A dehydroalanyl residue can capture the 5'-deoxyadenosyl radical generated from S-adenosylmethionine by pyruvate...

The glycyl radical (Gly-734) contained in the active form of pyruvate formate-lyase (PFL) of Escherichia coli is produced post-translationally by pyruvate formate-lyase-activating enzyme (PFL activase), employing adenosylmethionine (AdoMet) and dihydroflavodoxin as co-substrates. Previous 2H-labelings found incorporation of the pro-S hydrogen of Gly-734 into the 5'-deoxyadenosine co-product, indicating that a deoxyadenosyl radical intermediate, generated by reductive cleavage of AdoMet, serves as the actual H atom abstracting species in this system. We have now examined an octapeptide (Suc-Arg-Val-Pro-DeltaAla-Tyr-Ala-Val-Arg-NH2) that is analogous to the Gly-734 site of the PFL polypeptide but contains a dehydroalanyl residue (DeltaAla) in the glycyl position. Applied to the PFL activase reaction, this peptide becomes C-adenosylated at the olefinic beta carbon of DeltaAla. The modified peptide was isolated in micromol-quantities and characterized, after chymotryptic truncation, by MS and 2D NMR. PFL activase functions catalytically (kcat >/= 1 min-1) in the peptide modification reaction, which occurs with stoichiometric consumption of AdoMet. The mechanism appears to involve addition of the nucleophilic deoxyadenosyl radical to the electrophilic CC double bond of DeltaAla, followed by quenching of the peptide backbone-centered adduct radical by the buffer medium. The trapping-property of the DeltaAla residue should be exploitable in investigating of how the Fe4S4 protein PFL activase generates the highly reactive deoxyadenosyl radical.