What is the Difference Between Porphyrin and Protoporphyrin

The main difference between porphyrin and protoporphyrin is that porphyrin is a general term for a class of cyclic organic molecules, while protoporphyrin specifically refers to a stage in the synthesis of heme, lacking the central metal ion that characterizes the fully formed heme molecule.

Porphyrins and protoporphyrins are integral components in biochemistry. Protoporphyrin is a subtype within the broader category of porphyrins. Both are involved in essential processes such as oxygen transport (as seen in heme) and photosynthesis (as seen in chlorophyll).

Key Areas Covered

1. What is Porphyrin 
      – Definition, Features, Role
2. What is Protoporphyrin
      – Definition, Features, Role
3. Similarities Between Porphyrin and Protoporphyrin
      – Outline of Common Features
4. Difference Between Porphyrin and Protoporphyrin
      – Comparison of Key Differences
5. FAQ: Porphyrin and Protoporphyrin
      – Frequently Asked Questions

Key Terms

Porphyrin, Protoporphyrin

Difference Between Porphyrin and Protoporphyrin - Comparison Summary

What is Porphyrin

Porphyrins are a class of organic compounds with a distinct cyclic structure, characterized by a tetrapyrrole ring. The most well-known porphyrin is heme, a key component of hemoglobin responsible for oxygen transport in the blood. The core structure of porphyrins consists of four pyrrole rings linked by methine bridges, forming a planar macrocycle. The central cavity of this macrocycle can coordinate with metal ions, giving rise to metalloporphyrins. The most common metal associated with porphyrins is iron, as seen in heme, but other metals like magnesium, zinc, and copper can also bind to the porphyrin ring, influencing the properties and functions of the resulting complexes.

Porphyrin vs Protoporphyrin

Figure 1: Porphyrin

One of the remarkable features of porphyrins is their ability to undergo complexation reactions with various metals, leading to the formation of metalloporphyrin complexes. These complexes exhibit unique electronic, optical, and catalytic properties. For instance, the color of porphyrins is deeply influenced by the central metal ion and its oxidation state, making them valuable for applications such as dye-sensitized solar cells and as chromophores in sensors.

The aromatic nature of the porphyrin ring system contributes to its stability and electron delocalization. This aromaticity is a result of the cyclic conjugation of π-electrons, which imparts interesting electronic properties. Porphyrins are also known for their involvement in electron transfer processes. This makes them essential components in biological electron transport chains.

In addition to their biological relevance, porphyrins have applications in various fields, including catalysis, sensing, and medical imaging. Their ability to bind to specific molecules and ions makes them versatile tools for designing molecular sensors and diagnostic agents. Furthermore, the catalytic activity of metalloporphyrins has been harnessed for diverse chemical transformations, such as oxidation reactions in industrial processes and environmental remediation.

What is Protoporphyrin

Protoporphyrin is a crucial molecule in the realm of biochemistry, playing a central role in the synthesis of heme, an essential component of hemoglobin, myoglobin, and various cytochromes. Its significance lies in its ability to coordinate with iron ions, forming the heme prosthetic group. Furthermore, this coordination is pivotal for the function of hemoproteins involved in oxygen transport, storage, and electron transfer processes within living organisms.

The biosynthetic pathway leading to protoporphyrin involves a series of enzymatic reactions that take place in the mitochondria and cytoplasm. Various intermediates, such as aminolevulinic acid (ALA) and porphobilinogen, undergo sequential modifications, eventually yielding protoporphyrin IX. The incorporation of iron into protoporphyrin is a key step and is tightly regulated to ensure the proper functioning of heme-containing proteins.

Compare Porphyrin and Protoporphyrin

Figure 2: Protoporphyrin

Beyond its role in heme synthesis, protoporphyrin has garnered attention in medical applications. Protoporphyrin IX, in particular, exhibits fluorescence properties when exposed to certain wavelengths of light. This feature has also been exploited in photodynamic therapy (PDT) for the treatment of certain cancers and other diseases. Furthermore, in PDT, a photosensitizing agent, often derived from protoporphyrin, is administered and selectively accumulates in target tissues. Subsequent exposure to light activates the photosensitizer, leading to the generation of reactive oxygen species and cell death.

Similarities Between Porphyrin and Protoporphyrin

  • Porphyrin and protoporphyrin share a common cyclic structure.
  • Both compounds are biologically significant and play crucial roles in living organisms.

Difference Between Porphyrin and Protoporphyrin


Porphyrins are organic compounds composed of four pyrrole rings linked together, forming a macrocyclic structure. Protoporphyrins, on the other hand, are specific types of porphyrins that include a central metal ion, such as iron and are crucial components of heme groups found in hemoglobin and myoglobin.

Metal Ion

Moreover, porphyrins can bind to various metal ions, while protoporphyrins specifically bind to a metal ion at the center of the macrocycle.


Porphyrins can have different functions depending on the specific type and the metal ion they bind. Protoporphyrins, because of their role in heme, are crucial for oxygen binding and transport in organisms.

FAQ: Porphyrin and Protoporphyrin

Is protoporphyrin a porphyrin?

Yes. Protoporphyrin is a porphyrin.

Is Haem an example of protoporphyrin?

Yes. Haem is an example of protoporphyrin.

Is chlorophyll a porphyrin?

Chlorophyll’s central structure is an aromatic porphyrin or chlorin (reduced porphyrin) ring system with a sequestered magnesium atom.


Porphyrin is a general term for a class of cyclic organic molecules, while protoporphyrin specifically refers to a stage in the synthesis of heme, lacking the central metal ion that characterizes the fully formed heme molecule. Thus, this is the main difference between porphyrin and protoporphyrin.


1. “Protoporphyrin_IX.” Wikipedia. Wikipedia Foundation.
2. “Porphyrin.” Wikipedia. Wikipedia Foundation. 

Image Courtesy:

1. “Porphyrin” By Lukáš Mižoch – Own work (Public Domain) via Commons Wikimedia
2. “PPIXtransH” By Smokefoot – Own work (CC BY-SA 4.0) via Commons Wikimedia

About the Author: Hasini A

Hasini is a graduate of Applied Science with a strong background in forestry, environmental science, chemistry, and management science. She is an amateur photographer with a keen interest in exploring the wonders of nature and science.

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