The Bowman-Birk inhibitor reactive site loop sequence represents an independent structural beta-hairpin motif

We have determined the NMR structure in aqueous solution of a dis disulphid e-cyclised 11-residue peptide that forms a stable beta -hairpin, incorporat ing a type VIb beta -turn. The structure is found to be extremely well orde red for a short peptide, with the 30 lowest energy simulated annealing stru ctures having an average pairwise r.m.s. deviation of only 0.36 Angstrom ov er the backbone. AU but three side-chains adopt distinct conformations, all owing a detailed analysis of their involvement in cross-strand interactions . The peptide sequence analysed originates from a previously reported study , which identified potent inhibitors of human leukocyte elastase from scree ning a combinatorial peptide library based on the short protein beta -sheet segment that forms the reactive site loop of Bowman-Birk inhibitors. A det ailed comparison of the peptide's solution structure with the corresponding region in the whole protein structure reveals a very good correspondence n ot only for the backbone (r.m.s. deviation approximate to0.7 Angstrom) but also for the side-chains. This isolated beta -hairpin retains the biologica lly active "canonical conformation" typical of small serine proteinase inhi bitor proteins, which explains why it retains inhibitory activity. Since th e structural integrity is sequence-inherent and does not depend upon the pr esence of the remaining protein, this beta -hairpin represents an independe nt structural motif and so provides a useful model of this type of protein architecture and its relation to biological function. The relationship betw een the conformation of this beta -hairpin and its biological activity is d iscussed. (C) 2001 Academic Press.