Respiratory-chain NADH dehydrogenase (EC 1.6.5.3) [1,2] (also known as complex
I or NADH-ubiquinone oxidoreductase) is an oligomeric enzymatic complex
located in the inner mitochondrial membrane which also seems to exist in
the chloroplast and in cyanobacteria (as a NADH-plastoquinone oxidoreductase).
Among the 25 to 30 polypeptide subunits of this bioenergetic enzyme complex
there is one with a molecular weight of 49 Kd (in mammals), which is the third
largest subunit of complex I and is a component of the iron-sulfur (IP)
fragment of the enzyme. It seems to bind a 4Fe-4S iron-sulfur cluster. The 49
Kd subunit has been found to be:
- Nuclear encoded, as a precursor form with a transit peptide in mammals, and
in Neurospora crassa.
- Mitochondrial encoded in protozoan such as Paramecium (ORF 400), Leishmania
and Trypanosoma (MURF 3).
- Chloroplast encoded in various higher plants (ORF 392).
The 49 Kd subunit is highly similar to [3,4]:
- Subunit D of Escherichia coli NADH-ubiquinone oxidoreductase (gene nuoD).
- Subunit NQO4 of Paracoccus denitrificans NADH-ubiquinone oxidoreductase.
- Subunit 5 of Escherichia coli formate hydrogenlyase (gene hycE).
- Subunit G of Escherichia coli hydrogenase-4 (gene hyfG).
As as signature pattern we selected a highly conserved region, located in the
N-terminal section of this subunit.
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