Sulphur mobilisation for [Fe-S] cluster assembly by the essential SUF pathway in the Plasmodium falciparum apicoplast and its inhibition

Abstract

The plastid of the malaria parasite- the apicoplast is essential for parasite survival. It houses several pathways of bacterial origin that are considered as attractive sites for drug intervention. Among these is the SUF pathway of [Fe-S] cluster biogenesis. Although the SUF pathway is essential for apicoplast maintenance and parasite survival there has been limited biochemical investigation of its components and inhibitors of Plasmodium SUFs have not been identified. We report characterization of two proteins, Plasmodium falcipraum SufS and SufE, which mobilise sulphur in the first step of [Fe-S] cluster assembly, and confirm their exclusive localisation to the apicoplast. The cysteine desulphurase activity of PfSufS is greatly enhanced by PfSufE, and the PfSufS-PfSufE complex is detected in vivo. Structure modelling of the complex reveals proximal positioning of conserved cysteine residues of the two proteins that would allow sulphide transfer from the PLP-cofactor bound active site of PfSufS. Sulphide release from the L-cysteine substrate catalysed by PfSufS is inhibited by the PLP-inhibitor D-cycloserine that forms an adduct with PfSufS-bound PLP. D-cycloserine is also inimical to parasite growth with an IC50 close to that reported for Mycobacterium tuberculosis against which the drug is in clinical use. Our results establish function of two proteins that mediate sulphur mobilisation- the first step in the apicoplast SUF pathway, and provide rationale for drug design based on inactivation of the PLP-cofactor of PfSufS.