Nonheme Iron Enzymes

Introduction

Haem is not the only way to activate oxygen with iron. Many enzymes bind iron directly to amino-acid side chains, with no porphyrin ring at all. These nonheme iron enzymes hydroxylate amino acids in collagen, sense oxygen levels, cleave aromatic rings, make antibiotics, convert methane to methanol and generate the radical needed to build DNA. Without the rigid porphyrin, iron has more open coordination sites and a flexible ligand set, which opens up reaction types unavailable to haem.

Core explanation

The facial triad. The most common mononuclear nonheme iron site uses two histidines and one aspartate or glutamate, bound to one face of an octahedron. This 2-His-1-carboxylate facial triad leaves the three positions on the opposite face available for water, co-substrate and O₂. Iron is held as high-spin Fe(II) in the resting state.

2-Oxoglutarate-dependent oxygenases. This large family uses 2-oxoglutarate (2OG, also called α-ketoglutarate) as a co-substrate. The overall reaction is:

R–H + 2OG + O₂ → R–OH + succinate + CO₂

The mechanism is ordered:

1. 2OG binds to Fe(II) as a bidentate ligand through its keto oxygen and carboxylate, displacing water. 2. The primary substrate binds nearby (not to iron), triggering release of the last water ligand. The five-coordinate iron is now primed for O₂. This gating limits unproductive O₂ activation. 3. O₂ binds and attacks the keto carbon of 2OG. Decarboxylation releases CO₂ and succinate and forms an Fe(IV)=O (ferryl) intermediate. 4. The ferryl abstracts a hydrogen atom from R–H, giving Fe(III)–OH and a substrate radical. 5. Rebound of the hydroxyl gives R–OH and returns iron to Fe(II).

The two electrons needed to cleave O₂ come from the co-substrate, not from an external reductase as in P450. One O atom goes into succinate, the other into the product. The high-spin (S = 2) Fe(IV)=O intermediate was first trapped in the enzyme TauD.

Variations on the theme. Halogenases use the same chemistry but with chloride bound to iron; after hydrogen abstraction the substrate radical captures chlorine instead of hydroxyl. Extradiol catechol dioxygenases use Fe(II) and the facial triad to cleave aromatic rings. Intradiol dioxygenases instead use Fe(III) to activate the substrate rather than O₂. Lipoxygenases use an Fe(III)–OH to abstract hydrogen from fatty acids before O₂ adds.

Diiron enzymes. Some enzymes use two iron atoms bridged by carboxylates. In soluble methane monooxygenase , a diiron(II) site binds O₂ to form a peroxo species and then intermediate Q, a diiron(IV) unit capable of breaking the very strong C–H bond of methane (about 440 kJ mol⁻¹). Class I ribonucleotide reductase uses a diiron site to generate a stable tyrosyl radical that starts the conversion of ribonucleotides to deoxyribonucleotides. Hemerythrin , an O₂ carrier in some marine worms, binds O₂ reversibly at a diiron site as hydroperoxide.

Comparing with haem. Haem enzymes store the second oxidising equivalent on the porphyrin ring (Compound I). Nonheme enzymes lack this redox-active ligand, so they use co-substrates, a second iron or a nearby tyrosine to supply or store electrons.

Step-by-step reasoning

1. Identify the iron ligands: a facial triad, other protein ligands, or a bridged diiron unit. 2. Find the electron source for O₂ activation: 2OG, a reductant, a second metal or the substrate itself. 3. Identify the oxidant formed: Fe(IV)=O, diiron(IV), or Fe(III)–superoxo. 4. Follow hydrogen abstraction and the fate of the radical: rebound, halogen transfer, desaturation or ring cleavage.

Visual explanation

Draw an octahedron with iron at the centre. Shade one face and label its three corners His, His and Asp. On the opposite face, draw 2OG as a two-pronged ligand and an empty site where O₂ enters. Then draw an arrow to Fe(IV)=O with succinate and CO₂ released, and a final arrow to R–OH.

