- Biochemistry
As an example, move the substrate around on the enzyme until you find the active site. (The light blue atoms represent a Co-Enzyme; NADH aka vitamin B3/Niacin).

An enzyme is a specific protein which increases (catalyzes) or decreases the speed of a chemical reaction, but the enzyme itself is not changed. A substrate is a substance upon which an enzyme acts in a biochemical reaction.

Enzymes are globular proteins. Their folded conformation creates an area known as the active site. The nature and arrangement of amino acids in the active site make it specific for only one type of substrate.

These highly specific molecules are centrally responsible for the precise "clockwork-like" processes that occur in the human body. In our bodies, once the enzyme has bound its substrate (much like a key in a lock), a reaction occurs to modify the substrate in some way. The modified substrate is then released as a "product" and the Enzyme is free to catalyze another reaction.

Now let's move on to the Four types of Polymeric Biological macromolecules.