Homology modeling of wild type and pyrimethamine/cycloguanil-cross resistant mutant type Plasmodium falciparum dihydrofolate reductase. A model for antimalarial chemotherapy resistance

We propose a low-resolution model for both the wild type and the pyrimetham ine (Pyr)/cycloguanil (Cyc) cross- resistant mutant type Plasmodium falcipa rum DHFR (PfDHFR), based on homology modeling using chicken liver DHFR as a template. The built models contain five alpha -helices. eight beta -sheets , eight tight turns and several loops. The Ramachandran plot for the models shows 95.3 and 100% of the amino acid residues in the favorable regions fo r the whole enzymes and for the active sites, respectively. Furthermore, we made a preliminary analysis of the complexes Pyr/Cyc-wild DHFR and Pyr/Cyc -mutant DHFR in order to explain the probable mechanism of resistance. Our results show that the steric factor may be the main structural cause of P. falciparum resistance toward antifolate drugs. (C) 2001 Published by Elsevi er Science B.V.