A single
amino acid exchange between the major histocompatibility complex molecules
HLA-B(*)2705 and
HLA-B(*)2709 (Asp-116/His) is responsible for the emergence of distinct HLA-B27-restricted T cell repertoires in individuals harboring either of these two subtypes and could correlate with their differential association with the
autoimmune disease ankylosing spondylitis. By using fluorescence depolarization and pK(a) calculations, we investigated to what extent electrostatic interactions contribute to shape antigenic differences between these HLA molecules complexed with viral, self, and non-natural
peptide ligands. In addition to the established main anchor of
peptides binding to
HLA-B27,
arginine at position 2 (pArg-2), and the secondary anchors at the
peptide termini, at least two further determinants contribute to stable
peptide accommodation. 1) The interaction of Asp-116 with
arginine at
peptide position 5, as found in pLMP2 (RRRWRRLTV; viral) and pVIPR (RRKWRRWHL; self), and with
lysine in pOmega, as found in gag (KRWIILGLNK; viral), additionally stabilizes the B(*)2705 complexes by approximately 5 and approximately 27 kJ/mol, respectively, in comparison with B(*)2709. 2) The protonation state of the key residues Glu-45 and Glu-63 in the B-pocket, which accommodates pArg-2, affects
peptide binding strength in a
peptide- and subtype-dependent manner. In B(*)2705/pLMP2, protonation of Glu-45/Glu-63 reduces the interaction energy of pArg-2 by approximately 24 kJ/mol as compared with B(*)2705/pVIPR. B(*)2705/pVIPR is stabilized by a deprotonated Glu-45/Glu-63 pair, evoked by allosteric interactions with pHis-8. The mutual electrostatic interactions of
peptide and HLA molecule, including
peptide- and subtype-dependent protonation of key residues, modulate complex stability and antigenic features of the respective HLA-B27 subtype.