Lock and Key Theory: Emil Fisher proposed this hypothesis in 1894. According to this hypothesis the active site of the enzyme is like a ‘lock’ into which substrate fits like a ‘key’.i.e., the shape of the active site and the substrate molecules are complementary . So the enzyme molecule holds the substrate, molecule close together, forming the unusable intermediate compound, the enzyme substrate complex. It dissociates to form enzyme and products. 
Induced Fit Theory: also called as "a hand in a glove model" Daniel E.Koshland formulated this hypothesis in 1958. According to this hypothesis the active site does not have a rigid ‘lock and key’ conformation. The binding of the substrate molecule to the enzyme molecule induces to modify the shape of the active site so that it becomes complementary to the substrate molecule. This is called the induced fit . Induced fit is possible because of the flexibility of the protein molecules.
Lock and Key Theory vs Induced fit Theory Lock and Key Theory:
1. Active site is a single entity.
2. There is no separate regulatory site like allosteric site.
3. Active site is rigid and static.
4. Development of transition state is not considered.
5.It does not visualize the weakening of substrate bonds.
6. It does not explain the mechanism of non activity in case of competitive inhibitor, allosteric regulation, and some enzymes ability to bind to several substrate with different affinity. Induced fit Theory:
1. Active site is made of flexible entities
2. A separate catalytic group is visualized.
3. Active site is flexible.
4. It considers the development of transition state before the reactants undergo change. Both active site and substrate undergo changes to make optimal fit.
5. Catalytic group is believed to weaken the substrate bonds by nucleophilic and electrophilic attack.
6. It explains the a mechanism for non action over competitive inhibition and also allosteric regulation
ncG1vNJzZmivp6x7rq3JqKmdoZabsrOxzZycrGaTpLpwfo9qa2hoZGSxqrLFnqmeppOaeqOx07CcnqZdobykt4yapZ1lm5rGb7TTpqM%3D