Spin-spin interaction in ethanolamine deaminase.

Abstract

The adenosylcobalamin coenzyme-dependent ethanolamine deaminase from Salmonella typhimurium catalyzes the deamination of aminoethanol to acetaldehyde and ammonia. The radical intermediate observed during steady state turnover of substrate aminoethanol has been characterized by continuous wave electron paramagnetic resonance (EPR) spectroscopy [J. Am. Chem. Soc. 121 (1999) 10522]. This study presents simulations of EPR spectra of this radical intermediate. Quantitative fits to the EPR spectra are achieved with a model of isotropic exchange and magnetic dipolar interaction between the substrate-derived radical and the Co(II) in the corrin ring. The simulated parameters are compared with those of substrate analog 2-aminopropanol-derived radical in the same enzyme. The comparison confirms that the aminoethanol-derived product radical interacts more weakly with the Co(II) than the 2-aminopropanol-derived radical and suggests that the reduction of isotropic exchange between the aminoethanol-derived product radical and the Co(II) is probably due to orientational-dependent wave function overlap. Successful fits to the radical line shapes of different isotope substitutions unequivocally establish that the observed radical intermediate is an pi-electron-based product radical. The derived principal hyperfine values for the 13C(alpha) and 1H(alpha) nucleus are consistent with previous electron nuclear double resonance (ENDOR) studies on similar radicals, thus providing reliable experimental hyperfine coupling constants for comparison with quantum mechanical-based calculations to gain further insight into the molecular structure of the observed radical.