2XTT:
reference: The catalytic aspartate is protonated in the Michaelis complex formed between trypsin and an in vitro evolved substrate-like inhibitor: a refined mechanism of serine protease action., Wahlgren WY, Pal G, Kardos J, Porrogi P, Szenthe B, Patthy A, Graf L, Katona G, J Biol Chem. 2010 Nov 21. PMID: 21097875

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