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Iron Protoporphyrin (Heme) – Function and Importance

Iron protoporphyrin, commonly known as heme, is an iron-containing compound essential for oxygen transport in the blood and cellular energy production.

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Things worth knowing about "Iron Protoporphyrin"

Iron protoporphyrin, commonly known as heme, is an iron-containing compound essential for oxygen transport in the blood and cellular energy production.

What is Iron Protoporphyrin?

Iron protoporphyrin, also referred to as heme (or haem in British English) and chemically known as ferroprotoporphyrin IX, is a compound consisting of a protoporphyrin IX ring with a centrally coordinated iron(II) ion (Fe²⁺). It serves as the functional, iron-containing group of a variety of essential proteins called hemoproteins. Iron protoporphyrin is best known as the core component of hemoglobin in red blood cells, where it enables the transport of oxygen throughout the body.

Biological Functions

Iron protoporphyrin performs several critical functions in the human body:

Oxygen transport: As the prosthetic group of hemoglobin, iron protoporphyrin reversibly binds oxygen in the lungs and releases it to tissues and organs.
Electron transport: In cytochromes of the mitochondrial electron transport chain, heme facilitates the transfer of electrons essential for cellular energy production (ATP synthesis).
Enzymatic activity: Heme is a key component of enzymes such as catalase and peroxidase, which protect cells from oxidative damage.
Oxygen storage in muscle: As part of myoglobin in muscle tissue, iron protoporphyrin stores oxygen and releases it during periods of increased physical demand.

Biosynthesis of Iron Protoporphyrin

The biosynthesis of heme is a multi-step biochemical process taking place in both the mitochondria and the cytoplasm of the cell. Key steps include:

Formation of delta-aminolevulinic acid (ALA) from glycine and succinyl-CoA as the initial building block.
Sequential condensation and ring closure reactions producing porphobilinogen, then uroporphyrinogen, and ultimately protoporphyrin IX.
Insertion of a ferrous iron ion (Fe²⁺) into the completed porphyrin ring by the enzyme ferrochelatase.

Defects in any of these enzymatic steps lead to a group of metabolic disorders known as porphyrias.

Iron Protoporphyrin and Iron Metabolism

The majority of iron in the human body exists in bound form as iron protoporphyrin (heme iron), primarily within hemoglobin. Heme iron, derived from animal-based foods such as meat and fish, is absorbed in the intestine considerably more efficiently than non-heme iron found in plant-based foods. The bioavailability of heme iron is approximately 25-35 %, compared to only 2-20 % for non-heme iron.

Clinical Relevance

Iron Deficiency Anemia

When iron is insufficient, the body cannot synthesize adequate amounts of iron protoporphyrin, leading to reduced hemoglobin production and iron deficiency anemia. Typical symptoms include fatigue, pallor, shortness of breath, and diminished physical performance. A useful diagnostic marker in this context is zinc protoporphyrin (ZPP): when iron is unavailable, zinc is substituted into the porphyrin ring instead, resulting in elevated ZPP levels in the blood.

Porphyrias

Porphyrias are a group of inherited or acquired metabolic disorders caused by dysfunctional enzymes in the heme biosynthesis pathway. This leads to the accumulation of heme precursors in the body, causing symptoms such as severe abdominal pain, neurological disturbances, and photosensitivity of the skin.

Diagnostic Use

Measurement of iron protoporphyrin and related compounds -- such as free erythrocyte protoporphyrin (FEP) -- is used in laboratory diagnostics to identify iron deficiency, lead poisoning, and disorders of porphyrin metabolism.

Iron Protoporphyrin in the Diet

Heme iron is found exclusively in animal-derived foods, including:

Red meat (beef, lamb, pork)
Poultry
Fish and seafood
Organ meats such as liver and kidney

Vegetarians and vegans do not consume heme iron and must rely on non-heme iron from plant-based sources. The absorption of non-heme iron can be enhanced by consuming it alongside vitamin C.

References

Ponka, P. - Cell biology of heme. In: American Journal of the Medical Sciences, 318(4):241-256, 1999. PubMed PMID: 10522552.
World Health Organization (WHO) - Iron Deficiency Anaemia: Assessment, Prevention and Control. WHO/NHD/01.3, Geneva, 2001.
Puy, H., Gouya, L., Deybach, J.C. - Porphyrias. In: The Lancet, 375(9718):924-937, 2010. DOI: 10.1016/S0140-6736(09)61107-2.

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