2SPT:
reference: Differences in the metal ion structure between Sr- and Ca-prothrombin fragment 1., Seshadri TP, Skrzypczak-Jankun E, Yin M, Tulinsky A, Biochemistry 1994 Feb 8;33(5):1087-92. PMID: 8110739

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