| Comments |
P.217 bottom paragraph: "The above-discussed observations should also be reflected in differences in the activation parameters for the catalytic reaction. From Table 10.2, one can see that the activation energy, ΔG#, for cold-active enzymes, is systematically lower than that of their mesophilic counterparts. The rather moderate differences result from significant decreases of the activation enthalpies, ΔH#, partially compensated for by more negative values of the activation entropy term, TΔS#. This indicates that a much more important reordering of the ground states is required to reach the activated states and fits perfectly well with the more negative heat capacity changes seen during activation of cold-active enzymes as discussed above. The compensation effect observed between activation enthalpy and entropy could originate from the protein surface mobility (Isaksen et al. 2016). Indeed, at least in the case of psychrophilic and mesophilic trypsins, rigidification of the surface of the psychrophilic enzyme by applying positional restraints from the water-enzyme interface and inward yielded an increase of the activation enthalpy partially compensated by an increase of the activation entropy eventually turning the psychrophilic enzyme into a mesophilic one. In Table 10.2, the activation entropy terms, TΔS#, of three mesophilic enzymes, i.e. chitobiase, endonuclease and lysozyme, are positive. This could mean that, contrary to the other enzymes listed, the activated states are characterised by greater disorder than the ground state or, more probably, that the rearrangement of water molecules during the activation process gives rise to a positive value of the activation entropy." |