Geometry of Reactant Centers in the Co^II-Substrate Radical Pair State of Coenzyme B₁₂-Dependent Ethanolamine Deaminase Determined by Using Orientation-Selection-ESEEM Spectroscopy

Abstract

The distances and orientations among the C5‘ methyl group of 5‘-deoxyadenosine, the radical-bearing C1 carbon of the substrate radical, and the low spin (S=1/2) CoII in cob(II)alamin in the active site of coenzyme B12-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized in the CoII-substrate radical pair state by using two-pulse X-band electron spin−echo electron paramagnetic resonance (ESE-EPR) and electron spin−echo envelope modulation (ESEEM) spectroscopies in the disordered solid state. Our approach is based on the orientation-selection created in the EPR spectrum of the biradical by the axial electron−electron dipolar interaction. Simulation of the ESE-EPR line shape yielded CoII-radical exchange and dipole interaction terms, which were used to calculate the CoII-C1 distance of 11.1 Å and the dependence of the EPR line shape on the angle between the CoII-C1 axis and the magnetic field vector. ESEEM spectroscopy performed at four magnetic field values addressed the coupling between 2H in the C5‘ methyl group and the unpaired spin on C1. Global ESEEM simulations, weighted by the orientation dependence of the EPR line shape, were performed for the four magnetic fields. The C1−H distance and orientation with respect to the CoII-C1 axis are specified for each C5‘ methyl hydrogen atom. In the derived model of the active site, C5‘ is located close to the CoII-C1 axis (at distances of 7.8 Å and 3.3 Å from CoII and C1, respectively) while the C1−H−C5‘ angle for the strongly coupled hydrogen is 165°. The near collinearity of the cobalt−carbon bond axis, radical migration coordinate and hydrogen atom transfer coordinate suggests an economy of nuclear displacements within the active site that would minimize rate-slowing molecular reorganization during the long-range radical pair separation.