The mitochondrial cytochrome oxidase subunit III gene and protein sequence analysis in chloroquine-resistant Plasmodium falciparum

Chloroquine has been an important antimalarial drug for several decades. However, the rapid spread of resistance has severely limited its effectiveness. One hypothesis of the mechanism of resistance was thought to be rapid efflux of the drug out of the resistant parasites. This efflux was thought to relate with the energy-generating process, electron transport system (ETS). The subunit III (COIII) of cytochrome oxidase, enzyme complex IV in ETS, plays an important role as the site for proton transfer from mitochondrial inner membrane to intermembrane space creating proton motive force for ATP production. In this study, we performed an analysis of this gene in chloroquine-resistant P. falciparum compared with the sensitive one. The nucleotide sequencing was performed and the deduced amino acid sequence was analysed. Two point mutations of this COIII gene, T771C and A1484G, were found in chloroquine-resistant P. falciparum. These mutations changed amino acid codon I268V and F30S of COIII protein subunit. The amino acid substitution with the same chemical property, isoleucine to valine at the codon 268, did not alter secondary structure even though this codon is located at a highly conserved region. For F30S which is at hypervariable region, the replacement with different amino acids, phenylalanine to serine, caused only minor alterations of secondary structure. However, both mutations were not specific and consistent with the chloroquine-resistant strains examined. These mutations in COIII gene may not be involved in the mechanism of chloroquine resistance in P. falciparum.