Energetic asymmetry among hydrogen bonds in MHC class II-peptide complexes

Comparison of crystallized MHC class II-peptide complexes has revealed that , in addition to pocket interactions involving the peptide side chains, pep tide binding to MHC class II molecules is characterized by a series of hydr ogen bonds between genetically conserved amino acid residues in the class I I molecule and the main chain of the peptide. Many class II.peptide structu res have two sets of symmetrical hydrogen bonds at the opposite ends of the class II antigen-binding groove (beta -His-81, beta -Asn-82 vs. alpha -His -68, alpha -Asn-69). In this study, we alter these peripheral hydrogen bond s and measure the apparent contribution of each to the kinetic stability of peptide.class II complexes. Single conservative amino substitutions were m ade in the I-Ad protein to eliminate participation as a hydrogen bonding re sidue, and the kinetic stability of a diverse set of peptides bound to the substituted I-Ad proteins was measured. Although each hydrogen bond does co ntribute to peptide binding, our results point to the striking conclusion t hat those hydrogen bonds localized to the amino terminus of the peptide con tribute profoundly and disproportionately to the stability of peptide inter actions with I-Ad. We suggest that the peripheral hydrogen bonds at the ami no terminus of the bound peptide that are conserved in all class II.peptide crystal structures solved thus far form a cooperative network that critica lly regulates peptide dissociation from the class II molecule.