By Eben Labuschagne
Researchers at Stellenbosch University (SU) have identified three key proteins that could serve as a new target in the fight against Plasmodium falciparum. It is a parasite responsible for the high infectivity, survivability, and mortality rate of many malaria cases. Their research, published in July in The Protein Journal, marks an important contribution in the long battle against malaria, finding that a common plant compound disrupts proteins critical to the parasite’s survivability in the extreme conditions of the human body.
Lead researcher Dr Tawanda Zininga and colleague Francisca Magum Timothy successfully produced and characterised three proteins in the laboratory for the first time. These proteins had been previously identified, but their research provides the first comprehensive analysis of their structure and function. The proteins function independently of ATP, the cell’s primary energy currency, making them particularly important when the parasite faces extreme stress conditions where energy availability is limited.
The breakthrough is the identification of these three proteins as ‘small heat shock proteins’ that act as molecular chaperones for the malaria parasite. These proteins, (designated PfHsp20a, PfHsp20b, and PfHsp20c) are essential to the parasite’s survivability. In Plasmodium falciparum, certain ATP-dependent proteins must fold into correct, three-dimensional shapes as part of the parasite’s metabolic functionality, but unfavourable conditions may cause them to unfold or misfold. Should these misfolded proteins aggregate (join a clump of proteins as part of its metabolic functionality), the protein aggregation will not function properly, threatening the parasite’s metabolic function. Such misfolding typically occurs under stress conditions, such as temperature change, in which energy ATP is scarcer.
These heat-shock proteins, or sHsps, are produced under such stress conditions and they prevent improper protein aggregation. These sHsps act as holdases, which work to prevent misfolded proteins from aggregating and forming an essential quality control system. In this way they maintain the structural integrity of other protein aggregations, and so ensure they take the correct “pathways” in the cells, when the parasite encounters physiological stress as it moves between mosquito and human hosts.
The team found that quercetin, a flavonoid compound found in fruits and vegetables, significantly disrupts both the structure and function of the heat shock proteins. Laboratory tests showed that quercetin reduced the size of the heat-shock proteins’ oligomeric assemblies – multi-unit structures required for their chaperone activity – and reduced their ability to prevent protein aggregation.
Quercetin inhibited the growth of both drug-sensitive and drug-resistant Plasmodium falciparum strains in culture, with IC50 values of 5.4 μM and 7.8 μM respectively. While these values indicate lower potency than current front-line antimalarials, quercetin showed similar effectiveness against both strains, with a resistance index of 1,32. This suggests quercetin may target the same mechanism in both drug-sensitive and drug-resistant parasites, potentially through inhibition of the heat shock proteins.
These experiments indicated distinct responses to quercetin among the three heat shock proteins, with PfHsp20c demonstrating the highest sensitivity to inhibition during functional assays using malate dehydrogenase and citrate synthase as model substrates.
The proteins’ unique role in the parasite’s stress response network makes them attractive as drug targets. By disrupting these proteins, it may be possible to prevent the parasite from adapting to physiological challenges it encounters inside the human body. Regarding pharmacological usability, quercetin’s favourable safety profile and pharmacological properties indicate it may be a viable starting point for developing more potent derivatives. The research also provides a framework for screening other small molecules that might target these essential parasite proteins.
With malaria claiming hundreds of thousands of lives annually and drug resistance spreading, unequivocally identifying the parasite’s dependence on these molecular chaperones for survival marks necessary and important progress. While the availability and usability of quercetin as treatment material is indicative of developmental possibilities, there remains research to be done about whether or not these heat shock proteins can be selectively targeted outside of culture, and in the human body.As such, Zininga recommends that future research focuses on understanding how these proteins interact with other components of the parasite’s protein quality control machinery and on optimising quercetin-like compounds for enhanced antimalarial activity. The work provides the first direct evidence that Plasmodium falciparum‘s small heat shock proteins are both functionally important and pharmacologically vulnerable. The paper is called “Comparative Characterization of Plasmodium falciparum Small Heat Shock Proteins and Their Inhibition by Quercetin (3,3′,4′,5,7-Pentahydroxyflavone)”, published in The Protein Journal.
