Rhea is a freely available and comprehensive resource of expert-curated biochemical reactions. It has been designed to provide a non-redundant set of chemical transformations for applications such as the functional annotation of enzymes, pathway inference and metabolic network reconstruction. There are three types of reaction participants (reactants and products): Small molecules, Rhea polymers, Generic compounds.
All three types of reaction participants are linked to the ChEBI database (Chemical Entities of Biological Interest) which provides detailed information about structure, formula and charge. Rhea provides built-in validations that ensure both mass and charge balance of the reactions. We have populated the database with the reactions found in the enzyme classification (i.e. in the IntEnz and ENZYME databases), extending it with additional known reactions of biological interest. While the main focus of Rhea is enzyme-catalysed reactions, other biochemical reactions (including those that are often termed "spontaneous") also are included.