According to previous quantum mechanics/molecular mechanics (QM/MM) studies, camphor hydroxylation in cytochrome P450 is catalysed by a single water molecule which lowers the computed B3LYP/CHARMM barrier by about 4 kcal mol(-1). Gas-phase B3LYP model studies for a variety of different substrates show the generality of this effect. Its origin is an electrostatic enhancement of hydrogen bonding ...