13C-NMR studies on the reaction intermediates of porcine kidney D-amino acid oxidase reconstituted with 13C-enriched flavin adenine dinucleotide.
The 13C-NMR spectra of the reaction intermediates of D-amino acid oxidase (DAO) were measured with DAO reconstituted with FAD in which the 2-, 4-, 4a-, and 10a-positions of the isoalloxazine moiety were selectively 13C-enriched. The reaction intermediates used include charge-transfer complexes of the oxidized DAO with substrate intermediates and those of the reduced enzyme with substrate intermediates. For the former type of complex, the reaction intermediates with beta-cyano-D-alanine (D-BCNA) and D-proline were used, while for the latter the purple intermediates with D-alanine and D-proline were chosen. The 13C-resonances of 2-13C in the reaction intermediates with D-BCNA and D-proline were downfield-shifted by about 1 ppm relative to the free oxidized DAO. The 4-13C signal for the DAO-D-BCNA intermediate was observed at 1.2 ppm upfield from that of the oxidized DAO, though that for DAO-D-proline intermediate showed no shift. These results suggest modulation of the hydrogen bondings at C(2) = 0 and/or C(4) = 0 in these reaction intermediates. Comparison of the 13C-resonances of reduced DAO with those of free reduced FMN in the neutral and anionic forms indicate that FAD in reduced DAO is in the anionic reduced form. The 4a-13C resonance of reduced DAO is upfield-shifted by about 3 ppm from that of free reduced anionic FMN. Comparison of the 13C-resonances for the purple intermediates with those of reduced FMN and reduced DAO indicate unequivocally that FAD in the purple intermediate is in the anionic reduced state. The 4a-13C resonances for the purple intermediates were substantially upfield-shifted (by 2.4 ppm with D-alanine and 1.9 ppm with D-proline) relative to reduced DAO. This indicates that the electron density, and hence the nucleophilicity, of the 4a-carbon is elevated in the purple intermediate relative to free reduced DAO. This leads to a model in which the oxidative half reaction proceeds via the reaction of molecular oxygen at the 4a-position of the reduced FAD in the purple intermediate. This provides a rational molecular basis for the oxidative half reaction by way of the purple intermediate prior to product release rather than by way of free reduced enzyme after product release.
D000409:Alanine / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D003605:D-Amino-Acid Oxidase / Q000378:metabolism*; D005486:Flavin Mononucleotide / Q000378:metabolism; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism*; D007668:Kidney / Q000201:enzymology*; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D011392:Proline / Q000378:metabolism; D013053:Spectrophotometry; D013552:Swine
D005182:Flavin-Adenine Dinucleotide; D005486:Flavin Mononucleotide; C004631:3-cyanoalanine; D011392:Proline; D003605:D-Amino-Acid Oxidase; D000409:Alanine