We have followed this classical approach in the UniPathway project (a collaborative project with the INRIA and the LECA), and have, whenever possible, further refined this first-level segmentation to a second-level one, in order to split the pathways into linear segments (i.e. sub-networks without branches) called 'sub-pathways'. Such a fine-grained segmentation allows representation of pathway variants. Indeed, depending on the organism (or set of organisms), the chemical route from one compound to another can be performed in different ways. It is important to represent these variations within the same pathway since UniProtKB covers a large number of species. In addition, it offers a convenient way to label the enzymatic reactions that constitute a metabolic pathway by their relative position ('step') in the sub-pathway.
