The role played by protein motions in driving enzyme-catalysed reactions has attracted much attention over recent years.The physical steps during enzyme catalysis,substrate binding,product release or other conformational changes,are well described in terms of ms-s timescale dynamics.It has been suggested that enzyme dynamics and flexibility are important not just for the physical events but also for the chemical step itself.The potential involvement of protein motions in driving the chemistry of enzyme catalysed reactions is often studied for hydrogen transfer,where quantum mechanical tunnelling plays a role and the width of the reaction barrier is as important in determining the reaction rate as its height.