Calculation of the three-dimensional structures of two antigenic sequences, pro-pro-pro-pro-asn-pro-asn-asp-pro and pro-pro-pro-pro-asn-pro-asn-asp-pro-pro-pro, of the circumsporozoite protein from a malaria-causing Plasmodium.

Using the chain build-up procedure based on the program ECEPP, we have computed the lowest energy structures for two terminally blocked subsequences from the antigenic circumsporozoite protein of Plasmodium berghei, that is known to cause malaria in animals. The full antigenic sequence is an octapeptide proline-rich tandem repeat, (Pro-Pro-Pro-Pro-Asn-Pro-Asn-Asp)(2). We computed the structures for the first octapeptide plus one Pro from the second octapeptide, terminally blocked CH(3)CO-Pro-Pro-Pro-Pro-Asn-Pro-Asn-Asp-Pro-NHCH(3) as well as the first octpeptide with an additional three Pro residues from the adjoining unit, i.e., CH(3)CO-Pro-Pro-Pro-Pro-Asn-Pro-Asn-Asp-Pro-Pro-Pro-NHCH(3). We find that the first sequence adopts a number of different low energy structures, the most probable of which has a probability of occurrence of 56 %. Addition of two more Pro residues results in the adoption a single, unique lowest energy structure that has a probability of occurrence of over 95 % without solvation effects and 86 % when solvation effects are included in the calculations. We predict that this structure may be the one recognized as a major antigenic determinant.