REDOX COMPONENTS AND STRUCTURE OF THE RESPIRATORY NADH-UBIQUINONE OXIDOREDUCTASE (COMPLEX I)

The proton-pumping NADH:ubiquinone oxidoreductase is the first complex in the respiratory chains of many purple bacteria and of mitochondria of most eucaryotes. The bacterial complex consists of 14 different su bunits. The mitochondrial complex contains at least 29 additional prot eins that do not directly participate in electron transfer and proton translocation. We analysed electron micrographs of isolated and negati vely stained complex I particles from Escherichia coli and Neurospora crassa and obtained three-dimensional models of both complexes at medi um resolution. Both have the same L-shaped overall structure with a pe ripheral arm protruding into the aqueous phase and a membrane arm exte nding into the membrane. The two arms of the bacterial complex are onl y slightly shorter than those of the mitochondrial complex although th e protein mass of the former is only half of that of the latter. The p resence of a novel redox group in the membrane arm of the complex is d iscussed. This group has been detected in the N. crassa complex by mea ns of UV-visible spectroscopy. After reduction with an excess of NADH and reoxidation by the lactate dehydrogenase reaction, a reduced-minus -oxidized difference spectrum was obtained that cannot be attributed t o the known cofactors flavin mononucleotide (FMN) and the FeS clusters N1, N2, N3 and N4. Due to its positive midpoint potential the novel g roup is believed to transfer electrons from the FeS clusters to ubiqui none. Its role in proton translocation is discussed. (C) 1998 Elsevier Science B.V.