title: Contribution of the FAD binding site residue tyrosine 308 to the stability of pea ferredoxin-NADP+ oxidoreductase.
abstract: The contribution made by tyrosine 308 to the stability of pea ferredoxin-NADP+ reductase was investigated using site-directed mutagenesis. The phenol side chain of the invariant carboxyl terminal tyrosine is stacked coplanar to the isoalloxazine moiety of the FAD cofactor. Fluorescence measurements indicate that this interaction plays a significant role in FAD fluorescent quenching by the reductase apoprotein. Replacement of the tyrosine by tryptophan or phenylalanine caused only a minor increase in the quantum yields of bound FAD, whereas nonaromatic substitutions to serine and glycine resulted in a large fluorescent rise. Results from NADP+ titration experiments support a recent hypothesis [Karplus et al. (1991) Science 251, 60-66], suggesting that the phenol ring of Tyr 308 may fill the nicotinamide binding pocket in the absence of the nucleotide. The stability of the site-directed mutants, judged by thermal- and urea-induced denaturation studies, was lowered with respect to the wild-type enzyme. FNR variants harboring nonaromatic substitutions displayed more extensive destabilization. The decrease in thermodynamic stability correlated with the impairment of catalytic activities [Orellano et al. (1993) J. Biol. Chem 268, 19267-19273]. The results indicate that the presence of the electron-rich aromatic side chain adjacent to the isoalloxazine ring is essential for maximum stabilization of the FNR holoenzyme, resulting in a flavin conformation which optimizes electron flow between the prosthetic group and its redox partners.
authors.affiliation: Department of Biological Sciences, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Argentina.
authors.forename: N B
authors.initials: NB
authors.lastname: Calcaterra
authors.forename: G A
authors.initials: GA
authors.lastname: Picó
authors.forename: E G
authors.initials: EG
authors.lastname: Orellano
authors.forename: J
authors.initials: J
authors.lastname: Ottado
authors.forename: N
authors.initials: N
authors.lastname: Carrillo
authors.forename: E A
authors.initials: EA
authors.lastname: Ceccarelli
mesh_terms: D001665:Binding Sites; D004795:Enzyme Stability; D005287:Ferredoxin-NADP Reductase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism*; D006358:Hot Temperature; D009154:Mutation; D018532:Pisum sativum / Q000201:enzymology*; D011489:Protein Denaturation; D013050:Spectrometry, Fluorescence; D014443:Tyrosine / Q000737:chemistry / Q000378:metabolism*
chemical_list: D005182:Flavin-Adenine Dinucleotide; D014443:Tyrosine; D005287:Ferredoxin-NADP Reductase