Isomerization reactions are fundamental in biology, and isomers usually differ in their biological role and pharmacological effects. In this study, we have cataloged the isomerization reactions known to occur in biology using a combination of manual and computational approaches. This method provides a robust basis for comparison and clustering of the reactions into classes. Comparing our results with the Enzyme Commission (EC) classification, the standard approach to represent enzyme function on the basis of the overall chemistry of the catalyzed reaction, expands our understanding of the biochemistry of isomerization. The grouping of reactions involving stereoisomerism is straightforward with two distinct types (racemases/epimerases and cis-trans isomerases), but reactions entailing structural isomerism are diverse and challenging to classify using a hierarchical approach. This study provides an overview of which isomerases occur in nature, how we should describe and classify them, and their diversity.T he 3D structure and function of biomolecules are intimately linked. One of the most outstanding attributes of enzymes is their ability to recognize similar molecules, such as isomers, selectively. For example, glutamate racemase catalyzes the interconversion between the isomers L-glutamate and D-glutamate, with the first being one of the 20 amino acids used to build proteins, whereas the second is an essential component of bacterial cell walls (1). Isomers of the same drug are often distinguished; for example, the tragic story of thalidomide unveiled how subtle changes in the spatial arrangement of atoms can have drastic consequences in their biological effect (2).The isomerases, which catalyze these interconversions, are involved in the central metabolism of most living organisms and have important applications in organic synthesis, biotechnology, and drug discovery (3-5). In comparison to other classes, isomerases are a small class involving unimolecular reactions, which are easy to analyze manually. The study of the biological mechanisms of isomerases provided fundamental insights into the electrostatic principles of enzyme catalysis (6) and helped to reveal the connection between host-parasite interactions and cancer (7). The challenges of automatically detecting stereoisomerization in reactions also make their chemistry technically interesting (8-11).A standard description of the biological function of genes and proteins is essential to interpret and report the outcome of sequencing initiatives. Scientists have traditionally developed elaborate classification systems to group functions in a hierarchical manner. Among the existing approaches, enzyme function is probably the best described at the molecular level, due to the longstanding effort of a team of enzymologists from the Enzyme Commission (EC) of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB) to classify enzyme function systematically. The EC classification is the most widely used system and uses...