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The pathway of heme synthesis. Between the two arms of the pathway, depicting the isomer I and isomer III porphyrin, is a box indicating the number of carboxyl substituents for each porphyrin. ALA, δ-aminolevulinic acid; PBG, porphobilinogen; URO, uroporphyrin; COPRO, coproporphyrin; PROTO, protoporphyrin.
