Comparative Characterization of Plasmodium falciparum Small Heat Shock Proteins and Their Inhibition by Quercetin (3,3′,4′,5,7-Pentahydroxyflavone)

Plasmodium falciparum, the parasite responsible for the most severe form of malaria, encodes three small heat shock proteins (sHsps), PfHsp20a, PfHsp20b, and PfHsp20c, each containing a conserved α-crystallin domain (ACD). These class I sHsps are hypothesized to play critical roles in proteostasis under stress, yet their specific functions have remained poorly defined. In this study, all three sHsps were recombinantly expressed and purified for structural and functional characterization. Circular dichroism and thermal shift assays revealed distinct conformational properties, with PfHsp20a exhibiting the highest thermal and chemical stability. Functional assays using malate dehydrogenase and citrate synthase confirmed that all three isoforms possess autonomous chaperone activity, although with varying efficiency. Notably, the plant-derived flavonoid quercetin disrupted both the structure and function of the sHsps in a concentration-dependent manner, with PfHsp20c being the most sensitive. Quercetin also inhibited the growth of P. falciparum Nf54 and Dd2 strains in vitro with IC₅₀ values of 5.4 μM and 7.8 μM, respectively. These results provide the first direct evidence of independent chaperone activity in P. falciparum sHsps and highlight their vulnerability to small molecule inhibition. This establishes their potential as novel drug targets for antimalarial intervention.