13C- and 15N-NMR studies on medium-chain acyl-CoA dehydrogenase reconstituted with 13C- and 15N-enriched flavin adenine dinucleotide.

The 13C- and 15N-NMR spectra of porcine kidney medium-chain acyl-CoA dehydrogenase (MCAD) reconstituted with 13C- and 15N-enriched FADs were measured. The positions of selective enrichment were C(2), C(4), C(4 alpha), C(10 alpha), N(1), N(3), and N(5) of the isoalloxazine nucleus of FAD. The NMR signals of the labeled atoms were observed as broad but distinct peaks in each NMR spectrum. The chemical shift values of the 2-, 4-, 4 alpha-, and 10 alpha-13C for the oxidized form of MCAD were 159.5, 166.8, 141.1, and 155.5 ppm, respectively, relative to the methyl resonance of 3-(trimethylsilyl)propionic acid-d4, while those of 1-, 3-, and 5-15N for the oxidized form were 183.6, 161.1, and 334.7 ppm, relative to liquid ammonia, respectively. The upfield shift of 2-13C of MCAD relative to that of FMN in the aqueous medium and its downfield shift relative to that of tetraacetylriboflavin in an apolar medium imply that a weaker hydrogen bond exists between C(2) = O and apoMCAD or a water molecule than that of free FMN with a water molecule. That the 4-13C resonance was observed downfield-shifted relative to that of free FMN in aqueous solution suggests a strong hydrogen bond between C(4) = O and apoMCAD. The chemical shift for 4 alpha-13C in oxidized MCAD is considerably downfield-shifted from that of FMN or any other flavoprotein observed thus far, indicating a unique environment around this position in MCAD. The 1-15N resonance of MCAD was most upfield-shifted among the flavoproteins observed.(ABSTRACT TRUNCATED AT 250 WORDS)