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Entamoeba histolytica Phosphoserine aminotransferase (EhPSAT): insights into the structure-function relationship

Vibhor Mishra1, Vahab Ali2, Tomoyoshi Nozaki3 and Vinod Bhakuni1*

Author Affiliations

1 Division of Molecular and Structural Biology, Central Drug Research Institute, Chattar Manzil Palace, Council of scientific and industrial research (CSIR), Lucknow 226001, India

2 Department of Biochemistry, Rajendra Memorial Research Institute of Medical Sciences, AgamKuan, Patna 800007, India

3 Department of Parasitology, National Institute of Infectious diseases, 1-23-1 Toyama, Shinjuku-Ku, Tokyo 162-8640, Japan

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BMC Research Notes 2010, 3:52  doi:10.1186/1756-0500-3-52

Published: 3 March 2010



Presence of phosphorylated Serine biosynthesis pathway upstream to the de novo cysteine biosynthesis pathway makes PSAT a crucial enzyme. Besides this, phoshoserine produced by the enzyme can also be taken up directly by cysteine synthase as a substrate. PSAT is a PLP dependent enzyme where the cofactor serves as an epicenter for functional catalysis with the active site architecture playing crucial role in optimum function of the enzyme.


EhPSAT is a homodimer of molecular mass 86 kDa. To understand the structural modulations associated with pH dependent changes in functional activity of EhPSAT detailed biophysical studies were carried out. pH alterations had no significant effect on the secondary structure, cofactor orientation and oligomeric configuration of the enzyme however, pH dependent compaction in molecular dimensions was observed. Most interestingly, a direct correlation between pH induced modulation of functional activity and orientation of Trp 101 present in the active site of the enzyme was observed. Sodium halides nullified the pH induced global changes in the enzyme, however differential effect of these salts on the active site microenvironment and functional activity of the enzyme was observed.


The study unequivocally demonstrates that pH induced selective modification of active site microenvironment and not global change in structure or oligomeric status of the enzyme is responsible for the pH dependent change in enzymatic activity of PSAT.