Enzyme-catalyzed reactions occur over the following stages: 

1. The active site of enzyme and the substrate come in close proximity to each other. 
2. The substrate binds to the enzyme at a specially-shaped area known as the active site. Enzymes are extremely specific and do not bind with just any molecule. They only bind with a substrate that fits snugly into the active site.  
3. On binding with the substrate, the enzymes alter their shape to fit more tightly around the substrate. The substrate is broken down or combined with other molecules to create a new product called an enzyme-substrate complex. 
4. The enzyme then releases the product of the reaction and returns to its original shape. It is now free and ready to bind to another molecule of substrate.