6U1J:
reference: D-alanine-D-alanine ligase as a model for the activation of ATP-grasp enzymes by monovalent cations., Pederick JL, Thompson AP, Bell SG, Bruning JB, J Biol Chem. 2020 Apr 25. pii: RA120.012936. doi: 10.1074/jbc.RA120.012936. PMID: 32335509

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