ions and polar substances against their concentration gradients. For instance a sugar or an amino acid can be co-transported with a proton or a sodium ion. The energetically “downhill” flow of the proton or sodium ion into the cell drives the accumulation of the sugar or of the amino acid inside the cell. Sodium ion/proton exchangers are essential components of all cells. They are involved in the excretion of sodium ions, in the maintenance of the intracellular pH and of the cell volume. We study the sodium ion/proton exchanger NhaA from the bacterium Escherichia coli. We have been able to crystallize this membrane protein and to elucidate its structure (Hunte C., E. Screpanti, M. Venturi, A. Rimon, E. Padan and H. Michel, Nature 435, 1197-1202 (2005)). It shows a funnel type structure so that the transport of sodium ions and protons has to occur over rather short distances. The structure will be presented and the potential mechanisms of regulation and transport discussed.


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