This final chapter analyzes the chemical logic in the tetrapyrrole biosynthetic pathway to heme B with focus on how the aminomethylpyrrole heterocycle of prophobilinogen is assembled and then utilized to build the tetrapyrrole macrocycles of heme and chlorphyll. Enzymatic steps include the formation of the porphobilinogen building block by aldol condensative dimerization of 4-aminolevulinate, its chain extension to the linear tetrapyrrole hydroxybilane, and its enzyme-directed cyclization to uroporphyrinogen III. Subsequently the tailoring of the macrocycle periphery involves loss of the six of the eight peripheral carboxylates as CO₂, six-electron oxidation of the macrocycle to a 20 π-electron system and ferrous iron insertion to give heme B. The aminopyrrole framework of porphobilinogen serves both as electrophile via a C₂ azafulvene form and as a nucleophile at C₅. Fourteen of the starting 48.