Electron paramagnetic resonance evidence for a cysteine-based radical in pyruvate formate-lyase inactivated with mercaptopyruvate.

Pyruvate formate-lyase (PFL) is a glycyl radical-containing enzyme that catalyzes the reversible, nonoxidative conversion of pyruvate and CoA into acetyl-CoA and formate. The radical is located on the alpha-carbon of glycine 734 and is required for catalysis. Two cysteine residues, C418 and C419, are also essential for catalysis. Mercaptopyruvate, a biologically relevant pyruvate analog, is shown here to be a mechanism-based inactivator of PFL. Upon addition of mercaptopyruvate to active PFL, an EPR spectrum is generated which exhibits components from two sulfur-based radicals. For one of these radicals, a disulfide radical, the hyperfine coupling to a single beta-methylene hydrogen is resolved in features at g = 2.057 and 2.023. The effects of deuterium labeling of the enzyme on the EPR spectrum for this species are consistent with the new radical being on a cysteine residue, probably cysteine 418 or 419. This spectrum is not formed upon addition of the inactivator to mutant enzymes, C418S and C419S, indicating that both active site cysteines are required for formation of the new radicals. The identity of the second species is also ascribed to be a sulfur-based radical on the basis of the EPR feature found at g = 2.01. Our results constitute the first direct evidence of sulfur-based radical formation in an enzyme. A mechanism for formation of the cysteine-based disulfide radical is proposed which requires the participation of the two active site cysteines as well as the glycyl radical.