4HKA:
reference: Crystal structure of Drosophila melanogaster tryptophan 2,3-dioxygenase reveals insights into substrate recognition and catalytic mechanism., Huang W, Gong Z, Li J, Ding J, J Struct Biol. 2013 Mar;181(3):291-9. doi: 10.1016/j.jsb.2013.01.002. Epub 2013, Jan 17. PMID: 23333332

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