How important are entropic contributions to enzyme catalysis?

The idea that enzymes accelerate their reactions by entropic effects has pl ayed a major role in many prominent proposals about the origin of enzyme ca talysis. This idea implies that the binding to an enzyme active site freeze s the motion of the reacting fragments and eliminates their entropic contri butions, (DeltaS(cat)(double dagger))', to the activation energy, It is als o implied that the binding entropy is equal to the activation entropy, (Del taS(W)(double dagger))', of the corresponding solution reaction. It is, how ever, difficult to examine this idea by experimental approaches, The presen t paper defines the entropic proposal in a rigorous way and develops a comp uter simulation approach that determines (DeltaS(double dagger))'. This app roach allows us to evaluate the differences between (DeltaS(double dagger)) ' of an enzymatic reaction and of the corresponding reference reaction in s olution. Our approach is used in a study of the entropic contribution to th e catalytic reaction of subtilisin. It is found that this contribution is m uch smaller than previously thought. This result is due to the following: ( i) Many of the motions that are free in the reactants state of the referenc e solution reaction are also free at the transition state. (ii) The binding to the enzyme does not completely freeze the motion of the reacting fragme nts so that (DeltaS(double dagger))' in the enzymes is not zero. (iii) The binding entropy is not necessarily equal to (DeltaS(W)(double dagger))'.