The Biosynthesis of Thiol- and Thioether-Containing Cofactors and Secondary Metabolites Catalyzed by Radical S-Adenosylmethionine Enzymes [Enzymology]

December 4th, 2014 by Jarrett, J. T.

Sulfur atoms are present as thiol and thioether functional groups in amino acids, coenzymes, cofactors, and various products of secondary metabolic pathways. The biosynthetic pathways for several sulfur-containing biomolecules require the substitution of sulfur for hydrogen at unreactive aliphatic or electron-rich aromatic carbon atoms. Examples discussed in this review include biotin, lipoic acid, methylthioether modifications found in some nucleic acids and proteins, and thioether crosslinks found in peptide natural products. Radical S-adenosyl-L-methionine (SAM) enzymes use an iron-sulfur cluster to catalyze the reduction of SAM to methionine and a highly reactive 5'-deoxyadenosyl radical; this radical can abstract hydrogen atoms at unreactive positions, facilitating the introduction of a variety of functional groups. Radical SAM enzymes that catalyze sulfur insertion reactions contain a second iron-sulfur cluster that facilitates the chemistry, either by donating the cluster's endogenous sulfide or by binding and activating exogenous sulfide or sulfur-containing substrates. The use of radical chemistry involving iron-sulfur clusters is an efficient anaerobic route to the generation of carbon-sulfur bonds in cofactors, secondary metabolites, and other natural products.
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