Nuclear magnetic resonance studies of the old yellow enzyme

The apoenzyme of NADPH oxidoreductase, ‘old yellow enzyme’, was reconstituted with selectively 13 C‐enriched flavin mononucleotides and investigated by 13 C NMR spectroscopy. The 13 C NMR results confirm the results obtained by 15 N NMR spectroscopy and yield additional information about the coenzyme‐apoenzyme interaction. A strong deshielding of the C(2) and C(4) atoms of enzyme‐bound FMN both in the oxidized and reduced state is observed, which is supposed to be induced by hydrogen‐bond formation between the protein and the two carbonyl groups at C(2) and C(4) of the isoalloxazine ring system. The chemical shifts of all 13 C resonances of the flavin in the two‐electron‐reduced state indicate that the N(5) atom is sp 3 ‐hybridized. From 31 P NMR measurements it is concluded that the FMN phosphate group is not accessible to bulk solvent. The unusual 31 P chemical shift of FMN in old yellow enzyme seems to indicate a different binding mode of the FMN phosphate group in this enzyme as compared to the flavodoxins. The 13 C and 15 N NMR data on the old‐yellow‐enzyme–phenolate complexes show that the atoms of the phenolate are more deshielded whereas the atoms of the enzyme‐bound isoalloxazine ring are more shielded upon complexation. A non‐linear correlation exists between the chemical shifts of the N(5) and the N(10) atoms and the pK a value of the phenolate derivative bound to the protein. Since the chemical shifts of N(5), N(10) and C(4a) are influenced most on complexation it is suggested that the phenolate is bound near the pyrazine ring of the isoalloxazine system. 15 N NMR studies on the complex between FMN and 2‐aminobenzoic acid indicate that the structure of this complex differs from that of the old‐yellow‐enzyme–phenolate complexes.