Enzyme always acts on substance called substrate, which is specific.
Key Lock Theory
Fisher (1898) has given a key lock theory, which has supported by Paol Fibler & DD Woodsi, to define the mode of action.
According to this theory enzyme looks like a lock and lock has a specific site on which a substrate attaches called as active site.
Lock has a site for key, many keys can be inserted into it but it can be opened by a specific key like a lock enzyme has a site where many substances get chance to enter but has to react with only one that’s why it is known as specific.
Induce Fit Model
Koshland (1959) proposed the induce fit model to further classify it.
In induce fit model he states that, when a substance gets attached to the active site of an enzyme it induces changes int he structure of an enzyme.