Hypergraph representation of the Urea Cycle shown in Figure 1. Purple nodes represent compounds and grey nodes are hyperedges representing reactions. They specify the catalyzing enzyme as an attribute. For each reaction, the incoming arrows represent the input compounds and the outgoing arrows represent the output compounds. Note that a reversible reaction (e.g. reaction R00557) is represented by a forward reaction (grey node with label R00557) and a backward one (grey node with label R00557rev).