Enzyme dynamics and hydrogen tunnelling in a thermophilic alcohol dehydrogenase

Biological catalysts (enzymes) speed up reactions by many orders of magnitu de using fundamental physical processes to increase chemical reactivity. Hy drogen tunnelling has increasingly been found to contribute to enzyme react ions at room temperature(1), Tunnelling is the phenomenon by which a partic le transfers through a reaction barrier as a result of its wave-like proper ty(1-3). In reactions involving small molecules, the relative importance of tunnelling increases as the temperature is reduced(4). We have now investi gated whether hydrogen tunnelling occurs at elevated temperatures in a biol ogical system that functions physiologically under such conditions, Using a thermophilic alcohol dehydrogenase (ADH), we find that hydrogen tunnelling makes a significant contribution at 65 degrees C; this is analogous to pre vious findings with mesophilic ADH at 25 degrees C (ref. 5), Contrary to pr edictions for tunnelling through a rigid barrier, the tunnelling with. the thermophilic ADH decreases at and below room temperature. These findings pr ovide experimental evidence for a role of thermally excited enzyme fluctuat ions in modulating enzyme-catalysed bond cleavage.