As early as 1894, the German chemist Emil Fischer (1852–1919), who was later awarded the Nobel Prize (Fig. 2.4), postulated that enzymes recognize their sub- strates on a lock-and-key principle. If the active site of an enzyme lies in a dimple (cavity, crevice) on the molecule’s surface, the substrate molecule must fit accu- rately, just like a key into its lock. Even slightly modified molecules will no longer interact with the enzyme. Lock-and-key is a viable preliminary explanation for the high substrate specificity of enzymes. It also explains why the shape of competing enzyme inhibitors (e.g., penicillin) strongly resembles that of the original substrate. Like a skeleton key, they block the active site of an enzyme (competitive inhibition), thus preventing the substrate from locking on.
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