Describe what an enzyme is and explain how it works

An enzyme is a biological catalyst which can speed up the rate of chemical reactions, it is also a protein that can be reused.
The lock and key model explains how an enzyme works to catalyse reactions. The enzyme's active site is a specific shape to fit the substrate - the enzyme is the lock and the substrate is the key. Substrate molecules that are the wrong shape will not fit into the active site as their shape is not complementary and therefore no reaction will take place.