4TTG:
reference: Elucidating factors important for monovalent cation selectivity in enzymes: E. coli beta-galactosidase as a model., Wheatley RW, Juers DH, Lev BB, Huber RE, Noskov SY, Phys Chem Chem Phys. 2015 Apr 8;17(16):10899-909. doi: 10.1039/c4cp04952g. PMID: 25820412

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