Fig. 1

a 1,4-Butanediol is an example of a non-natural compound that is produced using a heterologous pathway generated by an pathway-identification algorithm. These heterologous genes from organisms differing from the host are dashed. Intermediates of the citric acid cycle were used as starting compounds in a pathway search restricted by path length toward the desired target, 1,4-butanediol. b Graph-based networks can be described as either directed, bipartite, or hypergraphs. In directed graphs, nodes (open circles) and edges (arrows) can represent either compounds or reactions. In the case of bipartite graphs, the pathway will be composed of nodes that alternate between compounds and reactions. For hypergraphs, groups of multiple compounds can be linked through a single hyperedge that may represent a reaction rule that links compounds that are associated with multiple additional reaction rules