Haem synthesis is a fundamental anabolic pathway in mammalian cells, particularly erythrocytes, that produces the prosthetic group essential for oxygen transport in haemoglobin and electron transfer in cytochromes. This essay outlines the compartmentalised eight-step pathway, its mitochondrial-cytosolic transitions, rate-limiting regulation, and physiological significance, drawing on established biochemical principles encountered during undergraduate study of intermediary metabolism.
The Eight-Step Pathway and Cellular Compartments
Synthesis begins and concludes within mitochondria while intermediate reactions occur in the cytosol. The initial and committed step condenses succinyl-CoA with glycine to form δ-aminolevulinic acid (ALA), catalysed by ALA synthase (ALAS). Two isoenzymes exist: the housekeeping ALAS1 in liver and the erythroid-specific ALAS2. Subsequent condensation of two ALA molecules yields porphobilinogen, mediated by ALA dehydratase. Four porphobilinogen units are then polymerised by porphobilinogen deaminase to hydroxymethylbilane, which is rearranged and cyclised by uroporphyrinogen III synthase into uroporphyrinogen III. Decarboxylation generates coproporphyrinogen III; this intermediate is transported back into mitochondria where coproporphyrinogen oxidase produces protoporphyrinogen IX. Final oxidation by protoporphyrinogen oxidase yields protoporphyrin IX, after which ferrochelatase inserts ferrous iron to complete haem (Berg et al., 2019).
Regulation and Feedback Control
Control primarily resides at ALAS1, which is repressed by haem or its oxidised derivative hemin; excess haem also inhibits ALA dehydratase and ferrochelatase, preventing toxic porphyrin accumulation. In erythroid cells, regulation is additionally exerted via iron-responsive elements in ALAS2 mRNA, linking haem production to iron availability. These mechanisms illustrate the cell’s capacity to match haem output to demand for haemoproteins while avoiding oxidative damage from free porphyrins.
Integration with Metabolism and Clinical Relevance
The pathway consumes glycine and succinyl-CoA, thereby connecting haem formation to the tricarboxylic acid cycle and one-carbon metabolism. Deficiencies at specific enzymatic steps produce porphyrias, each characterised by accumulation of distinct porphyrin intermediates and characteristic clinical photosensitivity or neurovisceral symptoms. Iron deficiency or lead poisoning, which inhibits ALA dehydratase and ferrochelatase, similarly disrupts haem production and leads to anaemia, underscoring the pathway’s sensitivity to environmental and nutritional factors.
In summary, haem synthesis demonstrates precise subcellular organisation, tight feedback regulation, and intimate linkage to central metabolic routes. Appreciating these features equips students to understand both normal oxygen-carrying capacity and the biochemical basis of several inherited and acquired disorders.
References
- Berg, J.M., Tymoczko, J.L., Gatto, G.J. and Stryer, L. (2019) Biochemistry. 9th edn. New York: W.H. Freeman.
- Voet, D. and Voet, J.G. (2011) Biochemistry. 4th edn. Hoboken: Wiley.
