A redetermination of the structure of the triple mutant (K53,56,120M) of phospholipase A₂ at 1.6 Å resolution using sulfur-SAS at 1.54 Å wavelength

The crystal structure of the triple mutant K53,56,120M of bovine pancreatic phospholipase A₂ has been redetermined using sulfur single-wavelength anomalous scattering. The synchrotron data were collected at λ = 1.54 Å and the crystal diffracted to 1.6 Å resolution. The program SOLVE was used to locate the heavy atoms and to estimate the initial phases and the resulting map was then subjected to RESOLVE. The output of 455 non-H atoms, including 12 S atoms, one calcium ion and one chloride ion, were then subjected to ARP/wARP followed by REFMAC. With the improved phases, the automatic model building successfully built more than 85% of the 123 residues, excluding the N- and C-­terminal residues. The final crystallographic R factor is 17.7% (R_free = 21.7%). The refined model consists of 954 non-H protein atoms, 165 water O atoms, three 2-methyl-2,4-pentanediol (MPD) molecules, one calcium ion and one chloride ion. The present work is yet another example that shows the utility of single-wavelength anomalous scattering data for solving a protein structure.