Folding dynamics and mechanism of beta-hairpin formation

Protein chains coil into alpha-helices and beta-sheet structures. Knowing the timescales and mechanism of formation of these basic structural elements is essential for understanding how proteins fold(1). For the past 40 years, alpha-helix formation has been extensively investigated in synthetic and natural peptides(2-5), including by nanosecond kinetic studies(6,7). In contrast, the mechanism of formation of beta structures has not been studied experimentally. The minimal beta-structure element is the beta-hairpin, which is also the basic component of antiparallel beta-sheets. Here we use a nanosecond laser temperature-jump apparatus to study the kinetics of folding a beta-hairpin consisting of 16 amino-acid residues. Folding of the hairpin occurs in 6 mu s at room temperature, which is about 30 times slower than the rate of alpha-helix formation(6,7). We have developed a simple statistical mechanical model that provides a structural explanation for this result, Our analysis also shows that folding of a beta-hairpin captures much of the basic physics of protein folding; including stabilization by hydrogen bonding and hydrophobic interactions, two-state behaviour, and a funnel-like, partially rugged energy landscape.