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The CXXC motif at the N terminus of an -helical peptide

Manchester Interdisciplinary Biocentre, The University of Manchester, Manchester M1 7ND, United Kingdom

(RECEIVED April 6, 2006; FINAL REVISION May 16, 2006; ACCEPTED May 16, 2006)

An active site containing a CXXC motif is always found in the thiol-disulphide oxidoreductase superfamily. A survey of crystal structures revealed that the CXXC motif had a very high local propensity (26.3 ± 6.2) for the N termini of -helices. A helical peptide with the sequence CAAC at the N terminus was synthesized to examine the helix-stabilizing capacity of the CXXC motif. Circular dichroism was used to confirm the helical nature of the peptide and study behavior under titration with various species. With DTT, a redox potential of E_o = –230 mV was measured, indicating that the isolated peptide is reducing in nature and similar to native human thioredoxin. The pK_a values of the individual Cys residues could not be separated in the titration of the reduced state, giving a single transition with an apparent pK_a of 6.74 (±0.06). In the oxidized state, the N-terminal pK_a is 5.96 (±0.05). Analysis of results with the modified helix-coil theory indicated that the disulfide bond stabilized the -helical structure by 0.5 kcal/mol. Reducing the disulfide destabilizes the helix by 0.9 kcal/mol.

Keywords: -helix; N-cap; N3; circular dichroism; protein folding; protein stability; CXXC; helix-coil theory

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