Kinetics of Photo-Induced Electron Transfer from High-Potential Iron-Sulfur Protein to the Photosynthetic Reaction Center of the Purple Phototroph Rhodoferax fermentans

• Published in: Proceedings of the National Academy of Sciences - PNAS 1996-07, Vol.93 (14), p.6998-7002
• Main Authors: Hochkoeppler, A., Zannoni, D., Ciurli, S., Meyer, T. E., Cusanovich, M. A., Tollin, G.
• Format: Article
• Language: English
• Subjects: Absorption spectra; Bacteria; Biochemistry; Cytochromes; Electrical phases; Electron transfer; Electron Transport; Electrostatics; Iron sulfur proteins; Iron-Sulfur Proteins - metabolism; Kinetics; Light; Mathematics; Models, Theoretical; Photooxidation; Photosynthesis; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - isolation & purification; Photosynthetic Reaction Center Complex Proteins - metabolism; Proteins; Rhodoferax fermentans; Rhodospirillaceae - growth & development; Rhodospirillaceae - metabolism; Spectrophotometry; Time Factors; Wavelengths
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.93.14.6998

The kinetics of photo-induced electron transfer from high-potential iron-sulfur protein (HiPIP) to the photosynthetic reaction center (RC) of the purple phototroph Rhodoferax fermentans were studied. The rapid photooxidation of heme c-556 belonging to RC is followed, in the presence of HiPIP, by a slower reduction having a second-order rate constant of 4.8 × 107 M-1· s-1. The limiting value of kobs at high HiPIP concentration is 95 s-1. The amplitude of this slow process decreases with increasing HiPIP concentration. The amplitude of a faster phase, observed at 556 and 425 nm and involving heme c-556 reduction, increases proportionately. The rate constant of this fast phase, determined at 425 and 556 nm, is ≈ 3 × 105 s-1. This value is not dependent on HiPIP concentration, indicating that it is related to a first-order process. These observations are interpreted as evidence for the formation of a HiPIP-RC complex prior to the excitation flash, having a dissociation constant of ≈ 2.5 μ M. The fast phase is absent at high ionic strength, indicating that the complex involves mainly electrostatic interactions. The ionic strength dependence of kobs for the slow phase yields a second-order rate constant at infinite ionic strength of 5.4 × 106 M-1· s-1 and an electrostatic interaction energy of -2.1 kcal/mol (1 cal = 4.184 J). We conclude that Rhodoferax fermentans HiPIP is a very effective electron donor to the photosynthetic RC.