ABSTRACT
The starting point for heme and chlorophyll synthesis is aminolaevulinic acid (ALA), which is made in animals from glycine and succinyl CoA by the enzyme ALA synthase. Two molecules of ALA then condense to form porphobilinogen in a reaction catalyzed by ALA dehydratase. Porphobilinogen deaminase catalyzes the condensation of four porphobilinogens to form a linear tetrapyrrole. This compound then cyclizes to form uroporphyrinogen III, the precursor of hemes, chlorophylls and vitamin B12. Further modifications take place to form protoporphyrin IX. The biosynthetic pathway then branches, and either iron is inserted to form heme, or magnesium is inserted to begin a series of conversions to form chlorophyll.
