Enzymes

Enzymes are proteins that control all of the chemical reactions in cells. They do so by acting as organic catalysts, lowering the activation energy needed for chemical reactions. Their action allows metabolic reactions to occur millions of times faster than they would proceed uncatalyzed. Enzymes are important parts of many metabolic pathways and are vital to both anabolic and catabolic reactions.

Enzymes catalyze reactions by binding with specific substrates. When enzyme-substrate complexes form, molecules of reactant substrates are positioned in a way that destabilizes bonds, allowing chemical reactions to proceed. Such destabilization is called the transition state of a chemical reaction. Substrate is converted to product and released by the enzyme. Enzymes are not used up or changed by the reactions they catalyze.

Enzyme function depends on the physical fit with the substrate. Substrates bind at an enzyme active site in a key-in-lock or hand-in-glove configuration. Misshapen enzymes are the cause of many genetic diseases in humans. It is possible for cells to control enzyme activity by altering the shape of the enzyme or by blocking the binding site.

Many enzymes function with coenzymes, non-protein molecules that must bind to the protein to make it a functional enzyme. Coenzymes often participate in the chemical reaction being catalyzed. Most participate in oxidation reduction reactions. Many vitamins act as co-enzymes, as do ATP and other nucleotides. One of the most important coenzymes is nicotinamide adenine dinucleotide (NAD+), which often acts as an electron acceptor in enzyme catalyzed oxidation reduction reactions.