Frago Susana, Lans Isaias, Navarro José A, Hervás Manuel, Edmondson Dale E, De la Rosa Miguel A, Gómez-Moreno Carlos, Mayhew Stephen G, Medina Milagros
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.
Biochim Biophys Acta. 2010 Feb;1797(2):262-71. doi: 10.1016/j.bbabio.2009.10.012. Epub 2009 Nov 10.
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 FMN has been replaced with analogues in which its 7- and/or 8-methyl groups have been replaced by chlorine and/or hydrogen.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.
黄素氧还蛋白（Fld）取代铁氧还蛋白（Fd）作为从光系统I（PSI）到铁氧还蛋白-NADP（+）还原酶（FNR）的电子载体。许多鱼腥藻Fld（AnFld）变体在与FNR和PSI的相互作用表面发生了取代，这表明极性和疏水残基对相互作用都不是关键的，特别是与FNR的相互作用。这表明Fld FMN辅因子暴露于溶剂的苯环表面可能对此有贡献。FMN已被其7-和/或8-甲基被氯和/或氢取代的类似物所取代。如预期的那样，由于其较低的负中点电位，氧化的Fld变体比Fld更有效地从还原的FNR接受电子。然而，与PSI的过程（包括首次在此描述的PSI对Fld半醌的还原）在涉及复杂重排和电子转移（ET）的步骤中受到阻碍。引入的基团，特别是氯，对FMN的吡嗪和嘧啶环有吸电子作用。这些变化反映在变体分子偶极矩的大小和取向上，这两个因素对于微调的PSI：Fld复合物的重排似乎都至关重要。与FNR的过程也略有调节。尽管观察到了位移，但偶极矩的负极指向包含FMN的表面，仍然允许形成能够有效进行ET的复合物。这与FNR：Fld相互作用中的几种替代结合模式一致。总之，Fld中的FMN不仅有助于氧化还原过程，而且有助于实现Fld与FNR和PSI的有效相互作用。
