Step 1:

Enzymes are basically protein molecules of high molecular masses. These form colloidal solutions when dissolved in water. These are complex, nitrogenous organic compounds produced by living plants and animals. Enzymes are also called ‘biochemical catalysts’.

Step 2:

Mechanism of enzyme catalysis:

On the surface of the enzymes, various cavities are present with characteristic shapes. These cavities possess active groups such as $-{\mathrm{NH}}_2$, −COOH, etc. The reactant molecules having a complementary shape fit into the cavities just like a key fits into a lock. This leads to the formation of an activated complex. This complex then decomposes to give the product.

Hence,

Thus, the enzyme-catalyzed reactions may be considered to proceed in two steps.

Step 1: Binding of the enzyme to substrate to form an activated complex.
E + S → ES^*

Step 2: Decomposition of the activated complex to form a product.
ES^* → E + P