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

A William Rutherford,Arezki Sedoud,José M Ortega,Mercedes Roncel,Fernando Guerrero,Diana Kirilovsky

doi:10.1074/jbc.m110.170126

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 ∼300 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.

Highlights

PCC 6803 have shown that both the cyt c550 and the 12-kDa protein stabilize the binding of the Ca2ϩ and ClϪ ions, which are essential for the oxygen-evolving activity of photosystem II (PSII), in a manner analogous to the extrinsic 17 and 24 kDa polypeptides of higher plants [14, 16, 17]
We determined an Em6 value of Ϫ240 mV for the soluble form of cyt c550 from the thermophilic cyanobacterium T. elongatus after its extraction from PSII [24]
Differential absorption spectra in the ␣-band region of the cytochromes were obtained by subtracting the absolute spectrum recorded at ϩ455 mV from those recorded during the course of the redox titration (Fig. 1A). This figure clearly shows that PSII core complexes contain two different components with absorption maxima in the ␣-band at 559 and 549 nm, which are progressively reduced during the course of titration

Summary

Introduction

We determined an Em6 value of Ϫ240 mV for the soluble form of cyt c550 from the thermophilic cyanobacterium T. elongatus after its extraction from PSII [24]. Such low redox potentials are well below the range normally expected for a mono-heme c-type cytochrome and seem incompatible with a redox function in PSII electron transfer. Some authors [13, 24, 26] have proposed that in conditions more native than isolated PSII core complexes, it is possible that the Em of cyt c550 may be even higher than Ϫ80 mV, and a redox function in the water oxidation complex could be conceivable

Results

Discussion

Conclusion