🌶️Pepperpedia
Community(soon)

Heme Synthesis

From Pepperpedia, the free peptide encyclopedia
Heme Synthesis
Properties
CategoryMechanisms
Also known asheme biosynthesis, porphyrin synthesis
Last updated2026-04-14
Reading time4 min read
Tags
mechanismmetabolism

Overview

Heme synthesis is an eight-step enzymatic pathway that builds the iron-containing porphyrin ring of heme from simple precursors: the amino acid glycine and the TCA cycle intermediate succinyl-CoA. Heme is an essential cofactor for hemoglobin, myoglobin, cytochromes of the respiratory chain, cytochrome P450 enzymes, catalase, peroxidases, nitric oxide synthase, and guanylyl cyclase. Heme synthesis is particularly active in developing erythrocytes (which produce enormous quantities of hemoglobin) and in liver (which synthesizes cytochromes for drug and endogenous metabolite processing).

The pathway is unusual in being distributed across two cellular compartments: it begins in the mitochondrion, moves to the cytoplasm for several steps, and returns to the mitochondrion for the final insertion of iron. This topology requires coordinated transport of intermediates and tight integration with mitochondrial function.

Inherited defects in heme synthesis produce the porphyrias — a family of diseases with variable presentations from acute neurovisceral attacks (acute intermittent porphyria) to severe photosensitivity (erythropoietic protoporphyria, porphyria cutanea tarda). Acquired defects (lead poisoning, iron deficiency) also affect the pathway.

Mechanism / Process

  1. Delta-aminolevulinic acid (ALA) synthesis. In mitochondria, ALA synthase (ALAS1 in nonerythroid tissues, ALAS2 in erythroid cells) condenses glycine and succinyl-CoA to form delta-aminolevulinic acid. This is the rate-limiting and regulated step. ALAS1 is feedback-inhibited by heme; ALAS2 is regulated by iron availability through a 5' iron-responsive element.

  2. ALA dehydratase. ALA is exported to the cytoplasm, where ALA dehydratase (ALAD) condenses two ALA molecules to form porphobilinogen (PBG). ALAD contains a zinc cofactor and is inhibited by lead.

  3. Hydroxymethylbilane synthesis. Porphobilinogen deaminase (hydroxymethylbilane synthase, HMBS) polymerizes four PBG molecules into the linear tetrapyrrole hydroxymethylbilane. Acute intermittent porphyria arises from HMBS deficiency.

  4. Uroporphyrinogen III synthesis. Uroporphyrinogen III synthase (UROS) cyclizes and isomerizes the tetrapyrrole to uroporphyrinogen III.

  5. Uroporphyrinogen decarboxylase. UROD decarboxylates all four acetyl side chains to methyl groups, producing coproporphyrinogen III. Deficiency causes porphyria cutanea tarda.

  6. Coproporphyrinogen oxidase. In the intermembrane space of mitochondria, CPOX decarboxylates two propionate side chains to vinyl groups, producing protoporphyrinogen IX.

  7. Protoporphyrinogen oxidase. PPOX oxidizes protoporphyrinogen IX to protoporphyrin IX.

  8. Ferrochelatase. The mitochondrial enzyme ferrochelatase (FECH) inserts ferrous iron into protoporphyrin IX to produce heme. Deficiency causes erythropoietic protoporphyria.

  9. Regulation. Heme represses ALAS1 transcription, destabilizes ALAS1 mRNA, and inhibits mitochondrial import of ALAS1. In erythroid cells, iron availability rather than heme regulates ALAS2 translation via iron regulatory proteins.

Key Players / Molecular Components

  • ALAS1 and ALAS2. Rate-limiting enzymes in nonerythroid and erythroid tissues.
  • ALAD, HMBS, UROS, UROD, CPOX, PPOX, FECH. Subsequent enzymes.
  • Heme oxygenase (HMOX1, HMOX2). Degrades heme to biliverdin, carbon monoxide, and iron.
  • Iron regulatory proteins. IRP1, IRP2; bind iron-responsive elements.
  • Mitochondrial importers. Mitoferrin delivers iron for ferrochelatase.

Clinical Relevance / Therapeutic Targeting

Porphyrias are classified as hepatic (acute intermittent porphyria, porphyria cutanea tarda, variegate porphyria, hereditary coproporphyria) or erythropoietic (congenital erythropoietic porphyria, erythropoietic protoporphyria). Treatments include avoiding triggers (drugs, alcohol, fasting), hemin infusions (suppress ALAS1), and newer therapies: givosiran is an siRNA targeting ALAS1 for acute hepatic porphyria. Lead poisoning inhibits ALAD and ferrochelatase, producing a characteristic porphyrin profile. Iron deficiency reduces heme synthesis in erythroid cells. Heme oxygenase induction by bilirubin or carbon monoxide has cytoprotective effects studied in cardiovascular and inflammatory disease.

Peptides That Target This Pathway

  • Hepcidin — iron-regulatory peptide that controls iron availability for heme synthesis.
  • Erythropoietin — drives erythroid proliferation and thus demand for heme synthesis.
  • Humanin — mitochondrial peptide with cytoprotective and iron-related effects.
  • MOTS-c — mitochondrial-derived peptide linked to metabolic regulation.
  • Thymosin beta-4 — supports erythroid maturation in some contexts.

Related entries

  • Amino Acid Catabolism— The pathways that dispose of dietary or mobilized amino acids by separating nitrogen for the urea cycle and channeling carbon skeletons into energy metabolism.
  • Bile Acid Synthesis— The hepatic conversion of cholesterol into bile acids, a major route of cholesterol disposal that produces signaling molecules regulating metabolism.
  • Mitochondrial Function— Mitochondrial function encompasses the energy-producing, signaling, and quality-control processes within mitochondria, including oxidative phosphorylation, ROS management, apoptosis regulation, and the emerging biology of mitochondrial-derived peptides.
  • Nucleotide Synthesis— The de novo and salvage pathways that produce the purine and pyrimidine nucleotides required for DNA, RNA, cofactors, and energy metabolism.