Electrostatic strengths of salt bridges in thermophilic and mesophilic glutamate dehydrogenase monomers

Here we seek to understand the higher frequency of occurrence of salt bridg es in proteins from thermophiles as compared to their mesophile homologs, m e focus on glutamate dehydrogenase, owing to the availability of high resol ution thermophilic (from Pyrococcus furiosus) and mesophilic (from Clostrid ium symbiosum) protein structures, the large protein size and the large dif ference in melting temperatures. We investigate the location, statistics an d electrostatic strengths of salt bridges and of their networks within corr esponding monomers of the thermophilic and mesophilic enzymes. We find that many of the extra salt bridges which are present in the thermophilic gluta mate dehydrogenase monomer but absent in the mesophilic enzyme, form around the active site of the protein, Furthermore, salt bridges in the thermosta ble glutamate dehydrogenase cluster within the hydrophobic folding units of the monomer, rather than between them. Computation of the electrostatic co ntribution of salt bridge energies by solving the Poisson equation in a con tinuum solvent medium, shows that the salt bridges in Pyrococcus furiosus g lutamate dehydrogenase are highly stabilizing, In contrast, the salt bridge s in the mesophilic Clostridium symbiosum glutamate dehydrogenase are only marginally stabilizing. This is largely the outcome of the difference in th e protein environment around the salt bridges in the two proteins. The pres ence of a larger number of charges, and hence, of salt bridges contributes to an electrostatically more favorable protein energy term. Our results ind icate that salt bridges and their networks mag have an important role in re sisting deformation/ unfolding of the protein structure at high temperature s, particularly in critical regions such as around the active site, Protein s 2000;38:368-383, Published 2000 Wiley-Liss, Inc.dagger.