Biosynthesis — enzymatic N₂ fixation
Nitrogenase: biological N₂ reduction
How the Nitrogenase works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: N₂, 8 H⁺, 8 e⁻, 16 MgATP, MoFe-protein (FeMo-co) + Fe-protein. Conditions: Aqueous, pH ~7, 25–30 °C, 1 atm N₂, strictly anaerobic. Bonds formed: 2 NH₃; H₂; 16 ADP + 16 Pi; bonds broken: N≡N; ATP (P–O). Stereochemistry: No stereocentres. Rate and selectivity: Slow (~5 s⁻¹); electron transfer from Fe-protein is rate-limiting.
At a glance
Step 1: N₂ binds the FeMo-cofactor
N₂ binds to the enzyme's iron-molybdenum cofactor (FeMo-co), a complex iron-sulfur-molybdenum cluster — the biological counterpart to N₂ binding on an iron catalyst surface in the industrial Haber process, just achieved at room temperature instead of high pressure and heat.
Step 2: Stepwise e⁻ / H⁺ transfers
A remarkable series of coupled proton and electron transfers (delivered stepwise from an external electron-transfer chain, ultimately powered by ATP hydrolysis) progressively reduce the bound N₂, breaking its formidable triple bond one electron pair at a time — mechanistically related to the stepwise surface hydrogenation of the Haber process, but happening at a single, precisely tuned metal cluster instead of an extended metal surface.
Step 3: 2 NH₃ and H₂ released
After eight total electron/proton transfers, two molecules of ammonia are released along with one equivalent of H₂ — nature's solution to the same nitrogen-fixation problem the Haber process solves industrially, achieved with a sophisticated metal cluster instead of brute-force heat and pressure.
Abbreviations and terms
- M
- Metal. A generic metal centre.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- ATP
- Adenosine triphosphate. The cell’s energy currency; transfers phosphate groups.
- ADP
- Adenosine diphosphate. ATP after losing one phosphate.
How it compares with related reactions
| Nitrogenase | Glycosyltransferase | Cytochrome P450 | |
|---|---|---|---|
| Reagents | N₂, 8 H⁺, 8 e⁻, 16 MgATP, MoFe-protein (FeMo-co) + Fe-protein | UDP-glucose donor, acceptor R–OH, glycosyltransferase (GT), Mn²⁺ or Mg²⁺ | Cytochrome P450 (heme-thiolate), O₂, NADPH (2 e⁻ via reductase), 2 H⁺, substrate R–H |
| Conditions | Aqueous, pH ~7, 25–30 °C, 1 atm N₂, strictly anaerobic | Aqueous buffer, pH ≈ 7.5, 37 °C | Aqueous, pH ~7.4, 37 °C, membrane-bound or soluble enzyme |
| Stereochemistry | No stereocentres | Inverting GT: single SN2-like step (α → β); retaining GT: double displacement, net retention | Rebound is fast, so configuration is often largely retained |
| Rate / selectivity | Slow (~5 s⁻¹); electron transfer from Fe-protein is rate-limiting | Metal-assisted UDP departure; kcat typically 1–100 s⁻¹ | Compound I formation fast; C–H abstraction is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Nitrogenase?
N₂, 8 H⁺, 8 e⁻, 16 MgATP, MoFe-protein (FeMo-co) + Fe-protein. Conditions: Aqueous, pH ~7, 25–30 °C, 1 atm N₂, strictly anaerobic.
Which bonds are formed and which are broken?
Formed: 2 NH₃; H₂; 16 ADP + 16 Pi. Broken: N≡N; ATP (P–O).
What is the stereochemical outcome?
No stereocentres
What controls the rate or the selectivity?
Slow (~5 s⁻¹); electron transfer from Fe-protein is rate-limiting
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make