DYNAMICAL STUDIES OF PEPTIDE MOTIFS IN THE PLASMODIUM-FALCIPARUM CIRCUMSPOROZOITE SURFACE PROTEIN BY RESTRAINED AND UNRESTRAINED MD SIMULATIONS

The immunodominant region on the circumsporozoite surface (CS) protein of the malaria parasite Plasmodium falciparum contains 37 repeated co pies of a asparagine-alanine-asparagine-proline (NANP) motif NMR studi es of linear synthetic peptides containing one, two or three repeat un its provided evidence for nascent type I beta-turns within the NPNA ca dence in aqueous solution. The beta-turns could be stabilised upon sub stituting proline for alpha-methylproline (p(Me)) in the dodecamer (NP (Me)NA)(3), without loss of the ability to elicit antibodies cross-rea ctive with P. falciparum sporozoites. In this work, four 4 ns MD simul ations of the dodecapeptide Acetyl-(NP(Me)NA)(3), in water, using NOE distance restraints, using (3)J-coupling constant restraints, using bo th these restraints and without restraints, were carried out to determ ine the conformations of this peptide in aqueous solution. An unrestra ined MD simulation of the unmethylated Ac-(NPNA)(3) peptide in water w as also carried out to investigate the effect of the additional methyl groups on the structure and dynamics of the peptide. The application of NOE distance restraints and (3)J-coupling constant restraints leads to contradictory results, probably due to different averaging time sc ales inherent to the measurement of these data, which exceed the 100 p s averaging applied in the simulations. The additional methyl groups l ead to more compact structures, which display enhanced local fluctuati ons. The central tetrapeptide adopts a type I beta-turn, while the out er motifs display more conformational variability. The three motifs in the methylated dodecamer peptide, however, adopt frequently in the di stance restrained MD simulation a compact structure such that the oute r motifs appear to form a hydrophobic core by stacking of their two pr oline rings. This arrangement also suggests how a peptide containing m ultiple tandemly linked copies of a stable beta-turn NPNA motif might adopt a folded stem-like structure, which conceivably may be of biolog ical relevance in the native CS protein. (C) 1997 Academic Press Limit ed.