3QK6:
reference: Structure of the Escherichia coli Phosphonate Binding Protein PhnD and Rationally Optimized Phosphonate Biosensors., Alicea I, Marvin JS, Miklos AE, Ellington AD, Looger LL, Schreiter ER, J Mol Biol. 2011 Dec 2;414(3):356-69. Epub 2011 Oct 12. PMID: 22019591

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