Figure 1

Connectivity distribution P(k) for the raw (rPIN), filtered (fPIN) and enzymes-only ePIN (A) as well as EIN, EIN from KEGG-Maps, mapEIN (B) and CIN (C). All distributions show approximately power law behavior P(k) ≈ k^-γ, where k is the degree of a node, and the slope γ representing the scale-free exponent given in the graphs with some noteworthy deviations for the rPIN and the EIN. The EIN*, a variant of EIN that includes interactions derived from relations (D) between small ubiquitously occurring molecules such as H⁺, NH₃, H₂O, CO₂ and metals as well as co-enzymes and co-substrates, such as CoA, NADH+, FAD, SAM (see Additional File 4), is characterized by a distribution deviating from the power law distribution (red oval) for high connectivity values, k. The solid lines correspond to linear regression lines applied to the raw data in log-log scale with the associated linear equation indicated in the graph.