A number of different eukaryotic oxidoreductases that require and bind a
molybdopterin cofactor have been shown  to share a few regions of sequence
similarity. These enzymes are:
Xanthine dehydrogenase (EC 184.108.40.206), which catalyzes the oxidation of
xanthine to uric acid with the concomitant reduction of NAD. Structurally,
this enzyme of about 1300 amino acids consists of at least three distinct
domains: an N-terminal 2Fe-2S ferredoxin-like iron-sulfur binding domain
(see <PDOC00175>), a central FAD/NAD-binding domain and a C-terminal Mo-
Aldehyde oxidase (EC 220.127.116.11), which catalyzes the oxidation aldehydes into
acids. Aldehyde oxidase is highly similar to xanthine dehydrogenase in its
sequence and domain structure.
Nitrate reductase (EC 18.104.22.168), which catalyzes the reduction of nitrate
to nitrite. Structurally, this enzyme of about 900 amino acids consists of
an N-terminal Mo-pterin domain, a central cytochrome b5-type heme-binding
domain (see <PDOC00170>) and a C-terminal FAD/NAD-binding cytochrome
Sulfite oxidase (EC 22.214.171.124), which catalyzes the oxidation of sulfite to
sulfate. Structurally, this enzyme of about 460 amino acids consists of an
N-terminal cytochrome b5-binding domain followed by a Mo-pterin domain.
There are a few conserved regions in the sequence of the molybdopterin-binding
domain of these enzymes. The pattern we use to detect these proteins is based
on one of them. It contains a cysteine residue which could be involved in
binding the molybdopterin cofactor.
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