Glu-108 is essential for subunit assembly and dimer stability of D-phosphoglycerate dehydrogenase from Entamoeba histolytica.

D-phosphoglycerate dehydrogenase catalyses the first step of phosphorylated serine biosynthesis pathway by oxidizing 3-phosphoglycerate, a glycolysis intermediate into phosphohydroxsy pyruvate. For Entamoeba histolytica this pathway is an integral part of the cysteine metabolism, which is considered to be vital for growth and survival of the parasite. Entamoeba histolytica D-phosphoglycerate dehydrogenase (EhPGDH) exists as a homodimer at pH 7. Mild acidic conditions induce significant changes in the functional and structural features of the protein as observed by enzymatic activity, spectropolarimetric measurements and fluorescence spectroscopy. Most interestingly the oligomeric status of the protein was lost and a functionally inactive monomer was stabilized at pH 5. Computational modeling and molecular dynamic simulations show that dimeric assembly of EhPGDH was stabilized with the help of several inter-subunit non-covalent interactions and subunit dissociation at pH 5 can be attributed to protonation of acidic amino acid residues present at the dimer interface. Site directed mutagenesis studies suggest that Glu-108 is essential for subunit assembly as the E108A mutant existed as monomer even at pH 7. The studies unequivocally show that the electrostatic interactions at the dimer interface play a crucial role in the stability of the protein and a complete dimer is essentially required for optimal enzymatic activity.