title: Understanding the folding mechanism of an alpha-helical hairpin.
abstract: The alpha-helical hairpin is the fundamental building block of the widespread helix-turn-helix DNA binding motif. With two antiparallel helices connected by a reverse turn, the alpha-helical hairpin structure may be regarded as a "supersecondary structural element" and, therefore, could exhibit rather unique folding properties. So far, the folding mechanism of alpha-helical hairpins has not been studied in detail and remains elusive. Herein, we examine the effects of the turn, the hydrophobic cluster, and a disulfide cross-linker on the folding kinetics of a designed alpha-helical hairpin, Z34C, using an infrared temperature-jump (T-jump) method in conjunction with site-specific mutagenesis. Our results show that Z34C folds with an ultrafast rate ( approximately 4.0 x 10(5) s(-1)) and support a folding mechanism in which the rate-limiting step corresponds to the formation of the reverse turn. On the other hand, the hydrophobic cluster and the disulfide cross-linker appear to largely stabilize the native state but not the folding transition state.
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Deguo
authors.initials: D
authors.lastname: Du
authors.forename: Feng
authors.initials: F
authors.lastname: Gai
mesh_terms: D002942:Circular Dichroism; D008958:Models, Molecular; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D011506:Proteins