Sodium Import By Rhodothermus Marinus Respiratory Complex I

Abstract

Complex I, NADH:quinone oxidoreductase, is the largest and the least understood membrane-bound complex of the aerobic respiratory chains. This enzyme couples the electron transfer from NADH to quinone to charge translocation across the cytoplasmatic or mitochondrial inner membrane. In this way it contributes to the membrane electrochemical potential that is the driving force for ATP synthesis. The mechanism of quinone reduction and its coupling to charge translocation is not known. Also, the nature of the ion that is translocated by this complex is not consensual, being proton and sodium ions possible candidates.
The study on ion translocation by complex I (NADH:menaquinone oxidoreductase) from the bacterium Rhodothermus marinus is being performed using membrane vesicles. Proton translocation was monitored by fluorescence spectroscopy and sodium ion transport by ²³Na-NMR spectroscopy. NADH-driven proton transport was observed to occur from the outside to the inside of the membrane vesicles, in accordance with the observed inside-positive membrane potential; sodium transport was also observed, but in the opposite direction. Specific inhibitors of complex I (rotenone) and of the dioxygen reductase (KCN) inhibited the proton and the sodium ion transport, but the KCN effect was totally reverted by the addition of a menaquinone analogue, indicating that both transports were catalyzed by complex I. The results show that complex I translocates sodium ions to the direction opposite to that of the establishment of membrane potential by proton translocation, what constitutes the first description of such a process. Moreover, studying the sodium dependences of the NADH oxidation and of proton and sodium transport activities allowed to propose a model for the mechanism of complex I in which the presence of two different energy coupling sites are suggested.

Short CV

2000-2005-Degree in Biochemistry by the Faculdade de Ciências da Universidade de Lisboa
2004-2005-Undergraduate training at the Metalloproteins and Bioenergetics Laboratory at Instituto de Tecnologia Química e Biológica under the supervision of Prof. Miguel Teixeira and Dra. Manuela Pereira
Since January of 2006 - PhD fellowship by Fundação para a Ciência e Tecnologia, on the “Study of proteins with NADH:quinone oxidoreductase activity – Type I and Type II” under the supervision of Prof. Miguel Teixeira and Dra. Manuela Pereira