Dual role of FMN in flavodoxin function: Electron transfer cofactor and modulation of the protein-protein interaction surface |
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Authors: | Susana Frago,José A. Navarro,Dale E. Edmondson,Carlos Gó mez-Moreno,Milagros Medina |
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Affiliation: | a Departamento de Bioquímica y Biología Molecular y Celular, Facultad de Ciencias, and Institute of Biocomputation and Physics of Complex Systems (BIFI). Universidad de Zaragoza, 50009 Zaragoza, Spain b Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla and CSIC, 41092 Sevilla, Spain c Department of Biochemistry, Emory University School of Medicine, Atlanta, GA 30322, USA d UCD School of Biomolecular and Biomedical Science, Conway Institute for Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland |
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Abstract: | Flavodoxin (Fld) replaces Ferredoxin (Fd) as electron carrier from Photosystem I (PSI) to Ferredoxin-NADP+ reductase (FNR). A number of Anabaena Fld (AnFld) variants with replacements at the interaction surface with FNR and PSI indicated that neither polar nor hydrophobic residues resulted critical for the interactions, particularly with FNR. This suggests that the solvent exposed benzenoid surface of the Fld FMN cofactor might contribute to it. FMN has been replaced with analogues in which its 7- and/or 8-methyl groups have been replaced by chlorine and/or hydrogen. The oxidised Fld variants accept electrons from reduced FNR more efficiently than Fld, as expected from their less negative midpoint potential. However, processes with PSI (including reduction of Fld semiquinone by PSI, described here for the first time) are impeded at the steps that involve complex re-arrangement and electron transfer (ET). The groups introduced, particularly chlorine, have an electron withdrawal effect on the pyrazine and pyrimidine rings of FMN. These changes are reflected in the magnitude and orientation of the molecular dipole moment of the variants, both factors appearing critical for the re-arrangement of the finely tuned PSI:Fld complex. Processes with FNR are also slightly modulated. Despite the displacements observed, the negative end of the dipole moment points towards the surface that contains the FMN, still allowing formation of complexes competent for efficient ET. This agrees with several alternative binding modes in the FNR:Fld interaction. In conclusion, the FMN in Fld not only contributes to the redox process, but also to attain the competent interaction of Fld with FNR and PSI. |
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Keywords: | AnFld, flavodoxin from Anabaena ApoFld, apoflavodoxin DDM, β-dodecyl maltoside ET, electron transfer ox, oxidised sq, semiquinone hq, hydroquinone Eox/sq, midpoint reduction potential for the ox/sq couple Esq/hq, midpoint reduction potential for the sq/hq couple Em, midpoint reduction potential for the 2-electron ox/hq reduction F, the Faraday constant FADS, FAD synthetase Fld, Fldox, Fldsq, Fldhq, flavodoxin and in its oxidised, semiquinone and hydroquinone states FMNox, FMNsq, FMNhq, FMN and in its oxidised, semiquinone and hydroquinone states FNR, FNRox, FNRsq, FNRhq, ferredoxin-NADP+ reductase and in its oxidised, semiquinone and hydroquinone states Ka, association constant Kd, dissociation constant Kr, re-arrangement constant kap, apparent observed rate constant ket, electron transfer rate constant k2, second order rate constant kobs, pseudo-first order observed rate constants PMS, phenazine methosulfate PSI, photosystem I RF, riboflavin r.m.s.d., root mean square deviation |
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