Importance of Contact Persistence in Denatured State Loop Formation: Kinetic Insights into Sequence Effects on Nucleation Early in Folding

Franco O. Tzul, Bruce E. Bowler

Research output: Contribution to journalArticlepeer-review

Abstract

Protein folding is dependent on the formation and persistence of simple loops early in folding. Ease of loop formation and persistence is believed to be dependent on the steric interactions of the residues involved in loop formation. We have previously investigated this factor in the denatured state of iso-1-cytochrome c using a five-amino-acid insert in front of a unique histidine in the N-terminal region of the protein. Previously, we reported that the apparent pKa values of loop formation for the most flexible (all Gly) and least flexible (all Ala) insert were, within error, the same. We evaluate whether this observation is due to differences in the persistence of loop contacts or due to effects of local sequence sterics and main-chain hydration on the persistence length of the chain. We also test whether sequence order affects loop formation. Here, we report kinetic results coupled to further mutagenesis of the insert to discern between these possibilities. We find that the amino acid-glycine versus alanine-next to the loop forming histidine has a dominant effect on loop kinetics and equilibria. A glycine in this position speeds loop breakage relative to alanine, resulting in less stable loops. At high percentage of Gly in the insert, rates of loop formation and breakage exactly compensate, leading to a leveling out in loop stability. Loop formation rates also increase with glycine content, inconsistent with poly-Gly segments being more extended than previously suspected due to main-chain hydration or local sterics. Unlike loop breakage rates, loop formation rates are insensitive to local sequence. Together, these observations suggest that contact persistence plays a more important role in defining the "folding code" than rates of loop formation.

Original languageEnglish
Pages (from-to)124-134
Number of pages11
JournalJournal of Molecular Biology
Volume390
Issue number1
DOIs
StatePublished - Jul 3 2009

Keywords

  • denatured states
  • loop formation
  • main-chain flexibility
  • protein folding
  • sequence composition

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