NADH:ubiquinone oxidoreductase

Complex I


NADH:ubiquinone oxidoreductase

A Home-page for Complex I has recently been set up by Drs. Akemi and Takao Yagi at Scripps, and contains a more extensive set of information and links.

The NADH:ubiquinone oxidoreductase (Complex I), provides the input to the respiratory chain from the NAD-linked dehydrogenases of the citric acid cycle. The complex couples the oxidation of NADH and the reduction of ubiquinone, to the generation of a proton gradient which is then used for ATP synthesis. The complex occurs in the mitochondria of eukaryotes and in the plasma membranes of purple photosynthetic bacteria, and the closely related respiratory bacteria. The close homology of sequences, function, and prosthetic groups shows a common ancestry. Mutations in this complex are associated with many disease conditions, including LEBER HEREDITARY OPTIC NEUROPATHY, MELAS SYNDROME, and ALTZHEIMER'S DISEASE.

A closely related set of sequences is found in chloroplasts; genes for 11 of the 14 minimal subunits are found in the plastid DNA of plants and in the genome of cyanobacteria. However, genes encoding subunits of the NADH dehydrogenase part of complex I are apparently missing in these species, so the complex might lack the NADH processing subunits. It is speculated that the chloroplast enzyme might use the quinone reductase function of the complex with a different reductant,- perhaps ferredoxin or NADPH.

Structure of the complex

This is one of the largest catalytic complexes, and our knowledge of the structure comes mainly from electron microscopy and biochemistry. Image averaging from negatively stained preparations of the protein reveal a complex shaped like an old boot.

Left: The complete complex. Right: the membrane associated "foot".

Samples of the images used to obtain the pictures above. Click for larger version


A model of the complex generated by image reconstruction. Click here to see a movie showing the complex rotating.


Images from the movie selected to provide stereo pairs for crossed-eye viewing. The two pairs provide approximately orthogonal views.
The images and movie are taken from the reference below, and from Hans Weiss' home page.

Ref.
Guénebaut, V., Vincentelli, R., Mills, D., Weiss, H. & Leonard, K. (1997) Three-dimensional structure of NADH-dehydrogenase from Neurospora crassa by electron microscopy and conical tilt reconstruction. J. Mol. Biol. (1997), 265, 409-418

The complex can be dissociated into two main sub-complexes, corresponding to the "ankle" of the boot, and the "foot" of the boot. The ankle is thought to protrude from the membrane so as to be predominantly in the aqueous phase (the matrix side of the mitochondrial membrane, - the N-phase (protochemically negative)), and contains the binding site for NAD(H), and the input electron transfer chain. This consists of a flavine mononucleotide (FMN) prosthetic group as the first acceptor of electrons from NADH, and iron sulfur centers N-1, N-3 and N-4. The sub-complex can be further dissociated into a flavoprotein and an iron protein. The foot (the hydrophobic protein) is membrane bound, and contains a catalytic site at which ubiquinone is reduced, and inhibitors bind, and several iron sulfur centers. There is a second catalytic site for ubiquinone reaction on the ankle, but this is seen as a separate activity only in the dissociated complex.


The complex from bacteria has a smaller size, and is lacking many of the subunits found in mitochondrial complexes. The picture above shows a comparison between electronmicrscopic reconstructions of the mitochondrial and bacterial enzymes.

Subunit composition

The minimal structural elements common to the mitochondrial and the bacterial complex are 14 polypeptides. Common prosthetic groups are 1 FMN and 6-8 iron-sulfur clusters as prosthetic groups. The mitochondrial complexes contain many additional accessory subunits for which no homologous counterparts exist in the bacterial complex; the bovine complex is estimated to contain 41 different polypeptides.
Subunit            Mr            Redox centers       Function

Mitochondria encoded (mainly hydrophobic fragment)

ND1               36                                Rotenone binding site (possibly UQ reductase site)
ND2               39
ND3               13.2
ND4               51.6
ND4L              10.7
ND5               66.9            
ND6               18.7                              Iron protein(?)             

Nuclear encoded

  A subunit       13		Iron protein	  
  B subunit       13
FP24KD subunit    23              N-1b 2Fe.2S(?)    Flavoprotein (2 FeS center look like 4Fe.4S from sequences)
  42KD subunit    42
IP49KD subunit    49              N-4 (4Fe.4S)      Iron pprotein
FP51KD subunit    51              FMN, N-3 (4Fe.4S) Flavoprotein, NAD-binding
  AGGG subunit    12.3

In the complex from Neurospora crassa mitochondria, the acyl carrier protein is an essential constituent of the peripheral (hydrophilic) domain, required for assembly of the complex.

A more recent summary of the complex I subunits is shown schematically below:



The association of gene products with the structure, and the redox centers associated with each subunit, are summarized in the pictures above.
Takao Yagi's summary of Complex I research.

Sequences of the minimal subunits

Sequences are available from the Caenorhabditis elegans and human sequence databases maintained by the Sanger Center. C. elegans is a simple nematode (thread worm) used as a "model" metazoan. The complete genome sequence is finished, and the human genome is nearing completion. There are links from this database into other sequence data bases, so that the family of NADH:Q oxidoreductases can be explored through sequence comparison. Alternatively, the Neuromuscular Mitochondrial Disorders page has easy links to the NCBI sequence database for the different subunits, arranged by disease

Mechanism

NADH:ubiquinone oxidoreductase catalyzes the oxidation of NADH, the reduction of ubiquinone, and the transfer of 4H+/NADH across the coupling membrane.
NADH + H+ + Q + 4H+N <==> NAD+ + QH2 + 4H+P
Neither the pathway of electron transfer, nor the mechanism (nor even stoichiometry) of proton transport are known with any certainty. However, a substantial body of research has established the main outlines of the electron transfer pathway through the enzyme, and there are a number of suggestions about the mechanism of coupling to proton movement. The redox centers are well characterized through spectroscopy and redox potentiometry (see Cramer and Knaff, chapter 4, for details).

Cartoon showing possible electron transfer pathway, and location of centers. The proton stoichiometry shown would not account for the 4H+ needed to explain the data. More recent models, and a comprehensive discussion, can be accessed through the complex 1 home-page.

Recent reviews

Weiss, H., Friedrich, T., Hofhaus, G. and Preis, D. (1991): The respiratory chain NADH dehydrogenase (Complex I) of mitochondria. Eur. J. Biochem. (Review) 197, 563-576.

Hofhaus, G., Weiss, H. and Leonard, K. (1991): Electron microscopic analysis of the peripheral and the membrane parts of mitochondrial NADH dehydrogenase (Complex I). J. Mol. Biol. 221, 1027-1043.

Friedrich, T., Steinmüller, K. & Weiss, H. (1995) The proton-pumping respiratory complex I of bacteria and mitochondria and is homologue of chloroplasts. FEBS Lett. (Minireview), 367, 107-111.

Yagi, T. (1993) Biochim. Biophys. Acta 1141, 1-17.

Ohnishi, T., ed. (1993) J. Bioenerg. Biomembr. 25, 325-391.

Dimroth, P. (1997) Biochim. Biophys. Acta 1318, 11-51.

Brandt, U. (1997) Biochim. Biophys. Acta 1318, 79-91.


©Copyright 1996, Antony Crofts, University of Illinois at Urbana-Champaign, a-crofts@uiuc.edu