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Mutational analysis of the N-capping box of the alpha-helix of chymotrypsin inhibitor 2.

The N-terminus of the helix of the chymotrypsin inhibitor 2 from barley (CI2) has an N-capping box (Ser at the first position in the helix and Glu at position 4) as well as a frequently found Glu at position 3. The energetic importance of this motif has been studied by determining the free energy of unfolding of the wild-type and protein mutants derived from those residues using guanidinium chloride-induced denaturation and differential scanning microcalorimetry. Mutating N-cap residue Ser31 to either Ala or Gly destabilizes CI2 by 0.8-1 kcal mol-1. Truncation of the box in the mutants SA31EA33EA34 or SG31EA33EA34 destablizes the protein by 1.5-2 kcal mol-1. The N-capping box is an important motif in stabilizing proteins and delineating the beginning of alpha-helices in the pathway of protein folding.