The role of the positive charge in trypsin specificity
Sanborn, Barbara Mortensen
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Enzymes are proteins which have the ability to catalyze chemical reactions and which do so with a high degree of specificity. Since trypsin has a sharp specificity for two positively charged substrates, arginine and lysine, an attempt was made to assess the role of the positive charge in determining this specificity. Both Pressman and Wilson did similar studies (Pressman on hapten-antibody reactions and Wilson on acetylcholinesterase reactions) in which they compared the hydrolysis of a substrate possessing a charged quaternary ammonium group with that of the same substrate in which the charged group had been replaced by a tertiary butyl group. In both cases, Ko/K+ was approximately eight where Ko is the dissociation constant for the uncharged substrate and K+ is the dissociation constant for the charged species. The free energy change associated with this change in K is about 1.5 kilocalories per mole and was attributed by both authors to the electrostatic contribution to the binding energy. Attempts to relate these changes in free energy to physical force expressions are discussed. Webb attempted to calculate group separation distances from the free energy changes using the expression for the potential energy of two separated charges corrected for ion atmosphere interactions. He tentatively concluded that for both hapten-antibody and cholinesterase-substrate interactions the groups involved are in direct contact without an intervening water molecule. [TRUNCATED]
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