Journey on the Radical SAM Road as an Accidental Pilgrim

Abstract

Radical S-adenosyl-L-methionine (SAM) enzymes catalyze a diverse group of complex transformations in all aspects of cellular physiology. These metalloenzymes bind SAM to a 4Fe− 4S cluster and reductively cleave SAM to generate a 5′deoxyadenosyl radical, which generally initiates the catalytic cycle by catalyzing a H atom to activate the substrate for subsequent chemistry. This perspective will focus on our discovery of several members of this superfamily of enzymes, with a particular emphasis on the current state of the field, challenges, and outlook.

Consecutive Methylation Catalyzed by TsrM, an Atypical Class B Radical SAM Methylase

“…The observation that the enzyme promiscuously further acts on the on‐pathway product adds new examples to the growing examples of radical SAM enzymes that simultaneously produce two or more distinct products from the same substrates. [ 19‐25 ]…”

“…[ 2‐3 ] The dAdo radical can also add to sp 2 carbons [ 4‐8 ] or heteroatoms [ 9‐12 ] to result in adenosylation reactions. Although in general radical SAM enzymes have evolved dedicated microenvironments to tune the reactivity of intermediates to achieve a specific catalytic outcome, promiscuous reactions have been observed in various systems, [ 13‐25 ] highlighting the remarkable catalytic plasticity of this superfamily of enzymes.…”

Twenty Years of Radical SAM! The Genesis of the Superfamily

“…Pioneering studies on KAM and pyruvate formate-lyase (PFL) indicated that their reactions had an unusual requirement for SAM. KAM catalyzes a 1,2-cross migration between the 2-amino group and the 3- proR hydrogen of lysine to afford β-lysine, a reaction that is typical of enzymes that use AdoCbl in catalysis. ,, In fact, a similar enzyme, d -lysine 5,6-aminomutase, which generates 2,5-diaminohexanoate, uses AdoCbl in catalysis . However, isolated KAM does not contain AdoCbl and its activity is not stimulated by treatment with exogenous AdoCbl.…”

“… 2 , 18 , 23 In fact, a similar enzyme, d -lysine 5,6-aminomutase, which generates 2,5-diaminohexanoate, uses AdoCbl in catalysis. 24 However, isolated KAM does not contain AdoCbl and its activity is not stimulated by treatment with exogenous AdoCbl. Instead, activity is stimulated by the addition of pyridoxal 5′-phosphate (PLP), a thiol-reducing agent, iron, and SAM to the reaction.…”

In Vitro Demonstration of Human Lipoyl Synthase Catalytic Activity in the Presence of NFU1

“…LS belongs to the radical S -adenosylmethionine (SAM) superfamily of enzymes, which use a [4Fe–4S] cluster cofactor to cleave SAM reductively to produce a 5′-deoxyadenosyl 5′-radical (5′-dA·). − In LS catalysis, the 5′-dA· is used to abstract hydrogen atoms (H·) from C6, and then from C8, of the octanoyllysyl residue to allow for sulfur attachment. − Unlike most radical SAM (RS) enzymes, which contain only one [4Fe–4S] cluster, LS contains two [4Fe–4S] clusters. ,− One cluster, termed the RS cluster, is found in all RS enzymes and is ligated by three cysteine residues in a conserved CxxxCxxC motif. SAM binds to the unligated iron ion of this cluster, which is a prerequisite for its reductive cleavage. ,− The second cluster, termed the auxiliary cluster, is bound by cysteines in an N-terminal CxxxxCxxxxxC motif and a C-terminal serine in a conserved RSSY motif (Figure ).…”