Oxidation equivalent accumulation

To account for the periodicity of four in the yield of O2 in a series of flashes (12-13), Kok and coworkers (14) proposed that photosystem II cycles through five states during flash illumination. These intermediate oxidation states are referred to as states (i = 0-4) with the subscript denoting the number of oxidizing equivalents accumulated. The sequential advancement of the S states occurs via the light-induced charge separation in photosystem II. 

As discussed in Chapter 22, the rate of electron donation by the S-states to Yz is dependent on the number of oxidizing equivalents accumulated in the S-states. In the sequence of Sq Si, Si >S2, S2->S3, (S4)->Sq transitions, the characteristic time for each successive transition (following a short flash) as measured by both EPR and optical spectroscopic methods (see Fig. 7 in Chapter 22) increases by a factor of 3.5, with values of 30, 110, 350 and 1300 jus, respectively. The progressive increase in the transition time was interpreted as a reflection of the increasing amount of positive charge stored in the S-state, thus raising the electrical repulsion between it and the positively charged Y ". ... 

Oxygen Evolution Requires the Accumulation of Four Oxidizing Equivalents in PSII... 

If the photosystem II reaction center transfers only one electron at a time, how does it assemble the four oxidizing equivalents needed for oxidation of H20 to 02 One possibility is that several different photosystem II reaction centers cooperate, but this seems not to happen. Instead, each reaction center progresses independently through a series of oxidation states, advancing to the next state each time it absorbs a photon. In this event 02 evolution would occur only when a reaction center has accumulated four oxidizing equivalents. Support for this conclusion comes from measurements made by Pierre Joliot of the amount of 02 evolved on each flash when chloroplasts are excited with a series of short flashes after a period of darkness. No 02 is released on the first or second flashes (fig. 15.21), but on the third flash, there is a burst of 02. After this, the amount of 02 released on each flash oscillates, going through a maximum every fourth flash. 

Accumulation of oxidizing equivalents on the donor side of PSII and water splitting the S-state cycle (Figure 3.4.2D). For water oxidation and 02 generation to occur, four electrons must be removed from two water molecules. As the RC only generates one electron hole per photon absorption, the PSII complex must store... 

The stoichiometries of the two reactions above raise an interesting mechanistic question how is the one-electron photochemistry in PS II coupled to the four-electron water-splitting chemistry The now-classical single turnover flash experiments of Joliot and co-workers [2], which showed period four oscillation in the O2 yield with flash number, provided a clearcut answer the PS II units function independently in accumulating the four oxidizing equivalents required to split water. This observation was quickly confirmed [3,4] and Kok provided the S-state model which is now widely used to summarize the situation ... 

Assumption (a), i.e., the concept of non-cooperativity, gives rise to the oscillatory pattern observed. If a complete independency of the electron transfer chains is postulated with respect to accumulation of oxidizing equivalents, partial inhibition which blocks a fraction of Oj-evolving centers should decrease the flash-induced Oj yield without affecting the pattern of oscillation. The same degree of inhibition was indeed observed for all flash yields in a sequence, when the number of active centers was decreased as much as 30-fold by DCMU, UV irradiation or Mn depletion [189]. 

The release of the four protons accompanying the oxidation of two molecules of water to evolve one molecule of oxygen, as shown in eq. (1) above, can, in principle, occur in either of two ways either stepwise, proton by proton, during the accumulation of the four oxidizing equivalents or in concert with... 

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