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Lock and key hypothesis. The hypothesis was put forwarded by Fischer (1894). According to this hypothesis the enzyme and its substrate have a complementary shape. The substrate and the active site of the enzyme have a shape such that the two fit into each other just like a lock and key. As a particular lock can be opened by a particular type of key and a particular enzyme can act on particular substrate due to their specificity, hence the name for the mechanism is given as Lock and Key Mechanism.

Lock and Key Hypothesis of Enzyme Action Complex