4DCV:
reference: Potassium Acts as a GTPase-Activating Element on Each Nucleotide-Binding Domain of the Essential Bacillus subtilis EngA., Foucher AE, Reiser JB, Ebel C, Housset D, Jault JM, PLoS One. 2012;7(10):e46795. doi: 10.1371/journal.pone.0046795. Epub 2012 Oct 8. PMID: 23056455

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