2J4R:
reference: Structure of the PPX/GPPA phosphatase from Aquifex aeolicus in complex with the alarmone ppGpp., Kristensen O, Ross B, Gajhede M, J Mol Biol. 2008 Feb 1;375(5):1469-76. Epub 2007 Dec 4. PMID: 18155044

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).