Enzymes are molecules responsible for sustaining life, consisting of mostly complex proteins. They accelerate metabolic reactions in the body, such as food digestion to to blood clotting to DNA synthesis.

Nearly all chemical reactions in cells require enzymes in order to function at a rate acceptable to sustain life. Enzymes work by grabbing on to a substrate at a specific area. Substrates are the biological molecule that the enzyme will work on. Enzymes are specific in the reactions they catalyze, and differentiate due to complementary shapes, charges, and other characteristics. The enzyme causes a process called catalysis to happen, which means that the substrate has changed, either by being broken down or combined with another molecule. The enzyme then lets go and is able to work on a different molecule. 

Enzymes are regulated by inhibitors (molecules that decrease enzyme activity) and activators (molecules that increase enzyme activity). Examples include drugs, poisons, temperature, or pressure.