5YRS:
reference: X-ray snapshot of HIV-1 protease in action: observation of tetrahedral intermediate and short ionic hydrogen bond SIHB with catalytic aspartate., Das A, Mahale S, Prashar V, Bihani S, Ferrer JL, Hosur MV, J Am Chem Soc. 2010 May 12;132(18):6366-73. PMID: 20397633

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