The secondary structure of ovomucoid and its domains as studied by circular dichroism

The secondary structure of chicken egg white ovomucoid and its domains was studied by means of circular dichroism (CD). The various domains (domain I, domains II . III, domains I . II, and domain III with and without carbohydrate) were prepared from the ovomucoid by cyanogen bromide cleavage and Staphylococcus aureus protease V-8 digestions. The carbohydrate moiety in the ovomucoid was isolated after its extensive pronase and endo-beta-N-acetyl-glucosaminidase H digestions. On the net CD spectra of polypeptide chain in the ovomucoid and domain preparations, which were obtained by subtracting the spectrum of the carbohydrate moiety from their spectra, the fractions of secondary structure were calculated by the method of Chang et al. (Chang, C.T., Wu, C.S.C. and Yang, J.T. (1978) Anal. Biochem. 91, 13-31). The estimated composition of the secondary structure in the ovomucoid was as follows: alpha-helix, 26%; beta-structure, 46%; beta-turn, 10%; and random coil, 18%. The secondary structure compositions estimated for the three domains suggested that domains I and II were homologous to one another but not to domain III in regard to the proportion of secondary structure content.