A comparative laser‐flash absorption spectroscopy study of Anabaena PCC 7119 plastocyanin and cytochrome c₆ photooxidation by photosystem I particles

Abstract

(Received (November 26, 1992) -EJB 92 1696/2Laser-flash absorption spectroscopy has been used to investigate the kinetics of electron transfer from reduced cytochrome c6 and plastocyanin, isolated from Anabaena PCC 7119, to oxidized P700 in photosystem-I particles isolated from the same cyanobacterium and from spinach. For all metalloproteins and photosystems, the observed rate constant has a non-linear protein-concentration dependence, thus suggesting complex formation preceding electron transfer. Plastocyanin and cytochrome c6 have similar association constants for complex formation with spinach photosystem I, but the copper protein exhibits a higher intracomplex-electron-transfer rate constant (twofold). With Anabaena photosystem I, the two redox proteins are more effective with respect to both complex formation (5 -10-fold) and electron transfer (1.5 -4-fold) than with the spinach photosystem. In all cases, the observed rate constants for electron-transfer monotonically decrease with increasing NaCl or MgC1, concentration. This is interpreted in terms of the involvement of attractive electrostatic interactions, which result in the initial collision complex having the most productive orientation for the electron transfer process, without a requirement for further reorientation. The magnitude of the response to MgC1, suggests the occurrence of specific ion effects as well. In the absence of added salts, the reduction rate of oxidized P700 increases with pH from approximately 6 to 8, but decreases slightly at pH 8.5.The soluble redox protein plastocyanin (10.5 m a ) , which contains a single Cu'+ active site with characteristic type-1 properties, functions as a mobile electron carrier between the membrane-bound cytochrome b& complex and the reaction center of photosystem I (PSI) within the electron-transport chain of organisms performing oxygenic photosynthesis [ 1, 21. In some eukaryotic algae and cyanobacteria, however, plastocyanin is functionally replaced, under conditions of copper deficiency and/or iron availability, by a soluble class-I c-type cytochrome referred to as cytochrome c6 (= 8.0 m a ) , which is absent in higher plants [3]. In spite of the very different redox cofactors they possess, the equivalent physiological role of these two proteins is reflected by their similar size, redox potentials and isoelectric points (PI) [4]. The two metalloproteins are acidic in green algae, but can be either basic or acidic in cyanobacteria. Within the same organism, however, cytochrome c, and plastocyanin exhibit almost identical PI values [41.Correspondence to G. Tollin, Department of Biochemistry, UniFax: f1602 621 9288. Abbreviations. PSI, photosystem I ; TSF-1, Triton-solubilized photosystem-I particles ; ROO', photooxidized P700; PI, isoelectric point; V,,, electrostatic interaction energy; k,, rate constant at infinite ionic strength; kobrr observed rate constant. The interaction between plastocyanin and PSI has been extensively studied in a wide variety of organisms, in spite of the difficulties and va...

A Single Arginyl Residue in Plastocyanin and in Cytochrome c6 from the Cyanobacterium Anabaenasp. PCC 7119 Is Required for Efficient Reduction of Photosystem I

“…Divalent cations like Mg 2ϩ have previously been reported to be specifically involved in Pc/PSI interactions in other organisms, but not Anabaena (8). As such, an analysis of the effect of Mg 2ϩ cations on the bimolecular rate constant of PSI reduction by WT and mutant Pcs was performed.…”

“…PCC 7119 was purified as described previously (8), with the following two exceptions. i) Pc samples were applied onto the CM-cellulose column after oxidation with potassium ferricyanide, and ii) elution of the adsorbed proteins was performed with a linear gradient of 2-30 mM potassium phosphate buffer, pH 7.0, containing 50 M potassium ferricyanide.…”

Detecting transient protein–protein interactions by X‐ray absorption spectroscopy: The cytochrome c6‐photosystem I complex

“…After concentration, a stock solution of 2 mM for Cc 6 was obtained. Protein concentration was determined by optical spectroscopy using a value of 26.5 mM À1 cm À1 for the Abbreviations: (Cc 6 ), Cytochrome c 6 ; EXAFS, extended X-ray absorption fine structure; FT, Fourier transform; PSI, photosystem I; PSI Hg , Hg-containing photosystem I; XANES, X-ray absorption near edge structure; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction molar extinction coefficient at 553 nm [9]. A 1.0 mM PSI Hg sample in 10 mM sodium phosphate pH 6.0, supplemented with 0.025% (w/v) bdodecyl-maltoside, was used.…”

“…Under these conditions, the percentage of Cc 6 bound to PSI Hg was estimated to be ca. 70%, according to the binding affinity constant calculated previously [9].…”

Transient Homodimer Interactions Studied Using the Electron Self-exchange Reaction

“…The ionic strength within the thylakoid lumen is estimated to be about 0.1-0.2 M, thus facilitating ESE by Debye screening. As well as general screening at this ionic strength, Coulombic repulsion between acidic PCu molecules will be diminished further by the presence in the thylakoid lumen of Mg 2ϩ ions at approximately mM concentrations (55), which have been implicated in catalysis of the oxidation of PCu I by PSI (56). The results presented in Fig.…”