在活細胞里催化化學反應的蛋白質。在必要的物理化學反應里酶是能夠加速形成和打斷化學連接的蛋白分子。酶可以是細胞內的，催化細胞內的反應如：新陳代謝、調節、信號傳導或合成過程；或細胞外的，在細胞間或血液里循環，并在那里催化對于多細胞生物的功能具有重要作用的反應。酶的催化中心是由一組氨基酸組成，其三維結構形成活性位點，允許它高效地結合或修飾底物。酶的作用一般分三步：結合底物；改變底物結構使其便于修飾；化學反應的完成修飾了底物并使酶恢復到原來的狀態。其它促成催化的因素來自結合水分子，金屬離子或其它因子。許多酶是由幾個基組成，有些具有不同區域形成的結合和催化功能。

A protein that catalyzes chemical reactions in living cells. Enzymes are protein molecules whose function it is to speed the making and breaking of chemical bonds required for essential physiochemical reactions. Enzymes are either intracellular, catalyzing reactions within the cell such as those involved in the metabolic, regulatory, signaling, or synthesis pathways; or extracellular, circulating in the spaces between cells or in the bloodstream, where they catalyze reactions critical to the functioning of a multicellular organism. The catalytic center of an enzyme is composed of a set of amino acids whose three-dimensional arrangement forms the “active-site” that allows them to efficiently bind and modify the substrate. Enzymes typically function in a three-step process: binding of the substrate (the molecule to which a chemical bond will either be added or cleaved); straining of the substrate into a conformation where it is susceptible to modification, (called the enzyme-substrate complex, in which the enzyme and substrate are joined by a transient covalent attachment); completion of the chemical reaction modifies the substrate and returns the enzyme to its initial state. Additional contributions to catalysis can come from bound water molecules, metal ions, or other factors. Many enzymes are composed of several subunits (domains), and some have binding (recognition) and catalytic (effector) functions encoded by different domains.