Asparagine and glutamine differ in their propensities to form specific side chain-backbone hydrogen bonded motifs in proteins

Abstract

Short range side chain-backbone hydrogen bonded motifs involving Asn and Gln residues have been identified from a data set of 1370 protein crystal structures (resolution ≤1.5 Å). Hydrogen bonds involving residues i - 5 to i + 5 have been considered. Out of 12,901 Asn residues, 3403 residues (26.4%) participate in such interactions, while out of 10,934 Gln residues, 1780 Gln residues (16.3%) are involved in these motifs. Hydrogen bonded ring sizes (C_n, where n is the number of atoms involved), directionality and internal torsion angles are used to classify motifs. The occurrence of the various motifs in the contexts of protein structure is illustrated. Distinct differences are established between the nature of motifs formed by Asn and Gln residues. For Asn, the most highly populated motifs are the C₁₀ (CO^δ_i...NH_{i + 2}), C₁₃ (CO^δ_i ...NH_{i + 3}) and C₁₇ (N^δH_i...CO_{i - 4}) structures. In contrast, Gln predominantly forms C₁₆ (CO^ε_i ...NH_{i - 3}), C₁₂ (N^εH_i...CO_{i - 2}), C₁₅ (N^εH_i...CO_{i - 3}) and C₁₈ (N^εH_i...CO_{i - 4}) motifs, with only the C₁₈ motif being analogous to the Asn C₁₇ structure. Specific conformational types are established for the Asn containing motifs, which mimic backbone β-turns and α-turns. Histidine residues are shown to serve as a mimic for Asn residues in side chain-backbone hydrogen bonded ring motifs. Illustrative examples from protein structures are considered.