The metabolism of living organisms can be understood as a network of biochemical reactions, generally catalyzed by enzymes. Dealing with this network as a whole is a complex task and a classical approach is to divide it into more manageable segments, called pathways. How this approach is applied is always somewhat arbitrary and depends upon the final usage of the data. Usually, a first level of segmentation is achieved on the basis of biological criteria. For instance, divisions could be achieved by considering the sub-network of all reactions involved in amino-acid biosynthesis or, more specifically, in l-lysine biosynthesis only, or even more specifically, in l-lysine biosynthesis via the AAA pathway. This results in a series of coarse- to fine-grained divisions (the coarsest is called a 'super-pathway').
