Replacement of Ser108 in Plasmodium falciparum enolase results in weak Mg(II) binding: Role of a parasite-specific pentapeptide insert in stabilizing the active conformation of the enzyme

Abstract

A distinct structural feature of Plasmodium falciparum enolase (Pfeno) is the presence of a five amino acid insert -104EWGWS108- that is not found in host enolases. Its conservation among apicomplexan enolases has raised the possibility of its involvement in some important physiological function(s). Deletion of this sequence is known to lower kcat/Km, increase Ka for Mg(II) and convert dimer into monomers (Vora HK, Shaik FR, Pal-Bhowmick I, Mout R & Jarori GK (2009) Arch Biochem Biophys 485, 128-138). These authors also raised the possibility of the formation of an H-bond between Ser108 and Leu49 that could stabilize the apo-Pfeno in an active closed conformation that has high affinity for Mg(II). Here, we examined the effect of replacement of Ser108 with Gly/Ala/Thr on enzyme activity, Mg(II) binding affinity, conformational states and oligomeric structure and compared it with native recombinant Pfeno. The results obtained support the view that Ser108 is likely to be involved in the formation of certain crucial H-bonds with Leu49. The presence of these interactions can stabilize apo-Pfeno in an active closed conformation similar to that of Mg(II) bound yeast enolase. As predicted, S108G/A-Pfeno variants (where Ser108-Leu49 H-bonds are likely to be disrupted) were found to exist in an open conformation and had low affinity for Mg(II). They also required Mg(II) induced conformational changes to acquire the active closed conformational state essential for catalysis. The possible physiological relevance of apo-Pfeno being in such an active state is discussed. Database Enolase, 2-phosphoglycerate dehydratase (EC4.2.1.11) The functional significance of a parasite specific five amino acid insert (-104EWGWS108-) in Plasmodium spp. enolase is explored. The stabilization of parasite apo-enolase in a closed active conformation is attributed to the ability of Ser108 to form hydrogen bonds with Leu49. Such a stabilization eliminates the need for Mg(II) induced conformational changes, as is the case with enolases from other species.