Comment on: “Computer Simulations Reveal an Entirely Entropic Activation Barrier for the Chemical Step in a Designer Enzyme”

Activation heat capacity has been proposed as an important factor in enzyme evolution and thermoadaptation. We previously demonstrated that the emergence of curved activity–temperature profiles during the evolution of a designer enzyme was due to the selective rigidification of its transition state ensemble that induced an activation heat capacity. Åqvist challenged our findings with molecular dynamics simulations suggesting that a change in the rate-limiting step underlies the experimental observations. As we describe here, Åqvist’s model is not consistent with the experimental trends observed for the chemical step of the catalyzed reaction (kcat). We suggest that this discrepancy arises because the simulations performed by Åqvist were limited by restraints and short simulation times, which do not allow sampling of the motions responsible for the observed activation heat capacity.