1GPH:
reference: Structure of the allosteric regulatory enzyme of purine biosynthesis., Smith JL, Zaluzec EJ, Wery JP, Niu L, Switzer RL, Zalkin H, Satow Y, Science 1994 Jun 3;264(5164):1427-33. PMID: 8197456

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