Most chemical reactions do not occur spontaneously in a cell. Instead, cells rely on proteins called enzymes to kickstart chemical reactions and speed them up, enabling cells to get the most out of the energy sources available to them. Enzymes have a unique way of kickstarting reactions.
They work by binding to one or more specific molecules called reactants, or substrates. Binding occurs at a special region on the enzyme called the active site. Once the substrates bind to the active site, they form what's called an enzyme-substrate complex. As the enzyme and substrates begin to react, some of the chemical bonds in the substrates begin to weaken, causing them to link together. Eventually, the chemical reactions at the active site lead to the formation of a different molecule.
This is referred to as the product. Once the reaction has occurred, the product is released from the active site. The enzyme returns to its original state and is free to react again with another set of substrates.