A substance (usually a protein) that speeds up, or catalyzes, a chemical reaction without being permanently altered or consumed. Enzymes carry out the thousands of chemical reactions that go on in a cell. Enzymes help make other molecules, including DNA. Enzymes also break food down and deliver and consume the energy that powers the cell. Other kinds of proteins, called regulatory proteins, preside over the many interactions that determine how and when genes do their work and are copied. Regulatory proteins also supervise enzymes and the give-and-take between cells and their environment.39