Heme biosynthesis enzymes (ALAS, ALAD, HMBS, UROS, UROD, CPOX, PPOX, FECH)

Heme biosynthesis enzymes consist of a sequence of eight enzymes that catalyze the production of heme from glycine and succinyl-CoA [StatPearls, 2023]. This pathway is essential for the formation of hemoproteins, including hemoglobin for oxygen transport, myoglobin for oxygen storage, and cytochromes for electron transport and drug metabolism [UniProt, 2024]. The pathway is primarily regulated at the rate-limiting step catalyzed by 5-aminolevulinate synthase (ALAS), which exists in two isoforms: ALAS1 (ubiquitous) and ALAS2 (erythroid-specific) [PubMed, PMID: 31747388]. Genetic mutations in any of these enzymes can lead to porphyrias, a group of rare metabolic disorders characterized by the accumulation of neurotoxic or phototoxic porphyrin precursors [NIDDK, 2022]. Pharmacological intervention often focuses on reducing the production of these toxic intermediates, such as using Givosiran to target ALAS1 mRNA or Hemin to exert feedback inhibition on the pathway [FDA, 2019]. Additionally, certain enzymes in this pathway are sensitive to environmental toxins; for example, lead potently inhibits ALAD and ferrochelatase, leading to clinical symptoms of lead poisoning [CDC, 2023].

Other names

Heme biosynthetic pathwayPorphyrin synthesis enzymes5-aminolevulinate synthase5-aminolevulinate dehydrataseHydroxymethylbilane synthaseUroporphyrinogen III synthaseUroporphyrinogen decarboxylaseCoproporphyrinogen III oxidaseProtoporphyrinogen IX oxidaseFerrochelatase

Therapeutic strategies include siRNA-mediated silencing of the rate-limiting enzyme ALAS1 to reduce toxic precursor accumulation, administration of exogenous heme to provide negative feedback on the pathway, and the use of enzyme inhibitors or substrate analogs to modulate metabolic flux [FDA, 2019; StatPearls, 2023].