“…5 D ). Recently, site-directed mutagenesis results have also demonstrated that interactions of Arg101 with a neighboring residue, Asn447, are important for formation and stabilization of the C4a-OOH flavin ( 84 ). Thr193 forms a hydrogen bond with the N5-H of the flavin isoalloxazine ring and interacts with Asp254, which in turn forms a salt bridge with Arg233 and Arg439 ( Fig.…”