Enzymes are globular protein molecules that act as catalysts by lowering the activation energy. Like all globular proteins, enzyme molecules are coiled into a three-dimensional shape with hydrophilic R groups on the outside of the molecule making them soluble in water. Enzymes control all metabolic and anabolic reactions, both intracellularly and extracellularly. A substrate is a molecule on which the enzyme acts. An enzyme combines with a substrate to form a short-lived enzyme-substrate complex. This simple reaction takes place in a small part of the enzyme called the active site. The shape of the active site allows the substrate to fit perfectly, it is held in place by temporary bonds that form between the substrate and some of the R groups