“All living things depend on millions of chemical reactions that happen constantly. Chemical reactions that keep you alive happen fast! When you eat food, breathe, play, and grow, all of these are chemical reactions, and they must take place quickly.
How does your body speed up these important reactions? The answer is enzymes. Enzymes in our bodies are catalysts that speed up reactions by helping to lower the activation energy needed to start a reaction. Each enzyme molecule has a special place called the active site where another molecule, called the substrate, fits. The substrate goes through a chemical reaction and changes into a new molecule called the product — sort of like when a key goes into a lock and the lock opens.
Since most reactions in your body’s cells need special enzymes, each cell contains thousands of different enzymes. Enzymes let chemical reactions in the body happen millions of times faster than without the enzyme. Because enzymes are not part of the product, they can be reused again and again.”
ACS Chemistry for Life (https://www.acs.org/education/outreach/celebrating-chemistry-editions/2021-ncw/enzymes.html)