ENZYME-ACTIVITY BELOW THE DYNAMICAL TRANSITION AT 220 K

Enzyme activity requires the activation of anharmonic motions, such as jumps between potential energy wells. However, in general, the forms and time scales of the functionally important anharmonic dynamics coup led to motion along the reaction coordinate remain to be determined. I n particular, the question arises whether the temperature-dependent dy namical transition from harmonic to anharmonic motion in proteins, whi ch has been observed experimentally and using molecular dynamics simul ation, involves the activation of motions required for enzyme function . Here we present parallel measurements of the activity and dynamics o f a cryosolution of glutamate dehydrogenase as a function of temperatu re. The dynamical atomic fluctuations faster than similar to 100 ps we re determined using neutron scattering. The results show that the enzy me remains active below the dynamical transition observed at similar t o 220 K, i.e., at temperatures where no anharmonic motion is detected. Furthermore, the activity shows no significant deviation from Arrheni us behavior down to 190 K. The results indicate that the observed tran sition in the enzyme's dynamics is decoupled from the rate-limiting st ep along the reaction coordinate.