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Alpha-Helix folding in the presence of structural constraints


Ihalainen, J A; Paoli, B; Muff, S; Backus, E H G; Bredenbeck, J; Woolley, G A; Caflisch, A; Hamm, P (2008). Alpha-Helix folding in the presence of structural constraints. Proceedings of the National Academy of Sciences of the United States of America (PNAS), 105(28):9588-9593.

Abstract

We have investigated the site-specific folding kinetics of a photoswitchable cross-linked alpha-helical peptide by using single (13)C = (18)O isotope labeling together with time-resolved IR spectroscopy. We observe that the folding times differ from site to site by a factor of eight at low temperatures (6 degrees C), whereas at high temperatures (45 degrees C), the spread is considerably smaller. The trivial sum of the site signals coincides with the overall folding signal of the unlabeled peptide, and different sites fold in a noncooperative manner. Moreover, one of the sites exhibits a decrease of hydrogen bonding upon folding, implying that the unfolded state at low temperature is not unstructured. Molecular dynamics simulations at low temperature reveal a stretched-exponential behavior which originates from parallel folding routes that start from a kinetically partitioned unfolded ensemble. Different metastable structures (i.e., traps) in the unfolded ensemble have a different ratio of loop and helical content. Control simulations of the peptide at high temperature, as well as without the cross-linker at low temperature, show faster and simpler (i.e., single-exponential) folding kinetics. The experimental and simulation results together provide strong evidence that the rate-limiting step in formation of a structurally constrained alpha-helix is the escape from heterogeneous traps rather than the nucleation rate. This conclusion has important implications for an alpha-helical segment within a protein, rather than an isolated alpha-helix, because the cross-linker is a structural constraint similar to those present during the folding of a globular protein.

We have investigated the site-specific folding kinetics of a photoswitchable cross-linked alpha-helical peptide by using single (13)C = (18)O isotope labeling together with time-resolved IR spectroscopy. We observe that the folding times differ from site to site by a factor of eight at low temperatures (6 degrees C), whereas at high temperatures (45 degrees C), the spread is considerably smaller. The trivial sum of the site signals coincides with the overall folding signal of the unlabeled peptide, and different sites fold in a noncooperative manner. Moreover, one of the sites exhibits a decrease of hydrogen bonding upon folding, implying that the unfolded state at low temperature is not unstructured. Molecular dynamics simulations at low temperature reveal a stretched-exponential behavior which originates from parallel folding routes that start from a kinetically partitioned unfolded ensemble. Different metastable structures (i.e., traps) in the unfolded ensemble have a different ratio of loop and helical content. Control simulations of the peptide at high temperature, as well as without the cross-linker at low temperature, show faster and simpler (i.e., single-exponential) folding kinetics. The experimental and simulation results together provide strong evidence that the rate-limiting step in formation of a structurally constrained alpha-helix is the escape from heterogeneous traps rather than the nucleation rate. This conclusion has important implications for an alpha-helical segment within a protein, rather than an isolated alpha-helix, because the cross-linker is a structural constraint similar to those present during the folding of a globular protein.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > Department of Biochemistry
07 Faculty of Science > Department of Biochemistry

07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:570 Life sciences; biology
540 Chemistry
Language:English
Date:15 July 2008
Deposited On:29 Oct 2008 14:30
Last Modified:05 Apr 2016 12:31
Publisher:National Academy of Sciences
ISSN:0027-8424
Additional Information:Copyright: National Academy of Sciences USA
Publisher DOI:10.1073/pnas.0712099105
PubMed ID:18621686
Permanent URL: http://doi.org/10.5167/uzh-4761

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