DISSECTION OF HELIX CAPPING IN T4 LYSOZYME BY STRUCTURAL AND THERMODYNAMIC ANALYSIS OF 6 AMINO-ACID SUBSTITUTIONS AT THR 59

Threonine 59, a helix-capping residue at the amino terminus of the longest helix in T4 phage lysozyme, was substituted with valine, alanine, glycine, serine, asparagine, and aspartic acid. The valine, alanine, and glycine replacements were observed to be somewhat more destabilizing than serine, asparagine, and aspartic acid. The crystal structures of the different variants showed that changes in conformation occurred at the site of substitution, including Asp 61, which is nearby, as well as displacement of a solvent molecule that is hydrogen-bonded to the gamma-oxygen of Thr 59 in wild-type lysozyme. Neither the structures nor the stabilities of the mutant proteins support the hypothesis of Serrano and Fersht (1989) that glycine and alanine are better helix-capping residues than valine because a smaller-sized residue allows better hydration at the end of the helix. In the aspartic acid and asparagine replacements the substituted side chains form hydrogen bonds with the end of the helix, as does threonine and serine at this position. In contrast, however, the Asp and Asn side chains also make unusually close contacts with carbon atoms in Asp 61. This suggests a structural basis for the heretofore puzzling observations that asparagine is more frequently observed as a helix-capping residue than threonine [Richardson, J. S., & Richardson, D. C. (1988) Science 240, 1648-1652] yet Thr --> Asn replacements at N-cap positions in barnase were found to be destabilizing [Serrano, L., & Fersht, A. R. (1989) Nature 342, 296-299]. Threonine and asparagine are shown to require different conformations of the backbone at the end of the a-helix in order to adopt optimal capping conformations. The energetic and structural consequences of substitutions at the N-cap position therefore depend not only on the nature of the substitution and the local environment in which it occurs but also on the backbone conformation that exists at the site of substitution.