Structural composition of βI‐ and βII‐proteins

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Abstract
Circular dichroism spectra of proteins are sensitive to protein secondary structure. The CD spectra of α‐rich proteins are similar to those of model α‐helices, but β‐rich proteins exhibit CD spectra that are reminiscent of CD spectra of either model β‐sheets or unordered polypeptides. The existence of these two types of CD spectra for β‐rich proteins form the basis for their classification as βI‐ and βII‐proteins. Although the conformation of β‐sheets is largely responsible for the CD spectra of βI‐proteins, the source of βII‐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 (P2) structure forms a significant fraction of the unordered conformation in globular proteins. We have compared the β‐sheet and P2 structure contents in β‐rich proteins to understand the origin of βII‐protein CD. We find that βII‐proteins have a ratio of P2 to β‐sheet content greater than 0.4, whereas for βI‐proteins this ratio is less than 0.4. The β‐sheet content in βI‐proteins is generally higher than that in βII‐proteins. The origin of two classes of CD spectra for β‐rich proteins appears to lie in their relative β‐sheet and P2 structure contents.