Radical SAM Enzyme Mechanisms in Biosynthetic Pathways

Radical S-adenosylmethionine (SAM) enzymes represent a large and versatile superfamily that harnesses the chemical potential of a [4Fe-4S] cluster and SAM to generate highly reactive radicals. These radicals initiate an array of regio- and stereospecific transformations in the biosynthesis of cofactors, antibiotics, peptide natural products and complex secondary metabolites. Central to the mechanism is the reductive cleavage of the SAM S–C5′ bond to form a 5′-deoxyadenosyl radical, often via an organometallic Fe–C intermediate denoted Ω. This intermediate controls the release of the radical equivalent, ensuring precision in hydrogen-atom abstraction from the substrate. Subsequent radical rearrangements or group transfers produce chemically challenging modifications, including carbon–carbon bond formation, methylation of unactivated centres and deamination. Auxiliary domains such as SPASM or Twitch extend catalytic scope by coordinating additional iron–sulfur clusters that guide peptide maturation or cofactor assembly. Collectively, radical SAM enzymes underpin many natural product pathways and offer templates for biocatalytic innovation in drug discovery and green chemistry.

Research from Nature Portfolio

A seminal study on the B12-dependent radical SAM methyltransferase TsrM has unveiled an unprecedented radical-based mechanism for C-methylation in thiostrepton A biosynthesis. Contrary to paradigms of substrate hydrogen-atom abstraction, TsrM exploits cob(II)alamin as a key intermediate and avoids typical radical-initiated deprotonation. Structural and kinetic evidence supports formation of methylcob(III)alamin and an alternative Fe-S-mediated transfer pathway, greatly expanding the known chemical repertoire of the radical SAM superfamily.

Radical SAM Enzyme Mechanisms in Biosynthetic Pathways publication trend

The graph below shows the total number of articles in radical sam enzyme mechanisms in biosynthetic pathways across all publications each year (not limited to Nature Index journals).

Technical terms

Radical SAM enzyme: Enzyme that uses SAM and a [4Fe-4S] cluster to generate radical intermediates for catalysis.

S-adenosylmethionine (SAM): Common co-substrate whose reductive cleavage yields radical species essential for diverse biosynthetic transformations.

[4Fe-4S] cluster: Iron–sulfur cofactor that supplies electrons to SAM, enabling homolytic bond cleavage and radical generation.

5′-deoxyadenosyl radical (5′-dAdo•): Highly reactive radical formed from SAM that abstracts hydrogen atoms from substrates.

Organometallic intermediate (Ω): Transient iron–carbon adduct formed during SAM cleavage, which regulates controlled radical release.

References

1. Mechanism of Radical Initiation in the Radical SAM Enzyme Superfamily. Annual Review of Biochemistry (2023).
2. Mechanistic Insights from the Crystal Structure and Computational Analysis of the Radical SAM Deaminase DesII. Advanced Science (2024).
3. Mechanistic Diversity of Radical S-Adenosylmethionine (SAM)-dependent Methylation*. Journal of Biological Chemistry (2014).
4. SPASM and Twitch Domains in S-Adenosylmethionine (SAM) Radical Enzymes*. Journal of Biological Chemistry (2014).
5. The thiostrepton A tryptophan methyltransferase TsrM catalyses a cob(II)alamin-dependent methyl transfer reaction. Nature Communications (2015).