M. Weyand et al., X-ray structure determination of a vanadium-dependent haloperoxidase from Ascophyllum nodosum at 2.0 angstrom resolution, J MOL BIOL, 293(3), 1999, pp. 595-611
The homo-dimeric structure of a vanadium-dependent haloperoxidase (V-BPO) f
rom the brown alga Ascophyllum nodosum (EC 1.1.11.X) has been solved by sin
gle isomorphous replacement anomalous scattering (SIRAS) X-ray crystallogra
phy at 2.0 Angstrom resolution (PDB accession code 1QI9), using two heavy-a
tom datasets of a tungstate derivative measured at two different wavelength
s. The protein sequence (SwissProt entry code P81701) of V-BPO was establis
hed by combining results from protein and DNA sequencing, and electron dens
ity interpretation. The enzyme has nearly an all-helical structure, with tw
o four-helix bundles and only three small beta-sheets. The holoenzyme conta
ins trigonal-bipyramidal coordinated vanadium atoms at its two active centr
es. Structural similarity to the only other structurally characterized vana
dium-dependent chloroperoxidase (V-CPO) from Curvularia inaequalis exists i
n the vicinity of the active site and to a lesser extent in the central fou
r-helix bundle.
Despite the low sequence and structural similarity between V-BPO and V-CPO,
the vanadium binding centres are highly conserved on the N-terminal side o
f an alpha-helix and include the proposed catalytic histidine residue (His4
18(V-BPO)/His(V-CPO)). The V-BPO structure contains, in addition, a second
histidine near the active site (His411(V-BPO)), which can alter the redox p
otential of the catalytically active VO2-O-2 species by protonation/deproto
nation reactions. Specific binding sites for the organic substrates, like i
ndoles and monochlordimedone, or for halide ions are not visible in the V-B
PO structure. A reaction mechanism for the enzymatic oxidation of halides i
s discussed, based on the present structural, spectroscopic and biochemical
knowledge of vanadium-dependent haloperoxidases, explaining the observed e
nzymatic differences between both enzymes. (C) 1999 Academic Press.