Caught in the act
نویسنده
چکیده
Along with replication and mutability, living beings are set apart from the mineral background they inhabit by their metabolism—their ability to catalyse chemical reactions. Since Linus Pauling first proposed that these reactions are made pos sible by enzymes that recognize and bind tightly to their substrates at a crucial transi tion point [1], it has become increasingly clear that understanding these reactions requires details of the precise molecular alignments that take place at the level of femtoseconds (10–15 s). This transition state is the ‘point of no return’ for colliding molecules in a chemi cal reaction. Beyond it, the reactants inevi tably go on to form new products; before it, the reaction does not take place. It lasts for tens to hundreds of femtoseconds, when the molecules are at a state of maximum energy from which they will fall either towards completing the reaction, or with equal likeli hood, away from it. The role of the enzyme is to enable the molecules to negotiate this energy summit and to reach the point of completing the reaction. Many processes, including protein fold ing and the splitting of water during photo synthesis, pass through more than one transition state. Unravelling them all is a challenging task, but the potential prizes are great and might include the ability to harness reactions to produce carbonneutral energy, for example, by mimicking or exploiting photosynthesis. There are also great thera peutic possibilities, as cell replication in cancer or metabolic processes in pathogens could be halted by intervening at transition states to block key reactions. This therapeutic avenue was first explored in 1986 by Richard Wolfenden, now at the University of North Carolina at Chapel Hill, USA, who calculated that con formational changes in the active site of an enzyme at the transition state should enable it to bind to the reactants with huge strength to overcome the energy barrier [2]. This, in turn, suggested that suitably designed ana logues, mimicking the reactants at the tran sition state, could intervene by binding to the enzyme during that brief window, thus rendering the enzyme ineffective.
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