Book of Abstracts: Albany 2007
June 19-23 2007
Structures and mechanisms of the complexes of the respiratory chain and of sodium/ proton antiporters
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. In the mitochondrial respiratory chain complexes I, III (cytochrome bc1 complex) and IV (cytochrome c oxidase) translocate (?pump?) protons across the mitochondrial membrane. The resulting 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. Cytochrome c oxidase 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 pumps four protons from the inner to the outer surface per reaction cycle enhancing the both electric voltage and pH difference. Despite the fact that various structures of cytochrome c oxidases are known the catalyzed reaction 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, 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 et al., 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.
References and Footnotes
M P I of Biophysics,