1B4K:
reference: High resolution crystal structure of a Mg2+-dependent porphobilinogen synthase., Frankenberg N, Erskine PT, Cooper JB, Shoolingin-Jordan PM, Jahn D, Heinz DW, J Mol Biol 1999 Jun 11;289(3):591-602. PMID: 10356331

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