Biophysical characterization of Entamoeba histolytica phosphoserine aminotransferase (EhPSAT): role of cofactor and domains in stability and subunit assembly.

We investigated the role of the cofactor PLP and its binding domain in stability and subunit assembly of phosphoserine aminotransferase (EhPSAT) from an enteric human parasite Entamoeba histolytica. Presence of cofactor influences the tertiary structure of EhPSAT because of the significant differences in the tryptophan microenvironment and proteolytic pattern of holo- and apo-enzyme. However, the cofactor does not influence the secondary structure of the enzyme. Stability of the protein is significantly affected by the cofactor as holo-enzyme shows higher T(m) and C(m) values for thermal and GdnHCl-induced denaturation, respectively, when compared to the apo-enzyme. The cofactor also influences the unfolding pathway of the enzyme. Although urea-dependent unfolding of both holo- and apo-EhPSAT is a three-state process, the intermediates stabilized during unfolding are significantly different. For holo-EhPSAT a dimeric holo-intermediate was stabilized, whereas for apo-EhPSAT, a monomeric intermediate was stabilized. This is the first report on stabilization of a holo-dimeric intermediate for any aminotransferase. The isolated PLP-binding domain is stabilized as a monomer, thus suggesting that either the N-terminal tail or the C-terminal domain of EhPSAT is required for stabilization of dimeric configuration of the wild-type enzyme. To the best of our knowledge, this is a first report investigating the role of PLP and various protein domains in structural and functional organization of a member of subgroup IV of the aminotransferases.