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 75 Kd (in mammals), which is the
largest subunit of complex I and is a component of the iron-sulfur (IP)
fragment of the enzyme. It seems to bind to two 4Fe-4S clusters (called N-3
and N-4).
The 75 Kd subunit is highly similar to [3,4]:
- Subunit γ of Alcaligenes eutrophus NAD-reducing hydrogenase (gene hoxU)
which also binds two 4Fe-4S cluster.
- Subunit NQO3 of Paracoccus denitrificans NADH-ubiquinone oxidoreductase.
- Subunit G of Escherichia coli NADH-ubiquinone oxidoreductase (gene nuoG).
The 75 Kd subunit and the bacterial hydrogenase γ subunit contains three
conserved clusters of cysteine residues which are most probably involved in
the binding of the iron-sulfur clusters. We have developed signature patterns
for these three regions.
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