Correlation of beta-bend conformations of tetrapeptides with their activities in CD4-receptor binding assays

Conformational analysis, based on ECEPP (Empirical Conformational Energy Program for Peptides) using the chain build-up procedure, was applied to determine the low-energy conformations for a series of tetrapeptides. The tetrapeptides are components of larger peptides which have been found to bind to the CD4 receptor of monocytes. Several previous studies have implicated the tetrapeptide units investigated here as being critical to the biological activities of the full peptides. Five such tetrapeptides were studied: Ser-Ser-Asn-Tyr (from ribonuclease A), Thr-Thr-Asn-Tyr (from peptide T, known to block human immunodeficiency virus from attaching to CD4+ T cells), Thr-Ile-Asn-Tyr (from polio virus coat protein, which is less active than the other peptides in binding to CD4 receptors), Ser-Ser-Ala-Tyr (from the gp 120 coat protein of human immunodeficiency virus, a variant of the peptide T sequence, active in blocking viral attachment to CD4+ cells), and the tetrapeptide from an active synthetic pentapeptide, Asn-Thr-Lys-Tyr (from Asn-Thr-Lys-Tyr-Thr). Using a 7 kcal/mol cutoff, the low-energy conformations for each peptide were computed. Approximately 20,000 conformations were computed for each tetrapeptide. Residue probability profiles were determined for each tetrapeptide. All tetrapeptides except for the polio sequence showed flexibility in the sense that many low-energy conformations were possible. In previous studies, it was postulated that the critical tetrapeptide units would adopt conformations similar to the one observed in a segment of ribonuclease A, residues 22-25, a beta-bend, which is part of an octapeptide segment (residues 19-26) that is homologous to the sequence of peptide T.(ABSTRACT TRUNCATED AT 250 WORDS)