Porphyrins and the Heme Group

Introduction

Heme is one of nature's most versatile cofactors. The same iron porphyrin carries oxygen in blood, passes electrons along respiratory chains and activates O₂ to oxidise unreactive substrates. The porphyrin ring provides a rigid, strongly binding platform, and the protein decides what happens above and below it. This page describes the macrocycle, how iron sits within it and how axial ligands switch its function.

Core explanation

The macrocycle. A porphyrin is built from four pyrrole rings joined at their α-carbons by four methine (=CH−) bridges. The resulting ring is planar and highly conjugated, with an 18-electron aromatic pathway. When the two inner N−H protons are removed, the dianion presents four nitrogen donors pointing inwards to a central cavity well matched to first-row transition-metal ions. The macrocyclic and chelate effects make metal binding extremely strong and kinetically inert.

Protoporphyrin IX. Biological heme b uses protoporphyrin IX, which carries four methyl, two vinyl and two propionate substituents around the rim. The propionates are usually oriented towards the protein surface, where they form salt bridges and hydrogen bonds. In cytochromes c, the vinyl groups are covalently linked to cysteine residues as thioether bonds, which fixes the heme to the protein.

Iron in the ring. The four porphyrin nitrogens occupy the equatorial positions of an octahedron. Iron commonly exists as Fe(II) or Fe(III), and in catalytic intermediates as Fe(IV). Low-spin iron is small enough to sit in the porphyrin plane, whereas high-spin Fe(II) is larger and tends to sit a few tenths of an ångström out of the plane towards a fifth ligand. This movement becomes the trigger for cooperative oxygen binding in haemoglobin.

Axial ligands decide the job. The two axial positions are where the protein expresses control: - Globins (myoglobin, haemoglobin): one "proximal" histidine, leaving the sixth site open for reversible O₂ binding. - Cytochromes: two strong axial ligands such as histidine/histidine or histidine/methionine keep the iron six-coordinate and low spin, so it only transfers electrons. - Cytochrome P450 and chloroperoxidase: a cysteine thiolate "pushes" electron density to iron, helping to cleave O−O bonds. - Peroxidases and catalase: a histidine or tyrosinate ligand with a polar distal pocket suited to hydrogen peroxide.

Distal pocket. Residues on the open side, such as the distal histidine in globins, form hydrogen bonds to bound ligands, discriminate between O₂ and CO, and help or hinder access. The protein also protects the heme from forming μ-oxo Fe−O−Fe dimers, which occur rapidly for free iron porphyrins in water and destroy O₂-binding ability.

Colour. Porphyrins absorb strongly near 400-430 nm (the Soret band, molar absorptivity above 10⁵ M⁻¹ cm⁻¹) and more weakly at 500-600 nm (Q bands). Changes in iron oxidation state, spin and axial ligation shift these bands, giving arterial blood its bright red colour and venous blood its darker hue, and making UV–visible spectroscopy a powerful tool for heme proteins.

Step-by-step reasoning

1. Identify the heme type (b, c or others) and whether it is covalently attached. 2. Determine the iron oxidation state and spin state from evidence. 3. Identify the proximal axial ligand. 4. Check whether the sixth site is occupied by a protein ligand, water, or open. 5. Use axial ligation and distal pocket polarity to predict the function: transport, electron transfer or catalysis.

Visual explanation

Draw a square of four pyrrole rings with the nitrogens pointing to a central Fe. Show the ring edge-on underneath: a flat line with iron slightly below it and a histidine imidazole beneath, and an O₂ molecule bound above at an angle. Add a second edge-on sketch with histidine above and methionine below for a cytochrome.

Real-world analogy

The porphyrin is like a standard power socket fitted into many different appliances. The socket always holds the same plug (iron) firmly, but the wiring above and below — the axial ligands and pocket — decides whether the appliance is a kettle, a lamp or a heater. The analogy is limited: the "wiring" also changes the iron's electronic state, not just what it is connected to.

Real-world example

Chlorophyll is a closely related magnesium chlorin, and vitamin B₁₂ contains cobalt in a corrin ring. Nature reuses the tetrapyrrole theme with different metals and ring saturation. The shared biosynthetic pathway branches at protoporphyrin IX: insertion of Fe²⁺ by ferrochelatase gives heme, and insertion of Mg²⁺ leads to chlorophyll.

Why?

Why do free iron porphyrins in water fail to carry oxygen reversibly? Fe(II)–O₂ reacts with a second Fe(II) porphyrin to form a peroxo-bridged dimer, which breaks down to Fe(III) species and a μ-oxo dimer. Globins bury the heme and sterically prevent two hemes approaching, blocking this irreversible oxidation.

Common misconception

"Heme always binds oxygen." Most hemes in cytochromes have both axial positions filled by protein ligands and never bind O₂; their job is electron transfer. Another error is to treat heme and haemoglobin as the same thing: heme is the cofactor, haemoglobin the whole protein.

Worked example

Question: A heme protein shows a sixth-site-free, high-spin Fe(II) centre with a proximal histidine. Suggest its likely function.

Reasoning: An open sixth site and a neutral histidine ligand match the globin pattern; electron-transfer cytochromes would be six-coordinate and low spin, and P450 would have a thiolate.

Answer: It is most likely an O₂-binding globin.

Quick check

1. Which axial ligand distinguishes cytochrome P450 from globins? Answer: P450 has a cysteine thiolate proximal ligand, whereas globins use a histidine.

Exam focus

Describe the porphyrin as a planar, aromatic, tetradentate dianionic ligand. Link axial ligand patterns to function. Explain in-plane versus out-of-plane iron with spin state, and name the Soret band as the intense near-UV absorption.

Advanced insight

Porphyrin rings are not innocent: they can be oxidised to a π-cation radical, storing an oxidising equivalent on the ligand. In peroxidase Compound I and P450 Compound I, the two-electron-oxidised state is described as Fe(IV)=O with a porphyrin or thiolate-influenced radical. Heme ruffling and saddling distortions imposed by the protein also shift potentials and spectra.

Summary

The porphyrin is a rigid aromatic macrocycle whose four nitrogens hold iron in the equatorial plane. Protoporphyrin IX iron forms heme b; heme c is covalently bound. Axial ligands and the distal pocket determine whether heme transports O₂, transfers electrons or activates oxidants. Spin state controls iron's position relative to the plane, and the intense Soret band makes heme proteins strongly coloured.

Practice questions

1. How many donor atoms does the porphyrin dianion provide and where are they located? Answer: Four nitrogen donors, one from each pyrrole ring, in the equatorial plane around the metal. 2. Why does high-spin Fe(II) sit out of the porphyrin plane? Answer: Its occupied e g-type orbitals make it too large for the central cavity, so it is displaced towards the axial ligand. 3. Give the typical axial ligands of cytochrome c and explain why they suit electron transfer. Answer: Histidine and methionine; both sites are filled, keeping iron six-coordinate and low spin, so it changes oxidation state with minimal structural change. 4. What role do globin proteins play in protecting heme? Answer: They bury the heme in a hydrophobic pocket and prevent two hemes meeting, blocking irreversible μ-oxo dimer formation and oxidation.