Illustrating the respective similarities found in alignable metabolic pathways. The chemical structures of the substrate and product of each enzyme are aligned to underline their respective similarities in the step catalyzed by the successive set of enzymes located in boxes numbered 1 to 3. The reaction modules described in the text are framed in light gray arrows labelled A (purine catabolism), B (pyrimidine catabolism), and C (pyrimidine anabolism). Although many reactions are reversible, the arrow orientation indicates the main direction found in vivo. The enzymes located in the same coloured box were found to be homologous. See list of abbreviations.