An organic radical in the lysine 2,3-aminomutase reaction

Lysine 2,3-aminomutase from Clostridium SB4 has been studied by electron paramagnetic resonance (EPR) spectroscopy at 77 K. Although the reaction catalyzed by this enzyme is similar to rearrangements catalyzed by enzymes requiring adenosylcobalamin, lysine 2,3-aminomutase does not utilize this cofactor. The enzyme instead contains iron-sulfur clusters, cobalt, and pyridoxal phosphate and is activated by S-adenosylmethionine. Subsequent to a reductive incubation procedure that is required to activate the enzyme, EPR studies reveal the appearance of an organic radical signal (g = 2.001) upon addition of both L-lysine and S-adenosylmethionine. The radical signal is complex, having multiple hyperfine transitions. The total radical concentration is proportional to enzyme activity and decreases in parallel with the approach to chemical equilibrium between alpha-lysine and beta-lysine. The signal changes over the time course of the reaction in a way that suggests the presence of more than one radical species, with different relative proportions of species in the steady state and equilibrium state. Isotopic substitution experiments show that unpaired spin density resides on the molecular framework of lysine and that solvent-exchangeable protons do not participate in strong hyperfine coupling to the radical. The results indicate that lysine radicals participate in the rearrangement mechanism.