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The chloroperoxidase (EC 1.11.1.-) from the fungus Curvularia inaequalis belongs
to a class of vanadium enzymes that oxidize halides in the presence of hydrogen
peroxide to the corresponding hypohalous acids. The 2.1 A crystal structure (R =
20%) of an azide chloroperoxidase complex reveals the geometry of the catalytic
vanadium center. Azide coordinates directly to the metal center, resulting in a
structure with azide, three nonprotein oxygens, and a histidine as ligands. In
the native state vanadium will be bound as hydrogen vanadate(V) in a trigonal
bipyramidal coordination with the metal coordinated to three oxygens in the
equatorial plane, to the OH group at one apical position, and to the epsilon 2
nitrogen of a histidine at the other apical position. The protein fold is mainly
alpha-helical with two four-helix bundles as main structural motifs and an
overall structure different from other structures. The helices pack together to
a compact molecule, which explains the high stability of the protein. An amino
acid sequence comparison with vanadium-containing bromoperoxidase from the
seaweed Ascophyllum nodosum shows high similarities in the regions of the metal
binding site, with all hydrogen vanadate(V) interacting residues conserved
except for lysine-353, which is an asparagine.
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