Structural and thermodynamic studies on a salt-bridge triad in the NADP-binding domain of glutamate dehydrogenase from Thermotoga maritima: Cooperativity and electrostatic contribution to stability

Cooperative interactions within ion-pair networks of hyperthermostable proteins are thought to be a major determinant for extreme protein stability. While the favorable thermodynamic contributions of optimized electrostatics in general as well as those of pairwise interactions have been documented, cooperativity between pairwise interactions has not yet been studied thermodynamically in proteins from hyperthermophiles. In this study we use the isolated cofactor binding domain of glutamate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima to analyze pairwise and cooperative interactions within the salt-bridge triad Arg190-Glu231-Lys193. The X-ray structure of the domain was solved at 1.43 Å and reveals the salt-bridge network with surrounding solvent molecules in detail. All three participating charges in the network were mutated to alanine in all combinations. The X-ray structure of the variant lacking all three charges reveals that the removal of the side chains has no effect on the overall conformation of the protein. Using solvent denaturation and thermodynamic cycles, the interaction energies between each pair of residues in the network were determined in the presence and in the absence of the third residue. Both the Arg190-Glu231 ion pair and the Lys193-Glu231 salt bridge in the absence of the third residue, contribute favorably to the free energy for unfolding of the domain in urea. Using guanidinium chloride as denaturant reveals a strong cooperativity between the two ion-pair interactions, the presence of the second ion pair converts the first interaction from destabilizing into stabilizing by as much as 1.09 kcal/mol. The different energetics of the salt-bridge triad in urea and GdmCl are discussed with reference to the observed anion binding in the crystal structure at high ionic strength and their possible role in a highly charged, high-temperature environment such as the cytoplasm of hyperthermophiles.