1.4.99.6 A46G mutation in loop L1. Probability of being in open conformation is higher than that of wild-type, yielding an increased level of solvent exposure of the active site. The flavin fluorescence intensity is about 2fold higher than in wild-type. Loop L1 is important for substrate capture and catalysis -, 740089 1.4.99.6 E87L kcat/Km pH-profile of the E87L mutant indicates only a single unprotonated group is required for maximal activity with D-arginine. E87 is the unprotonated group on the enzyme that binds cationic substrates 733215 1.4.99.6 H48F residue H48 is not responsible for the observed pKa value 733215 1.4.99.6 S45A mutation in loop L1. Probability of being in open conformation is higher than that of wild-type, yielding an increased level of solvent exposure of the active site. The flavin fluorescence intensity is about 2fold higher than in wild-type. Loop L1 is important for substrate capture and catalysis -, 740089 1.4.99.6 Y249F mutant protein shows both an active yellow FAD (Y249F-y) and an inactive chemically modified green 6-hydroxy-FAD cofactor. Variants show no differences in the overall protein structure and fold. Variant Y249F-y displays an alternative conformation for some active site residues and the flavin cofactor. Y249F-y with FAD samples a metastable conformational state, not available to the wild-type enzyme. The alternate conformation in the Y249F-y enzyme is responsible for the higher spin density at the C6 atom of flavin, consistent with the formation of 6-hydroxy-FAD 762738 1.4.99.6 Y249F steady-state kinetic parameters similar to those of the wild-type enzyme. Rate constants for flavin reduction (kred) with D-leucine are 3fold smaller than the wild-type value with similar pKa values for an unprotonated group of about 10.0. Cleavage of the substrate NH and CH bonds in the enzyme variant occurs in synchronous fashion, which can be reconciled with a hydride transfer mechanism 733354 1.4.99.6 Y53F steady-state kinetic parameters similar to those of the wild-type enzyme. Rate constants for flavin reduction (kred) with D-leucineare 3fold smaller than the wild-type value with similar pKa values for an unprotonated group of about 10.0. Cleavage of the substrate NH and CH bonds in the enzyme variant occurs in synchronous fashion, which can be reconciled with a hydride transfer mechanism 733354