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Radical Reactivity: Electrophilic Radicals

Radical Reactivity: Electrophilic Radicals

Radicals adjacent to electron‐withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For example, the malonate radical, in which the radical center is flanked by two electron‐withdrawing groups, reacts readily with butyl vinyl ether, which consists of an electron‐donating oxygen substituent. The reaction between electrophilic malonate radical and nucleophilic vinyl ether is favored because the radical has a low‐energy SOMO, which interacts...

Radical Reactivity: Overview

Radical Reactivity: Overview

Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired molecule. These three...

Radical Reactivity: Nucleophilic Radicals

Radical Reactivity: Nucleophilic Radicals

Radicals adjacent to electron-donating groups are called nucleophilic radicals. These radicals readily react with electrophilic alkenes. The SOMO–LUMO interactions are the driving force for the reaction, where the high-energy SOMO of the electron-rich, nucleophilic radicals interacts with the low-energy LUMO of the electron-deficient, electrophilic alkenes. Such SOMO–LUMO interactions are the basis of reactive radical traps, affecting the selectivity in radical reactions. For instance, consider...

Radical Reactivity: Steric Effects

Radical Reactivity: Steric Effects

The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
Along with electronic factors, steric factors also account...

Radical Formation: Addition

Radical Formation: Addition

Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an unpaired...

Phase II Reactions: Methylation Reactions

Phase II Reactions: Methylation Reactions

Methylation is a phase II biotransformation process involving the attachment of a methyl group to a substrate. Enzymes known as methyltransferases orchestrate this reaction.
The mechanism of methylation unfolds in two stages. The first stage sees a methyltransferase enzyme facilitating the transfer of a methyl group from S-adenosylmethionine (SAM) to the substrate, forming S-adenosylhomocysteine (SAH). The second stage involves further metabolism of SAH into homocysteine, which can be recycled...

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Related Experiment Video

Updated: Jun 2, 2026

Sequence-specific Labeling of Nucleic Acids and Proteins with Methyltransferases and Cofactor Analogues

Sequence-specific Labeling of Nucleic Acids and Proteins with Methyltransferases and Cofactor Analogues

Published on: November 22, 2014

Structural basis for methyl transfer by a radical SAM enzyme.

Amie K Boal¹, Tyler L Grove, Monica I McLaughlin

¹Department of Molecular Biosciences, Northwestern University, Evanston, IL 60208, USA.

Science (New York, N.Y.)

|April 30, 2011

Summary

Radical S-adenosyl-L-methionine (SAM) enzymes RlmN and Cfr methylate 23S ribosomal RNA. Structural studies reveal RlmN uses a single SAM molecule and active site to perform its complex methylation reaction.

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Antibody-Free Assay for RNA Methyltransferase Activity Analysis

Antibody-Free Assay for RNA Methyltransferase Activity Analysis

Published on: July 9, 2019

Area of Science:

Biochemistry
Molecular Biology
Structural Biology

Background:

Radical S-adenosyl-L-methionine (SAM) enzymes RlmN and Cfr are crucial for methylating 23S ribosomal RNA.
This methylation occurs at the C2 or C8 position of adenosine 2503, impacting ribosome function.

Purpose of the Study:

To elucidate the structural mechanisms by which RlmN utilizes S-adenosyl-L-methionine (SAM) for ribosomal RNA methylation.
To understand how RlmN achieves its complex two-step methylation process within a single active site.

Main Methods:

X-ray crystallography was employed to determine the structures of RlmN and its complex with SAM.
Structural analysis focused on the coordination of the [4Fe-4S] cluster and the positioning of key residues and SAM.

Main Results:

Crystal structures revealed that a single SAM molecule coordinates the [4Fe-4S] cluster in RlmN.
The conserved Cys(355) residue is S-methylated and positioned close to the SAM methyl group, indicating a shared binding site for both methylation steps.
This suggests RlmN employs structural economy by utilizing one site for both SAM-dependent reactions.

Conclusions:

RlmN efficiently catalyzes the two-step methylation of 23S ribosomal RNA by utilizing a single active site for SAM binding and methyl transfer.
The enzyme harnesses the distinct reactivities of SAM within a structurally conserved binding pocket, showcasing an elegant biochemical mechanism.