Geometry of Reactant Centers in the CoII-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 A 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 coup...