The enzyme–substrate complex (ES) is the reversible complex formed when an enzyme binds its substrate at the active site. Within this complex the substrate is converted into product, after which the complex dissociates to release the product and regenerate the free enzyme. The formation of this complex is the first step of enzyme-catalysed reactions.

酶–底物复合物(ES)是酶在活性部位结合其底物时形成的可逆复合物。在该复合物中,底物被转化为产物,随后复合物解离,释放产物并再生游离的酶。该复合物的形成是酶催化反应的第一步。