Tryptophan zippers: Stable, monomeric β-hairpins

Abstract

A structural motif, the tryptophan zipper (trpzip), greatly stabilizes the β-hairpin conformation in short peptides. Peptides (12 or 16 aa in length) with four different turn sequences are monomeric and fold cooperatively in water, as has been observed previously for some hairpin peptides. However, the folding free energies of the trpzips exceed substantially those of all previously reported β-hairpins and even those of some larger designed proteins. NMR structures of three of the trpzip peptides reveal exceptionally well-defined β-hairpin conformations stabilized by cross-strand pairs of indole rings. The trpzips are the smallest peptides to adopt an unique tertiary fold without requiring metal binding, unusual amino acids, or disulfide crosslinks.

Cross-Strand Coupling and Site-Specific Unfolding Thermodynamics of a Trpzip β-Hairpin Peptide Using 13C Isotopic Labeling and IR Spectroscopy

“…Studies of isolated, water-soluble -hairpins can provide valuable insight into properties of -sheet structure and folding since -hairpins are considered to act as possible nucleation sites for protein folding. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Additionally, their detailed structural interactions can be relevant in developing an understanding of the mechanism for forming various -sheet structures such as found in many (amyloid-like) neurodegenerative diseases in which protein aggregation is an important pathology. [15][16][17][18] During the past decade, there have been many reports discussing de novo designed, water-soluble -hairpin peptide systems, as well documented in several reviews.…”

“…Trpzip has been reported to be the smallest peptide sequence that has a tertiary structure without disulfide bonds or metal binding. 1 The folding of -hairpins has been proposed to be initiated either by a sequence of residues with high propensities to form turn structures which then bring the -strands together ("zipping" mechanism) 35 or by nonbonded interactions between opposing residues on the strands that lead to hydrophobic collapse and enable cross-strand hydrogen bond formation and subsequent turn formation. 10 Trpzip is an excellent model for study of such a hydrophobic collapse, which has been shown to contribute to its structural stability and the heterogeneity of its folding dynamics.…”

“…It has a unique electronic circular dichroism (ECD) spectrum, arising from the exciton coupling of neighboring tryptophan residues, which can be used as a probe of Trp-Trp contact, and provide a measure of its tertiary structural stability. 1 Both static and dynamic fluorescence spectra can also monitor change of the tryptophan environment, 29,32 while IR absorption for the amide I band can be used to follow correlated secondary structural changes and dynamics during the unfolding process. 23,24,26,29,31,34,36,37 Among the original trpzip sequences, trpzip2 has been more widely studied, partly due to simplicity and increased solubility.…”

“…The resulting thermal denaturation curves are often characterized by very broad transitions. [43][44][45][46][47] Increase in the number of interstrand hydrophobic interactions has been shown to stabilize -hairpin structures and result in somewhat more sigmoidal denaturation profiles in trpzips; 1,30,48 however, there can be a tendency for such peptide sequences to aggregate. 24 Due to their small size, trpzips have also been a subject of many theoretical studies.…”

Secondary structure prediction of beta-hairpin peptide tryptophan zipper-I

“…Trpzip-I molecule, a 12-residue small peptide, forms a stable β-hairpin structure in aqueous solution [11]. Moreover, NMR data of that molecule is unusually of high quality for a short peptide indicating a highly ordered structure.…”