Hydrogen bonding/salt bridge network at the site of p.Lys250Asn ADNIV mutation.

<p><b>A</b>. In the crystal structure of calpain-9, Lys252 forms a salt bridge with Glu241, which in turn hydrogen bonds with the backbone nitrogen of Arg237 and locks the flexible gating loop in place. <b>B</b>. The same set of interactions are observed in the homology model of calpain-5, where Lys250 is well positioned to form a salt bridge with Asp239, which in turn can form a hydrogen bond with the backbone nitrogen of Val235. <b>C</b>. The p.Lys250Asn (purple) mutation modeled here indicates that Asn250, with altered hydrogen bonding capability, will interact differently with Asp239 and affect the conformation and flexibility that this catalytically important gating loop adopts. Changes in the conformation of the gating loop would subsequently alter enzymatic activity.