Enthalpic and entropic consequences of the removal of disulfide bridges inribonuclease A

In this study a careful analysis of the enthalpic and entropic effects asso ciated with the removal of each of the four disulfide bridges in ribonuclea se A is accomplished. The fundamental role of disulfides for the stability of the native structure is emphasized by the strong decrease in denaturatio n temperature, at least 20 degreesC. In contrast to the traditional belief, the destabilization in two out of four mutant forms is driven by enthalpic factors, related to the loss or weakening of several van der Waals interac tions among side-chains in the protein close-packed interior. Theoretical r elations, derived in the assumption that the insertion of a disulfide does affect only the entropy of the denatured state by lowering its conformation al freedom, prove to be unable to reproduce and explain the experimental re sults. Such failure is rationalized by taking into account the complexity o f the protein structure in both the native and denatured states, and the la rge Variety of stabilizing and destabilizing interactions involved. (C) 200 0 Elsevier Science B.V. All rights reserved.