Structural insights into the protease-like antigen Plasmodium falciparum SERA5 and its noncanonical active-site serine.

The sera genes of the malaria-causing parasite Plasmodium encode a family of unique proteins that are maximally expressed at the time of egress of parasites from infected red blood cells. These multi-domain proteins are unique, containing a central papain-like cysteine-protease fragment enclosed between the disulfide-linked N- and C-terminal domains. However, the central fragment of several members of this family, including serine repeat antigen 5 (SERA5), contains a serine (S596) in place of the active-site cysteine. Here we report the crystal structure of the central protease-like domain of Plasmodium falciparum SERA5, revealing a number of anomalies in addition to the putative nucleophilic serine: (1) the structure of the putative active site is not conducive to binding substrate in the canonical cysteine-protease manner; (2) the side chain of D594 restricts access of substrate to the putative active site; and (3) the S(2) specificity pocket is occupied by the side chain of Y735, reducing this site to a small depression on the protein surface. Attempts to determine the structure in complex with known inhibitors were not successful. Thus, despite having revealed its structure, the function of the catalytic domain of SERA5 remains an enigma.

Structure of Leishmania mexicana phosphomannomutase highlights similarities with human isoforms.

Phosphomannomutase (PMM) catalyses the conversion of mannose-6-phosphate to mannose-1-phosphate, an essential step in mannose activation and the biosynthesis of glycoconjugates in all eukaryotes. Deletion of PMM from Leishmania mexicana results in loss of virulence, suggesting that PMM is a promising drug target for the development of anti-leishmanial inhibitors. We report the crystallization and structure determination to 2.1 A of L. mexicana PMM alone and in complex with glucose-1,6-bisphosphate to 2.9 A. PMM is a member of the haloacid dehalogenase (HAD) family, but has a novel dimeric structure and a distinct cap domain of unique topology. Although the structure is novel within the HAD family, the leishmanial enzyme shows a high degree of similarity with its human isoforms. We have generated L. major PMM knockouts, which are avirulent. We expressed the human pmm2 gene in the Leishmania PMM knockout, but despite the similarity between Leishmania and human PMM, expression of the human gene did not restore virulence. Similarities in the structure of the parasite enzyme and its human isoforms suggest that the development of parasite-selective inhibitors will not be an easy task.

Structure of glyceraldehyde-3-phosphate dehydrogenase from Plasmodium falciparum.

The malaria parasite Plasmodium falciparum is responsible for about two million deaths annually, making it important to obtain information about enzymes from this organism that represent potential drug targets. The gene for P. falciparum glyceraldehyde-3-phosphate dehydrogenase (PfGAPDH) has been cloned and the protein expressed as a hexahistidine-tagged recombinant protein in Escherichia coli. The recombinant protein has been crystallized and its three-dimensional structure determined. One molecule of the cofactor NAD+ is bound to each of the four subunits in the tetrameric enzyme. The major structural feature distinguishing human GAPDH from PfGAPDH is the insertion of a dipeptide (-KG-) in the so-called S loop. This insert, together with other characteristic single-amino-acid substitutions, alters the chemical environment of the groove that encompasses the R dyad and that links adjacent cofactor-binding sites and may be responsible for the selective inhibition of the enzyme by ferriprotoporphyrin IX.