Real-world analogy

A haem enzyme is like a power tool with a built-in battery (the porphyrin stores charge). A nonheme iron enzyme is like a tool that needs a disposable cartridge each time: 2OG is consumed in every cycle to power the oxygen-activation step.

Real-world example

Collagen prolyl hydroxylase is a 2OG-dependent enzyme that adds hydroxyl groups to proline, stabilising the collagen triple helix. If the iron is accidentally oxidised to Fe(III) in an uncoupled cycle, ascorbate (vitamin C) reduces it back. This is why vitamin C deficiency leads to weak collagen and the symptoms of scurvy. Related prolyl hydroxylases act as cellular oxygen sensors that control the hypoxia response.

Why?

Why does substrate binding trigger loss of water before O₂ binds? If O₂ could react with iron and 2OG without substrate present, the ferryl would form with nothing to oxidise and could damage the enzyme. Coupling the open site to substrate binding ensures oxidising power is generated only when it can be used.

Common misconception

"Without a porphyrin, iron cannot reach the Fe(IV) state." Nonheme enzymes form Fe(IV)=O units routinely. They simply use different sources of electrons, and their ferryls are typically high-spin rather than the intermediate-spin species found in haem enzymes.

Worked example

Question: A 2OG-dependent hydroxylase is supplied with ¹⁸O₂. Where does the label appear, and how many electrons does 2OG supply?

Reasoning: O₂ is split; one atom is inserted into the substrate hydroxyl and the other ends up in succinate carboxylate. Oxidative decarboxylation of 2OG to succinate plus CO₂ is a two-electron oxidation, providing the two electrons that, with the two from R–H, complete the four-electron reduction of O₂.

Answer: One ¹⁸O in R–OH and one in succinate; 2OG supplies two electrons.

Quick check

1. Which three amino-acid ligands form the common nonheme facial triad, and what does the triad leave available? Answer: Two histidines and one aspartate or glutamate; they leave three sites on the opposite face open for water, 2OG and O₂.

Exam focus

Draw the facial triad and describe its open sites. Write the 2OG-oxygenase equation and outline the mechanism through Fe(IV)=O, hydrogen abstraction and rebound. Compare with P450: electron source, spin state and where the second oxidising equivalent is stored. Name a diiron enzyme and its role.

Advanced insight

Nonheme ferryl reactivity depends strongly on spin state: high-spin (S = 2) Fe(IV)=O units have low-lying acceptor orbitals that make hydrogen abstraction easier than for S = 1 synthetic analogues. Mössbauer spectroscopy, with isomer shifts near 0.3 mm s⁻¹, identifies these intermediates. The balance between hydroxyl rebound and alternative radical pathways lets related enzymes perform hydroxylation, halogenation, desaturation and ring formation from a common intermediate.

Summary

Nonheme iron enzymes bind iron to protein side chains, often through a 2-His-1-carboxylate facial triad that leaves three sites open. 2OG-dependent oxygenases use co-substrate decarboxylation to generate a high-spin Fe(IV)=O intermediate that hydroxylates or halogenates C–H bonds. Diiron enzymes such as methane monooxygenase and ribonucleotide reductase activate O₂ with two metal ions. Substrate-triggered gating prevents wasteful oxidant formation.

Practice questions

1. Write the overall equation for a 2OG-dependent hydroxylation. Answer: R–H + 2OG + O₂ → R–OH + succinate + CO₂. 2. How does a nonheme halogenase produce R–Cl instead of R–OH? Answer: Chloride is bound to the iron; after hydrogen abstraction the substrate radical captures the chlorine ligand rather than the hydroxyl. 3. Why do vitamin C deficiencies affect collagen? Answer: Ascorbate reduces inactive Fe(III) in prolyl hydroxylase back to Fe(II); without it, proline hydroxylation fails and collagen is unstable. 4. What oxidant does soluble methane monooxygenase use to break the C–H bond of methane? Answer: Intermediate Q, a diiron(IV) species formed from O₂ at the carboxylate-bridged diiron site.