Analysis of the kinetic and redox properties of the NADH peroxidase R303M mutant: correlation with the crystal structure.

The crystal structure of the flavoprotein NADH peroxidase shows that the Arg303 side chain forms a hydrogen bond with the active-site His10 imidazole and is therefore likely to influence the catalytic mechanism. Dithionite titration of an R303M mutant [E(FAD, Cys42-sulfenic acid)] yields a two-electron reduced intermediate (EH(2)) with enhanced flavin fluorescence and almost no charge-transfer absorbance at pH 7.0; the pK(a) for the nascent Cys42-SH is increased by over 3.5 units in comparison with the wild-type EH(2) pK(a) of Cys42-SOH. The crystal structure of the R303M peroxidase has been refined at 2.45 A resolution. In addition to eliminating the Arg303 interactions with His10 and Glu14, the mutant exhibits a significant change in the conformation of the Cys42-SOH side chain relative to FAD and His10 in particular. These and other results provide a detailed understanding of Arg303 and its role in the structure and mechanism of this unique flavoprotein peroxidase.