Electron Transfer from the Bound Iron–Sulfur Clusters to Ferredoxin/Flavodoxin: Kinetic and Structural Properties of Ferredoxin/Flavodoxin Reduction by Photosystem I
In oxygen-evolving organisms, Photosystem I (PS I) catalyzes the light-driven reduction of ferredoxin, a small acidic soluble protein containing a low-potential [2Fe–2S] cluster. Under conditions of iron deprivation, flavodoxin, which contains a FMN cofactor, can replace ferredoxin in some algae and cyanobacteria. The reduction kinetics of ferredoxin and flavodoxin by (PS I) have been studied over the last 10 years with the unique technique of flashabsorption spectroscopy. This chapter describes and discusses the kinetic aspects of these processes, using data obtained in vitro with wild type systems as well as with mutants. A detailed summary of all the available kinetic data concerning the effects of the mutations is provided. In the case of the three stromal subunits PsaC, PsaD, and PsaE, this allows one to define structurally the region of the reaction center that is involved in ferredoxin docking, using the 2.5 Å structure of PS I from the cyanobacterium Thermosynechococcus elongatus.
KeywordsDocking Site Electron Trans Ionic Strength Dependence Extrinsic Subunit Cyanobacterial Photosystem
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