Improved 5-step modeling of the Photosystem II S-state mechanism in cyanobacteria

Abstract

We present a model of the S-state mechanism, as well as an improved eigenvalue analysis, that integrate into a coherent ensemble several features found since the S-state model was initially developed. These features include the presence of S-1, deactivations in the dark interval between flashes, and the change in the number of active PS II centers by photoinhibition or photoactivation. A new feature is the capacity to predict the steady-state distribution of S-states under conditions of steady photoinhibition or photoactivation. The improved eigenvalue analysis allowed the calculation of the initial S-state distribution. In addition, the model resolved 'true' photochemical misses from apparent misses due to deactivations in the dark interval between flashes. The model suggested that most of the misses that are commonly reported are due to deactivations, and not to an intrinsic inefficiency of the photochemical mechanism of PS II. Because models that allow double-hits encompassing the S2 to S3 transition often predict negative initial quantities of S2 in cyanobacteria, our proposed model specifically prohibited them. The model accounts for inhomogeneous misses and a steady-state distribution of the type (S2)≈(S1)>(S3)≈(S0). This 5-step model uses only 4 probabilities, and is therefore easy to handle. The use of this model is critical for the analysis of several cyanobacterial strains, as well as for any species that show non-negligible deactivations in the dark interval between flashes.

Glutamate-354 of the CP43 polypeptide interacts with the oxygen-evolving Mn4Ca cluster of photosystem II: a preliminary characterization of the Glu354Gln mutant

“…Polarization of the electrode (0.73 V ) was initiated 10 s before the initiation of data acquisition and the flash sequence (15 flashes at a frequency of 4 Hz) was initiated 333 ms after that. Analysis of S-state parameters was performed according to Lavorel (1976) and Meunier et al (1995).…”

Misses during Water Oxidation in Photosystem II Are S State-dependent

“…1). These concepts are routinely used in the analysis of the S cycle, mostly in flash-induced oxygen evolution studies where it was introduced (19,20,(22)(23)(24)(25)(26)(27). They have also been used in the analysis of S cycle intermediates studied with EPR spectroscopy (28,35,36), transient optical spectroscopy, variable fluorescence (21,(41)(42)(43), and even EXAFS (30,31,33,34), reflecting S state-dependent structural changes.…”

“…S state-dependent misses were predicted from known equilibrium constants on the donor and acceptor sides of PSII (17,20) and, more recently, estimated from analysis of the flash-induced variable fluorescence yield (21). Despite this, an S state-independent miss parameter is used predominantly during analysis of the S cycle intermediates (19,(22)(23)(24)(25)27). There are two reasons for this (20).…”

“…The first is experimental, and it was found that an equal miss parameter in each S transition can provide a satisfactory fit to the data when only one parameter is measured (20,24) (also see the average miss parameter in Table 2). The second reflects the fact that finding the S state dependence of the miss factor and the individual S state composition after a flash from the oxygen-evolving step alone is an underdetermined problem (25,27). It does not allow separate and direct determination of the misses in each S transition.…”

“…In most studies, the S state cycle advancement has been probed by flash oxygen evolution measurements (19,20,(22)(23)(24)(25)(26)(27) which directly probe only the oxygen release in the S 3 3[S 4 ]3 S 0 step. The measured oxygen yield is then used to determine the miss factor which represents the average miss per state transition during the full S cycle because none of the other S state transitions can be measured directly.…”