Biosynthesis — radical 1,2-migration
Vitamin B12-dependent radical rearrangement
How the Vitamin B12-dependent radical rearrangement 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: Adenosylcobalamin (AdoCbl) cofactor; methylmalonyl-CoA mutase; substrate methylmalonyl-CoA. Conditions: Aqueous, pH ~7, 37 °C, enzyme active site. Bonds formed: 5′-deoxyadenosyl radical; C–C (migrated); Co–C (regenerated); bonds broken: Co–C (homolysis); substrate C–H; migrating C–C. Stereochemistry: Stereospecific 1,2-acyl shift inside the enzyme. Rate and selectivity: Substrate binding triggers Co–C homolysis (~10¹² acceleration).
At a glance
Step 1: Co–C homolysis
Adenosylcobalamin's weak Co–C bond homolyzes, generating a stabilized cob(II)alamin and a highly reactive 5′-deoxyadenosyl radical — one of the few enzymatic cofactors that generates a genuine carbon radical on demand.
Step 2: H abstraction, then radical 1,2-acyl shift
This radical abstracts a hydrogen atom from the substrate (methylmalonyl-CoA), generating a substrate radical that then undergoes an unusual radical 1,2-migration — a carbonyl-containing group migrates from one carbon to an adjacent radical center, a radical-based cousin of the ionic 1,2-shifts seen in the pinacol and Meinwald rearrangements.
Step 3: Radical relay returns H; cobalt recaptures
The rearranged radical abstracts a hydrogen back from the adenosyl group, regenerating the adenosyl radical to be recaptured by cobalt and giving succinyl-CoA — this remarkable radical relay lets the enzyme perform a skeletal rearrangement that would be essentially impossible through ordinary ionic chemistry.
Abbreviations and terms
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
- CoA
- Coenzyme A. Thiol carrier that forms high-energy thioesters (acetyl-CoA).
How it compares with related reactions
| Vitamin B12-dependent radical rearrangement | Cytochrome P450 | DNA glycosylase | |
|---|---|---|---|
| Reagents | Adenosylcobalamin (AdoCbl) cofactor; methylmalonyl-CoA mutase; substrate methylmalonyl-CoA | Cytochrome P450 (heme-thiolate), O₂, NADPH (2 e⁻ via reductase), 2 H⁺, substrate R–H | DNA glycosylase (e.g. uracil-DNA glycosylase), H₂O |
| Conditions | Aqueous, pH ~7, 37 °C, enzyme active site | Aqueous, pH ~7.4, 37 °C, membrane-bound or soluble enzyme | Physiological pH, 37 °C |
| Stereochemistry | Stereospecific 1,2-acyl shift inside the enzyme | Rebound is fast, so configuration is often largely retained | Inversion or retention at C1′ depending on enzyme |
| Rate / selectivity | Substrate binding triggers Co–C homolysis (~10¹² acceleration) | Compound I formation fast; C–H abstraction is rate-limiting | Base flipping and oxocarbenium-like TS control rate |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Vitamin B12-dependent radical rearrangement?
Adenosylcobalamin (AdoCbl) cofactor; methylmalonyl-CoA mutase; substrate methylmalonyl-CoA. Conditions: Aqueous, pH ~7, 37 °C, enzyme active site.
Which bonds are formed and which are broken?
Formed: 5′-deoxyadenosyl radical; C–C (migrated); Co–C (regenerated). Broken: Co–C (homolysis); substrate C–H; migrating C–C.
What is the stereochemical outcome?
Stereospecific 1,2-acyl shift inside the enzyme
What controls the rate or the selectivity?
Substrate binding triggers Co–C homolysis (~10¹² acceleration)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make