PULSED ELECTRON-PARAMAGNETIC-RESONANCE STUDIES OF THE LYSINE 2,3-AMINOMUTASE SUBSTRATE RADICAL - EVIDENCE FOR PARTICIPATION OF PYRIDOXAL 5'-PHOSPHATE IN A RADICAL REARRANGEMENT

The role of pyridoxal 5'-phosphate (PLP) in the radical-mediated amino group migration catalyzed by lysine 2,3-aminomutase from Clostridia SB4 has been investigated by electron spin echo envelope modulation (ESEEM) spectroscopy, This pulsed electron paramagnetic resonance (EPR) method was used to estimate the distance between the unpaired electron in the alpha-radical of beta-lysine, a steady-state intermediate in the reaction, and deuterium at the C4' position of the cofactor, PLP. [4'-H-2]PLP was synthesized and exchanged into the enzyme, The steady-state radical was generated in the labeled samples and in samples with unlabeled PLP by addition of L-lysine . H2SO4 to activated enzyme. ESEEM spectra of the samples prepared with [4'-?H]PLP exhibited distinctive low-frequency modulations that were not present in spectra of matched samples with unlabeled PLP. Fourier transformation of the modulations yielded a prominent doubler signal centered about the Larmor frequency of deuterium. The magnitude of the doublet splitting of the H-2 ESEEM signal exhibited angle selection across the CW EPR powder pattern. The observed angle selection, as well as simulation of the time domain spectra, indicated that the doublet splitting was due to the combined effects of the H-2 hyperfine and nuclear quadrupole interactions. The influences of the quadrupole interaction and of isotropic and dipolar hyperfine interactions were explored by simulations of the ESEEM spectra. The analysis indicates a distance of <3.5 Angstrom between the H-2 at C4' of PLP and the radical center at C alpha lysine. The data are most compatible with an aldimine linkage between PLP and the beta-nitrogen of beta-lysine. These data support the proposed radical rearrangement mechanism, wherein PLP promotes the reaction by formation of an aldimine linkage to the migrating nitrogen.