Abstract
In order to gain deeper insights into the functional sites of human placental alkaline phosphatase, the structures of the enzyme with the putative regulators L-Phe, pNPP and 5'-AMP [Llinas et al. (2005), J. Mol. Biol. 350, 441-451] were re-refined. Significant variations in ligand positioning and identity were found compared with the previous report. The multiple corrections to the model improved the phases and the electron-density maps, allowing the modeling of omitted side chains and multiple disordered residues. These improvements led to a change in the position of L-Phe at the peripheral binding site, which appeared to be reversed. The structure with pNPP contained only p-nitrophenol in three distinct sites, while the structure with 5'-AMP contained the p-nitrophenyl group in two of the sites instead of 5'-AMP. Comparison of the re-refined models shows a consistent pattern of interactions at the peripheral site.