posted on 1996-09-11, 00:00authored byMark R. Bray, Robert J. Deeth
The suggestion that hydroxide is coordinated to the oxidised
molybdenum site in xanthine oxidase (XnO) is
tested theoretically by computing the structures of a range of four-,
five-, and six-coordinate active site models.
The local density approximation of density functional theory has
been used with the two experimentally verified
singly bonded sulfur ligands modeled by both dithiolene,
[SRCCRS]2- (R = H and CH3), and thiolate,
[CH3S]-
groups. Both ligand types give virtually identical results for
analogous species. Based on a comparison of the
computed M−L distances and those reported in recent EXAFS studies, it
is concluded that both four- and six-coordination are unlikely since the optimized Mo−S contacts are too
short or too long respectively. Of the
five-coordinate MoOS(SR)2X models, the ones with X =
[OH]- give computed M−L bond lengths in
excellent
agreement with the reported EXAFS data while X = H2O,
NH3, [CH3S]-, and
O2- give relatively poor agreement.
The theoretical results imply that the active site represents a
stable, preferred geometry rather than some imposed
entatic state.