4DUW:
reference: Structural explanation for allolactose (lac operon inducer) synthesis by lacZ beta-galactosidase and the evolutionary relationship between allolactose synthesis and the lac repressor., Wheatley RW, Lo S, Jancewicz LJ, Dugdale ML, Huber RE, J Biol Chem. 2013 May 3;288(18):12993-3005. doi: 10.1074/jbc.M113.455436. Epub, 2013 Mar 13. PMID: 23486479

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