Answer:
the lock and key model.
Explanation:
The reason why this model for enzymes is called as lock and key model is because like a specific key fits into a particular lock meant for only that key, in a similar way a substrate also fits in an enzyme active site which is specifically meant for that particular substrate only.
Enzymes are very specific for their substrates and only a substrate which has a particular shape corresponding to the active site of that enzyme can get inside the enzyme for catalysis.
This model is extremely useful for blocking enzymatic activities. In order to block the enzyme we can use a structural analogue of that enzyme which rather than getting converted into products inhibits the enzyme and restricts its activity.
For example: Cholesterol biosynthesis can be restricted easily by using statin drugs which are structural analogues of HMG CoA. Statins bind HMG CoA reductase and stop its structural analogue which is HMG CoA from binding this enzyme. By doing this HMG CoA is not converted into mevalonate and hence cholesterol biosynthesis is halted.
