Reaction of the Co(II)-substrate radical pair catalytic intermediate in coenzyme B12-dependent ethanolamine ammonia-lyase in frozen aqueous solution from 190 to 217 K

The decay kinetics of the aminoethanol-generated Co^II-substrate radical pair catalytic intermediate in ethanolamine ammonia-lyase from Salmonella typhimurium have been measured on timescales of <10⁵ s in frozen aqueous solution from 190 to 217 K. X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy of the disordered samples has been used to continuously monitor the full radical pair EPR spectrum during progress of the decay after temperature step reaction initiation. The decay to a diamagnetic state is complete and no paramagnetic intermediate states are detected. The decay exhibits three kinetic regimes in the measured temperature range, as follows. i), Low temperature range, 190 ≤ T ≤ 207 K: the decay is biexponential with constant fast (0.57 ± 0.04) and slow (0.43 ± 0.04) phase amplitudes. ii), Transition temperature range, 207 < T < 214 K: the amplitude of the slow phase decreases to zero with a compensatory rise in the fast phase amplitude, with increasing temperature. iii), High temperature range, T ≥ 214 K: the decay is monoexponential. The observed first-order rate constants for the monoexponential (k_obs,m) and the fast phase of the biexponential decay (k_obs,f) adhere to the same linear relation on an lnk versus T⁻¹ (Arrhenius) plot. Thus, k_obs,m and k_obs,f correspond to the same apparent Arrhenius prefactor and activation energy (logA_app,f (s⁻¹) = 13.0, E_a,app,f = 15.0 kcal/mol), and therefore, a common decay mechanism. We propose that k_obs,m and k_obs,f represent the native, forward reaction of the substrate through the radical rearrangement step. The slow phase rate constant (k_obs,s) for 190 ≤ T ≤ 207 K obeys a different linear Arrhenius relation (logA_app,s (s⁻¹) = 13.9, E_a,app,s = 16.6 kcal/mol). In the transition temperature range, k_obs,s displays a super-Arrhenius increase with increasing temperature. The change in E_a,app,s with temperature and the narrow range over which it occurs suggest an origin in a liquid/glass or dynamical transition. A discontinuity in the activation barrier for the chemical reaction is not expected in the transition temperature range. Therefore, the transition arises from a change in the properties of the protein. We propose that a protein dynamical contribution to the reaction, which is present above the transition temperature, is lost below the transition temperature, owing to an increase in the activation energy barrier for protein motions that are coupled to the reaction. For both the fast and slow phases of the low temperature decay, the dynamical transition in protein motions that are obligatorily coupled to the reaction of the Co^II-substrate radical pair lies below 190 K.