An NMR study on the beta-hairpin region of barnase

The beta-hairpin of barnase (residues Ser92-Leu95) has been proposed in theoretical and protein engineering studies to be an initiation site for folding [1]. There is evidence for residual structure in this region from NMR studies of the denatured protein under different denaturing conditions [2,3]. A more detailed analysis is possible by NMR studies of isolated fragments.

Protons of fragments B(80-110) and B(69-110) in 6 M urea have non-random chemical shifts. Non-native long-range and medium-range NOE contacts with the aromatic moiety of Trp94 indicate that it is involved in a beta-turn-like or alpha-helix-like conformation. Also, the sidechains of Trp71, Tyr79, Phe82, Tyr90, Tyr97, His102, Tyr103 and Phe106 show non-native hydrophobic contacts. Non-random conformational shifts and sequential NN(i,i+1) NOE contacts are clustered to one of the beta-strands and one of the loop regions.

The hairpin region of barnase adopts beta-turn-like or alpha-helix-like conformations, which are weakly populated even in 6 M urea. The hairpin region is a potential nucleation site in folding that may consolidate on docking with the first alpha-helix. The other residues that have conformational preferences from a beta-strand and one of the loop regions in the native intact protein, but they do not constitute a nucleation site.