A High Redox Potential Form of Cytochrome c550 in Photosystem II from Thermosynechococcus elongatus

Abstract

Cytochrome c 550 (cyt c 550 ) is a component of photosystem II (PSII) from cyanobacteria, red algae, and some other eukaryotic algae. Its physiological role remains unclear. In the present work, measurements of the midpoint redox potential (E m ) were performed using intact PSII core complexes preparations from a histidine-tagged PSII mutant strain of the thermophilic cyanobacterium Thermosynechococcus (T.) elongatus. When redox titrations were done in the absence of redox mediators, an E m value of ؉200 mV was obtained for cyt c 550 . This value is ϳ300 mV more positive than that previously measured in the presence of mediators (E m ‫؍‬ ؊80 mV). The shift from the high potential form (E m ‫؍‬ ؉200 mV) to the low potential form (E m ‫؍‬ ؊80 mV) of cyt c 550 is attributed to conformational changes, triggered by the reduction of a component of PSII that is sequestered and out of equilibrium with the medium, most likely the Mn 4 Ca cluster. This reduction can occur when reduced low potential redox mediators are present or under highly reducing conditions even in the absence of mediators. Based on these observations, it is suggested that the E m of ؉200 mV obtained without mediators could be the physiological redox potential of the cyt c 550 in PSII. This value opens the possibility of a redox function for cyt c 550 in PSII.In all photosynthetic oxygen-evolving organisms, the primary steps of light conversion take place in a large pigmentprotein complex named PSII, 2 which drives light-induced electron transfer from water to plastoquinone with the concomitant production of molecular oxygen (for review, see Ref.1). The reaction center of PSII is made up of two membranespanning polypeptides, D1 and D2, which bind four chlorophylls, two pheophytins, two quinones, Q A and Q B (the primary and secondary quinone acceptors of the reaction centre of PSII), a non-heme iron atom, and a cluster made up of four manganese ions and one calcium ion. In green algae and higher plants, three extrinsic proteins are associated to reaction center in water-splitting active PSII complexes: 23-24, 16 -18, and 33 kDa proteins, whereas in cyanobacteria, red algae and some other eukaryotic algae, cyt c 550 , 12 kDa and 33 kDa proteins are found. The three-dimensional structure of PSII confirmed that cyt c 550 binds on the lumenal membrane surface in the vicinity of the D1 and CP43 (2-6).Cyt c 550 , encoded by the psbV gene, is a monoheme protein with a molecular mass of Ϸ 15 kDa and an isoelectric point between 3.8 and 5.0 (7, 8). The recent resolution of the threedimensional structure of the soluble form of cyt c 550 from three cyanobacteria, Synechocystis sp. PCC 6803 (9), Arthrospira maxima (10), and Thermosynechococcus elongatus (11) has confirmed a previously proposed bis-histidine coordinated heme that is very unusual for monoheme c-type cytochromes (8,11,12). Crystal structures of both isolated and PSII-bound forms of cyt c 550 show that the protein presents a hydrophobic inner core typical of monoheme cytochromes c, with three helices...

The photosynthetic cytochrome c 550 from the diatom Phaeodactylum tricornutum

“…Redox titration of Phaeodactylum Cc 550 established a midpoint redox potential (E m,7 ) value of -190±12 mV (Figure S1, supplementary section) which did not significantly change in the pH range 5-7 (data not shown). This potential value, although maintaining the typical negative redox potential, is significantly more positive than those described in cyanobacteria for Cc 550 in solution (-250 to -300 mV) (Navarro et al 1995;Roncel et al 2003;Guerrero et al 2011).…”

“…Cyanobacterial Cc 550 shows intriguing structural and biophysical properties. In addition to the unusual bis-histidinyl axial heme coordination, the protein has a very low midpoint redox potential (Em) when purified as the soluble form (from -250 to -314 mV) (Alam et al 1984;Navarro et al 1995;Roncel et al 2003), but much more positive potential values were obtained for the Cc 550 bound to PSII (from -80 to +200 mV) (Roncel et al 2003;Guerrero et al 2011). This latter fact allowed proposing a possible role for Cc 550 as an electron donor to the Mn 4 CaO 5 cluster in a photoprotective cycle (Guerrero et al 2011).…”

“…In addition to the unusual bis-histidinyl axial heme coordination, the protein has a very low midpoint redox potential (Em) when purified as the soluble form (from -250 to -314 mV) (Alam et al 1984;Navarro et al 1995;Roncel et al 2003), but much more positive potential values were obtained for the Cc 550 bound to PSII (from -80 to +200 mV) (Roncel et al 2003;Guerrero et al 2011). This latter fact allowed proposing a possible role for Cc 550 as an electron donor to the Mn 4 CaO 5 cluster in a photoprotective cycle (Guerrero et al 2011). On the other hand, the EPR spectra of the different cyanobacterial Cc 550 studied in the oxidized form are typical of a low-spin heme with bis-histidine coordination (Roncel et al 2003;Kerfeld et al 2003).…”

Metalloproteins Containing Cytochrome, Iron–Sulfur, or Copper Redox Centers

“… 285 − 288 When complexed with PSII, more positive values of reduction potentials have been determined. 288 , 289 A reduction potential of +200 mV in PS- C550 cytochrome from Thermosynechococcus elongates has recently been reported, 185 which suggests a possible role of this cytochrome in ET in PSII, despite a long distance (∼22 Å) between the PS- C550 cytochrome and its nearest redox center, the Mn 4 Ca cluster. 290 …”

Photoelectron Generation by Photosystem II Core Complexes Tethered to Gold Surfaces

“…elongatus cells used in FIRe measurement: FIRe measurements on whole cells of T. elongatus were conducted on a novel Histagged CP43 mutant strain, [24] similar to WT* and equivalent to that described in Refs. [25] and [26].…”