Salvaging the Thermodynamic Destabilization of Interface Histidine in Transmembrane β-Barrels.

The ability of histidine to participate in a wide range of stabilizing polar interactions preferentially populates this residue in functionally important sites of proteins. Histidine possesses an amphiphilic and electrostatic nature that is essential for amino acids residing at membrane interfaces. However, the frequency of occurrence of histidine at membrane interfaces, particularly transmembrane β-barrels, is lower than those of other aromatic residues. Here, we carry out comprehensive energetic measurements using equilibrium folding of the outer membrane enzyme PagP to address the contribution of a C-terminal interface histidine to barrel stability. We show that placing histidine at the C-terminus universally destabilizes PagP by 4.0-8.0 kcal mol irrespective of the neighboring residue. Spectroscopic and electrophoretic measurements indicate that the altered stability may arise from a loss of barrel compaction. Isoleucine, methionine, and valine salvage this destabilization marginally (in addition to tyrosine, which shows an exceptionally high folding free energy value), when placed at the penultimate position, at the expense of an altered folding pathway. Double-mutant cycle analysis indicates that the coupling energy between the terminal and penultimate residues in PagP-XH increases when the level of intrinsic destabilization by the terminal H is high. Our observations that neighboring residues cannot salvage the energetic destabilization of histidine may explain why histidine is less abundant at membrane interfaces.