Electrostatics of mesophilic and psychrophilic trypsin isoenzymes: Qualitative evaluation of electrostatic differences at the substrate binding site

A qualitative evaluation of electrostatic features of the substrate binding region of seven isoenzymes of trypsin has been performed by using the cont inuum electrostatic model for the solution of the Poisson-Boltzmann equatio n, The sources of the electrostatic differences among the trypsins have bee n sought by comparative calculations on selective charges: all charges, con served charges, partial charges, unique cold trypsin charges, and a number of charge mutations. As expected, most of the negative potential at the S-1 region of all trypsins is generated from Asp(189), but the potential varie s significantly among the seven trypsin isoenzymes, The three cold active e nzymes included in this study possess a notably lower potential at and arou nd the S-1-pocket compared with the warm active counterparts; this finding may be the main contribution to the increased binding affinity. The source of the differences are nonconserved charged residues outside the specificit y pocket, producing electric fields at the S-1-pocket that are different in both sign Land magnitude. The surface charges of the mesophilic trypsins g enerally induce the S-1 pocket positively, whereas surface charges of the c old trypsins produce a negative electric field of this region, Calculations on mutants, where charged amino acids were substituted between the trypsin s, showed that mutations in Loop2 (residues 221B and 224) and residue 175, in particular, were responsible for the low potential of the cold enzymes. (C) 2000 Wiley-Liss, Inc.