Histidine-aromatic interactions in barnase. Elevation of histidine pKa and contribution to protein stability

Aromatic side-chains are found in the vicinity of histidine residues in many proteins and protein complexes. We have studied the interaction between a histidine residue (His18) and aromatic residues at position 94 in barnase. Three different techniques have been applied to show that Trp94 interacts more strongly with the protonated form of His18. The aromatic-histidine interaction stabilizes the protonated form of histidine by 0.8 to 1 kcal mol-1 relative to the unprotonated and, thereby, increases its pKa value. This was shown indirectly from the pH dependence of the stability of the wild-type protein and the mutant Trp94----Leu; and directly from the difference in pKa of His18 between wild-type barnase and the same mutant protein, and from double-mutant cycles that measure the total interaction energy of Trp94 with His18 at both low and high pH. When Trp94 is replaced by other aromatic amino acids, the strength of the interaction decreases in the series His-Trp greater than His-Tyr greater than His-Phe. The interaction is not masked by high salt concentrations. The raising of the pKa value of His18 by interaction with Trp94 is shown to be consistent with solution studies with model compounds. The histidine-aromatic interaction could have implications in binding and catalysis for modulation of the histidine pKa value.