4ME4:
reference: Crystal structure of an HD-GYP domain cyclic-di-GMP phosphodiesterase reveals an enzyme with a novel trinuclear catalytic iron centre., Bellini D, Caly DL, McCarthy Y, Bumann M, An SQ, Dow JM, Ryan RP, Walsh MA, Mol Microbiol. 2014 Jan;91(1):26-38. doi: 10.1111/mmi.12447. Epub 2013 Nov 24. PMID: 24176013

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