EWGWS insert in Plasmodium falciparum ookinete surface enolase is involved in binding of PWWP containing peptides: Implications to mosquito midgut invasion by the parasite

Functions of tryptophan residues in EWGWS insert of Plasmodium falciparum enolase

Plasmodium falciparum enolase (Pfeno) is a dimeric enzyme with multiple moonlighting functions. This enzyme is thus a potential target for anti-malarial treatments. A unique feature of Pfeno is the presence of a pentapeptide insert ¹⁰⁴ EWGWS ¹⁰⁸. The functional role of tryptophan residues in this insert was investigated using site-directed mutagenesis. Replacement of these two Trp residues with alanines (or lysines) resulted in a near complete loss of enolase activity and dissociation of the normal dimeric form into monomers. Molecular modeling indicated that ³⁴⁰R forms π-cation bonds with the aromatic rings of ¹⁰⁵W and ⁴⁶Y. Mutation induced changes in the interactions among these three residues were presumably relayed to the inter-subunit interface via a coil formed by ⁴⁶Y : ⁵⁹Y, resulting in the disruption of a salt bridge between ¹¹R : ⁴²⁵E and a π-cation interaction between ¹¹R : ⁵⁹Y. This led to a drop of ~ 4 kcal·mole^-1 in the inter-subunit docking energy in the mutant, causing a ~ 10³ fold decrease in affinity. Partial restoration of the inter-subunit interactions led to reformation of dimers and also restored a significant fraction of the lost enzyme activity. These results suggested that the perturbations in the conformation of the surface loop containing the insert sequence were relayed to the interface region, causing dimer dissociation that, in turn, disrupted the enzyme's active site. Since Plasmodium enolase is a moonlighting protein with multiple parasite-specific functions, it is likely that these functions may map on to the highly conserved unique insert region of this protein.

ENZYMES

Enolase(EC4.2.1.11).

Immunolocation and enzyme activity analysis of Cryptosporidium parvum enolase

Background

Enolase is an essential multifunctional glycolytic enzyme that is involved in many biological processes of apicomplexan protozoa, such as adhesion and invasion. However, the characteristics of enolase in Cryptosporidium parvum, including the location on the oocyst and the enzyme activity, remain unclear.

Methods

The C. parvum enolase gene (cpeno) was amplified by RT-PCR and sequenced. The deduced amino acid sequence was analysed by bioinformatics software. The gene was expressed in Escherichia coli BL21 (DE3) and purified recombinant protein was used for enzyme activity analysis, binding experiments and antibody preparation. The localisation of enolase on oocysts was examined via immunofluorescence techniques.

Results

A 1,350 bp DNA sequence was amplified from cDNA taken from C. parvum oocysts. The deduced amino acids sequence of C. parvum enolase (CpEno) had 82.1% homology with Cryptosporidium muris enolase, and 54.7–68.0% homology with others selected species. Western blot analysis indicated that recombinant C. parvum enolase (rCpEno) could be recognised by C. parvum-infected cattle sera. Immunolocalization testing showed that CpEno was found to locate mainly on the surface of oocysts. The enzyme activity was 33.5 U/mg, and the Michaelis constant (K_m) was 0.571 mM/l. Kinetic measurements revealed that the most suitable pH value was 7.0–7.5, and there were only minor effects on the activity of rCpEno with a change in the reaction temperature. The enzyme activity decreased when the Ca²⁺, K⁺, Mg²⁺ and Na⁺ concentrations of the reaction solution increased. The binding assays demonstrated that rCpEno could bind to human plasminogen.

Conclusion

This study is the first report of immunolocation, binding activity and enzyme characteristics of CpEno. The results of this study suggest that the surface-associated CpEno not only functions as a glycolytic enzyme but may also participate in attachment and invasion process of the parasite.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2200-y) contains supplementary material, which is available to authorized users.