Charge Separation, Stabilization, and Protein Relaxation in Photosystem II Core Particles with Closed Reaction Center

Abstract

The fluorescence kinetics of cyanobacterial photosystem II (PSII) core particles with closed reaction centers (RCs) were studied with picosecond resolution. The data are modeled in terms of electron transfer (ET) and associated protein conformational relaxation processes, resolving four different radical pair (RP) states. The target analyses reveal the importance of protein relaxation steps in the ET chain for the functioning of PSII. We also tested previously published data on cyanobacterial PSII with open RCs using models that involved protein relaxation steps as suggested by our data on closed RCs. The rationale for this reanalysis is that at least one short-lived component could not be described in the previous simpler models. This new analysis supports the involvement of a protein relaxation step for open RCs as well. In this model the rate of ET from reduced pheophytin to the primary quinone Q(A) is determined to be 4.1 ns(-1). The rate of initial charge separation is slowed down substantially from approximately 170 ns(-1) in PSII with open RCs to 56 ns(-1) upon reduction of Q(A). However, the free-energy drop of the first RP is not changed substantially between the two RC redox states. The currently assumed mechanistic model, assuming the same early RP intermediates in both states of RC, is inconsistent with the presented energetics of the RPs. Additionally, a comparison between PSII with closed RCs in isolated cores and in intact cells reveals slightly different relaxation kinetics, with a approximately 3.7 ns component present only in isolated cores.

Quenching in Arabidopsis thaliana Mutants Lacking Monomeric Antenna Proteins of Photosystem II

“…3 were fitted to the experimental data that allowed for dissection of PSI and PSII kinetics in the fluorescence decay (32). These kinetic models for PSI and PSII are based on previous results from time-resolved measurements and fluorescence lifetime analysis of isolated PSI (44) and PSII particles (45)(46)(47)(48)(49) of vascular plants and cyanobacteria. The adequacy of these models has been discussed extensively by Holzwarth et al (32).…”

Time-resolved fluorescence measurements on leaves: principles and recent developments

“…For this reason, it should be avoided at the experimental level by using carefully designed measuring conditions, such as use of proper cuvettes that allow sample refreshment, use of sufficiently low excitation intensities (see “Time-resolved fluorescence methods” and “How to achieve well-defined photosynthetic states?” sections for more details).PSII kinetics depends strongly on the measuring conditions: Charge separation in PSII with open RCs are much faster than with closed PSII RCs, resulting in substantially slower fluorescence kinetics in the closed state as compared to the open state. For example, isolated PSII cores in open and closed states have the reported average lifetimes of 65 ps and 850 ps, respectively (Miloslavina et al 2006; Szczepaniak et al 2009). Therefore, in order to be able to interpret the obtained results unambiguously, it is important to measure in a well-defined state of PSII: either at F 0 (all PSII RCs open), or F m (all PSII RCs closed) (or the corresponding F 0 ′ or F m ′ conditions).…”

A Comparison Between Plant Photosystem I and Photosystem II Architecture and Functioning

“…Although a limitation imposed by excited state diffusion was observed in these studies, it was concluded that this process plays a minor role in determining the effective trapping rate with respect to the kinetic bottleneck at the level of charge separation events [201, 209, 211]. On the other hand considerable evidence has also accumulated indicating that there is a partial but significant diffusion limited component in PSII, which accounts for 20-30% of the overall trapping time.…”