Redox components and structure of the respiratory NADH:ubiquinone oxidoreductase (complex I)

Abstract

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 subunits. The mitochondrial complex contains at least 29 additional proteins that do not directly participate in electron transfer and proton translocation. We analysed electron micrographs of isolated and negatively stained complex I particles from Escherichia coli and Neurospora crassa and obtained three-dimensional models of both complexes at medium resolution. Both have the same L-shaped overall structure with a peripheral arm protruding into the aqueous phase and a membrane arm extending into the membrane. The two arms of the bacterial complex are only slightly shorter than those of the mitochondrial complex although the protein mass of the former is only half of that of the latter. The presence of a novel redox group in the membrane arm of the complex is discussed. This group has been detected in the N. crassa complex by means 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 to the known cofactors flavin mononucleotide (FMN) and the FeS clusters N1, N2, N3 and N4. Due to its positive midpoint potential the novel group is believed to transfer electrons from the FeS clusters to ubiquinone. Its role in proton translocation is discussed.

A Central Functional Role for the 49-kDa Subunit within the Catalytic Core of Mitochondrial Complex I

“…By virtue of its pH-dependent redox potential (21) it has been suggested to be an important component of the proton-translocating machinery (22, 23). There has been conflicting evidence whether this redox group resides in the TYKY (24) or in the PSST subunit (25). In either case one would not have expected mutations in the 49-kDa subunit to have pronounced and specific effects on iron-sulfur cluster N2, as was observed in this study.…”

Function of Conserved Acidic Residues in the PSST Homologue of Complex I (NADH:Ubiquinone Oxidoreductase) from Yarrowia lipolytica

“…However, the location of iron-sulfur center N-2 is still controversial because two approaches to resolve this issue gave contradictory results. Sitedirected mutagenesis in E. coli suggests the PSST homolog (14), and a similar study in Rhodobacter capsulatus (15) suggests the TYKY homolog as the most likely candidates to bind N-2.…”

The Mitochondrial Oxidative Phosphorylation Proteome of Chlamydomonas reinhardtii Deduced from the Genome Sequencing Project

“…These subunits bind the FMN and the eight iron-sulfur clusters of the enzyme. Seven of these subunits are the homologs of the hydrophobic ND1, 2, 3, 4, 4L, 5, and 6 subunits encoded in the mitochondrial genome of eukaryotes (Dupuis et al, 1998;Friedrich et al, 1998). The additional subunits present in mitochondrial complex I, the so-called supernumerary subunits, are considered to participate in the assembly, the stability, or the regulation of the enzyme (Friedrich et al, 1998;Heazlewood et al, 2003a).…”

“…S. cerevisiae cannot, however, be considered as the reference simply because its mitochondria are deprived of complex I. Nevertheless, mitochondria of other fungi such as Neurospora crassa (Friedrich et al, 1998) and Yarrowia lipolytica (Kerscher et al, 2001) that possess a complex I have been successfully used to study the structure and function of this enzyme.…”