MS05-P08 Mechanistic study of human phosphoserine phosphatase (hPSP) via crystallization with intermediatesHuman phosphoserine phosphatase (hPSP) catalyses the third and final step of the serine pathway, which is the transformation of O-Phospho-L-Serine into L-Serine. This Mg2+ enzyme is part of the Haloacid Dehalogenase superfamily of enzymes and possesses a Rossman-like domain in order to orient the substrate within the active site. This protein is also a therapeutic target in colorectal cancer 
In this work, hPSP was produced, purified and crystallized in various conditions. In order to design inhibitors and gain a better understanding of the catalytic mechanism, we co-crystallized the enzyme in presence of different substrates and inhibitors. Crystals were obtained and data were collected at SOLEIL Synchrotron in France.
After resolution, we obtained a co-crystal of hPSP with phosphate as a ligand at a resolution of 1.5 Å. We also resolved a dynamic loop that was absent in the other high-resolution structure of hPSP (PDB: 1NNL) . This loop was also described by Kim et al with a resolution range going from 2.5 to 2.8 Å but was placed in a different manner (PDB: 1L8L & 1L8O) . The dynamic properties of this chain indicates a possible induced fit in order to open and close the active site during reaction.
We will complete those results thanks to other co-crystallization assays of hPSP with ligands. Moreover, we will apply dynamic simulations to elucidate the catalytic mechanism of this enzyme.
Acknowledgment. The authors thank members of namedic who generated namedic library used for inhibition and crystallization assays, in particular Prof B. Masereel and L. Pochet. Authors also acknowledge SOLEIL for provision of synchrotron radiation facilities and we would like to thank Leonard Chavas for assistance in using beamline Proxima I.
 Li, X. et al (2016). Biochemical and Biophysical Research Communications, 477, 633-639
 Peeraer, Y. et al (2003). Acta Crystallographica Section D Biological Crystallography, D59, 971-977
 Kim, H-Y. et al (2002). Journal of Biological Chemistry, 277, 46651-46658.Keywords: Phosphoserine phosphatase, protein crystallization, co-crystallization