Thermodynamic cycles as probes of structure in unfolded proteins

William A. McGee, Frederico I. Rosell, John R. Liggins, Sofia Rodriguez-Ghidarpour, Yaoguang Luo, Jie Chen, Gary D. Brayer, A. Grant Mauk, Barry T. Nall

Research output: Contribution to journalArticlepeer-review

25 Scopus citations

Abstract

The relationship between structure and stability has been investigated for the folded forms amid the unfolded forms of iso-2 cytochrome c and a variant protein with a stability-enhancing mutation, N52I iso-2. Differential scanning calorimetry has been used to measure the reversible unfolding transitions for the proteins in both heme oxidation states. Reduction potentials have been measured as a function of temperature for the folded forms of the proteins. The combination of measurements of thermal stability and reduction potential gives three sides of a thermodynamic cycle and allows prediction of the reduction potential of the thermally unfolded state. The free energies of electron binding for the thermally unfolded proteins differ from those expected for a fully unfolded protein, suggesting that residual structure modulates the reduction potential. At temperatures near 50 °C the N52I mutation has a small but significant effect on oxidation state-sensitive structure in the thermally unfolded protein. Inspection of the high- resolution X-ray crystallographic structures of iso-2 and N52I iso-2 shows that the effects of the N52I mutation and oxidation state on native protein stability are correlated with changes in the mobility of specific polypeptide chain segments and with altered hydrogen bonding involving a conserved water molecule. However, there is no clear explanation of oxidation state or mutation induced differences in stability of the proteins in terms of observed changes in structure and mobility of the folded forms of the proteins alone.

Original languageEnglish (US)
Pages (from-to)1995-2007
Number of pages13
JournalBiochemistry
Volume35
Issue number6
DOIs
StatePublished - Feb 13 1996

ASJC Scopus subject areas

  • Biochemistry

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