The role of disulfide bond formation in the structural transition observed in the intermediate filaments of developing hair

Hair keratin is a composite structure in which intermediate filaments (IF) are embedded in a protein matrix. During the early stages of development in the hair follicle the redox potential is such that the cysteine residues in the IF are maintained in a reduced form. However, at a late stage of development the redox potential changes to produce an oxidizing environment and the IF undergo a structural transition involving both molecular slippage and radial compaction. In our earlier study the changes in the molecular parameters were estimated from knowledge of the sites of artificially induced crosslinks, and it was noted that the changes in these parameters realigned many of the cysteine residues to positions more favorable to disulfide bond formation. As the energy involved in the formation of disulfide bonds is much greater than that of hydrogen bonds or van der Waals interactions the structural transition is likely to be dominated by the requirement that the bonded cysteine residues occur at closely equivalent axial positions. This criterion was used in the present study to obtain more precise values for the molecular parameters in the oxidized fiber than has hitherto been possible. A comparison of the sequences of hair keratins and epidermal keratins suggests that the slippage observed in trichocyte IF during keratinization does not occur in epidermal IF.