Correlating C-H bond cleavage with molybdenum reduction in xanthine oxidase

We have performed a computational study of substrate C-H bond activation in enzymes of the XO family. The C-H H-atom for all XO substrates studied is transferred to the terminal sulfido at the transition state with near neutral charge, and this is consistent with both Mo=S π→ C-H σ* and C-H σ→Mo=S π* donor-acceptor interactions activating the C-H bond. A C-H bond scission and Mo reduction appear to be highly correlated along the reaction coordinate for all XO substrates studied, with Mo reduction being a continuous and exponential function of C-H bond breaking along the reaction coordinate.