Reaction Mechanism of Cobalt-Substituted Homoprotocatechuate 2,3-Dioxygenase: A QM/MM Study

The
reaction mechanisms of cobalt-substituted homoprotocatechuate
2,3-dioxygenase (Co-HPCD) with electron-rich substrate homoprotocatechuate
(HPCA) and electron-poor substrate 4-nitrocatechol (4NC) were investigated
by quantum mechanical/molecular mechanical (QM/MM) calculations. Our
results demonstrated that the Co-O2 adducts has doublet
ground state with a CoIII-O2•– character when 4NC was used as the substrate, in good agreement
with the EPR spectroscopic experiment. The reactive oxygen species
is the doublet CoIII-O2•– for Co-HPCD/4NC and the quartet SQ•↑-CoII-O2•–↓ species
for Co-HPCD/HPCA, indicating that the substrate plays important roles
in the dioxygen activation by Co-HPCD. B3LYP was found to overestimate
the rate-limiting barriers in Co-HPCD. TPSSh predicts barriers of
21.5 versus 12.0 kcal/mol for Co-HPCD/4NC versus Co-HPCD/HPCA, which
is consistent with the fact that the rate of the reaction is decreased
when the substrate was changed from HPCA to 4NC.