We have performed a computational study of substrate CÀH bond activation in enzymes of the XO family. The CÀH 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 Mo¼S p ! CÀH s* and CÀH s ! Mo¼S p* donorÀacceptor interactions activating the CÀH bond. A CÀH 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 CÀH bond breaking along the reaction coordinate.

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