Structural composition of βI- and βII-proteins

Circular dichroism spectra of proteins are sensitive to protein secondary structure. The CID spectra of a-rich proteins are similar to those of model alpha-helices, but P-rich proteins exhibit CD spectra that are reminiscent of CD spectra of either model P-sheets or unordered polypeptides. The existence of these two types of CD spectra for P-rich proteins form the basis for their classification as beta(I)- and beta(II)-proteins. Although the conformation of P-sheets is largely responsible for the CD spectra of beta(I)-proteins, the source of beta(II)-protein CD, which resembles that of unordered polypeptides, is not completely understood. The CD spectra of unordered polypeptides are similar to that of the poly(Pro)II-helix, and the poly(Pro)II-type (P-2) Structure forms a significant fraction of the unordered conformation in globular proteins. We have compared the P-sheet and P-2 structure contents in P-rich proteins to understand the origin of beta(II)-protein CD. We find that beta(II)-proteins have a ratio of P-2 to P-sheet content greater than 0.4, whereas for PI-proteins this ratio is less than 0.4. The beta-sheet content in beta(I)-proteins is generally higher than that in beta(II)-proteins. The origin of two classes of CD spectra for P-rich proteins appears to lie in their relative P-sheet and P-2 structure contents.