S-adenosylmethionine-dependent reduction of lysine 2,3-aminomutase and observation of the catalytically functional iron-sulfur centers by electron paramagnetic resonance

Lysine 2,3-aminomutase catalyzes the interconversion of L-α-lysine and L-β-lysine. The enzyme contains an iron-sulfur cluster with unusual properties, and it requires pyridoxal-5'-phosphate (PLP) and S- adenosylmethionine (AdoMet) for activity. The reaction proceeds by a substrate radical rearrangement mechanism, in which the external aldimine formed between PLP and lysine is initially converted into a lysyl-radical intermediate by hydrogen abstraction from C3. The present research concerns the mechanism by which a hydrogen-abstracting species is generated at the active site of lysine 2,3-aminomutase. Earlier tritium tracer experiments have implicated the 5'-deoxyadenosyl moiety of AdoMet in this process. AdoMet is here shown to interact with the iron-sulfur cluster at the active site of Clostridial lysine 2,3-aminomutase. Reduction of the iron-sulfur cluster from its EPR-silent form [4Fe-4S]²⁺ to the fully reduced form [4Fe-4S]¹⁺ requires the presence of either AdoMet or S-adenosylhomocysteine (SAH) and a strong reducing agent such as dithionite or deazariboflavin and light. The reduced forms are provisionally designated E-[4Fe-4S]¹⁺/AdoMet and E-[4Fe- 4S]¹⁺/SAH, and they display similar low-temperature EPR spectra centered at g(av) = 1.91. The reduced form E-[4Fe-4S]¹⁺/AdoMet is fully active in the absence of any added reducing agent, whereas the form E-[4Fe-4S]¹⁺/SAH is not active. It is postulated that the active form E-[4Fe-4S]¹⁺/AdoMet is in equilibrium with a low concentration of a radical-initiating form that contains the 5'-deoxyadenosyl radical. Initiation of the radical rearrangement mechanism is postulated to take place by action of the 5'- deoxyadenosyl radical in abstracting a hydrogen atom from carbon-3 of lysine, which is bound as its external aldiminine with PLP. This process accounts for the results of tritium tracer experiments, it explains the radical rearrangement mechanism, and it rationalizes the roles of AdoMet and the [4Fe-4S] cluster in the reaction.