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Loop entropy and cytochrome c stability.

Liping Wang1, Edna V Rivera, Maria G Benavides-Garcia

  • 1Center for Biomolecular Structure Analysis, Department of Biochemistry, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA.

Journal of Molecular Biology
|September 27, 2005
PubMed
Summary
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The loop entropy model was tested using mutant cytochrome c proteins with varying loop lengths. Protein stability decreased with longer loops, but the observed effect was smaller than predicted, suggesting compensatory interactions.

Area of Science:

  • Protein biophysics
  • Molecular biology
  • Thermodynamics

Background:

  • The loop entropy model posits that longer protein loops incur greater entropic costs for closure.
  • Cytochrome c, a model protein, features loops connecting helical structures around a heme core.

Purpose of the Study:

  • To experimentally validate the loop entropy model using engineered protein variants.
  • To quantify the relationship between loop length and protein stability in cytochrome c.

Main Methods:

  • Construction of mutant cytochrome c proteins with inserted (Gly)n or (Thr)n segments of varying lengths (n=4-20).
  • Differential scanning calorimetry (DSC) to measure protein stability.
  • Analysis of stability data using the loop entropy model.

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Main Results:

  • Protein stability significantly decreased as the length of the inserted (Gly)n or (Thr)n loop segments increased.
  • The experimental data fit the loop entropy model but showed a weaker dependence of loop closure probability on loop length than predicted by polymer theory.
  • A discrepancy suggests that unfavorable loop entropy is partially counteracted by interactions within the unfolded protein.

Conclusions:

  • The loop entropy model provides a quantitative framework for understanding loop length-stability relationships.
  • Interactions, potentially involving molecular crowding, may offset entropic penalties, influencing loop closure.
  • Engineered loop mutants can serve to probe folding signals that promote loop closure.