Proc. Natl. Acad. Sci. USA Vol. 90, pp. 1834-1838, March 1993 Biophysics Oxygen evolution in photosynthesis: From unicycle to bicycle (photosytem II/S states/quinone acceptors/mism) VLADIMIR P. SHINKAREV* AND COLIN A. WRAIGHTt Department of Plant Biology, University of Illinois, Urbana, IL 61801 Communicated by Jack Meyers, October 30, 1992 (receivedfor review August 10, 1992) ABSTRACT Flash-induced oxygen evolution in the thyla- is mainly attributed to a double turnover of the RC induced koids of plants and algae exhibits damped oscillations with during the tail of the actinic flash (3, 4). period four. These are wel described by the S-state model of The nature of misses has not been mechanistically defined Kok et al. [Kok, B., Forbush, B. & McGloin, M. (1970) (see, however, ref. 9). In the analysis of Joliot and Kok (3), Photochem. Photobiol. 11, 457-4751, with damping provided misses were suggested to be due either to the fraction ofRCs by empirical misses and double hits in the reaction center of in which a photochemical transition does not occur or to photosystem II. Here we apply a mechanistic interpretation of back-reactions that annihilate the effect ofthe previous flash. misses as mainly determined by reaction centers that are Although equal misses for each transition give adequate inactive at the time of the flash due to the presence ofeither P+ fitting of the observed oxygen evolution, many authors have or QA according to the electron transfer equilibria on the suggested that they may be different for each S state (e.g., donor and acceptor sides of the reaction center. Caation of refs. 3, 9-12) and, indeed, some improvement of the fit is misses on this basis, using known or estimated values of the seen. However, these models have been essentially phenom- equilibrium constants for electron transfer between the S states enological in nature. and tyrosine Yz, between Yz and P680, as well as between the Here we suggest that misses are substantially determined acceptor plastoquinones, allows a natural description of the by the fraction of RCs that have either P+ or QA before each flah number dependence of oxygen evolution. The calulated flash, due to the reversibility of the electron transfer reac- misses are different for each flash-induced reaction center tions. With this underlying mechanism, the miss factor be- transition. Identificationofthis mechanism underlying the miss comes a fundamentally informative parameter for the oxy- factor for each transition leads to the recogition of two gen-evolving process. Calculation of misses from this stand- different reaction sequence cydes of photosystem H, with point, using available values for the equilibrium constants, different transition probabilities, producing an intrinsic het- gives good predictions for flash-induced oxygen evolution. erogeneity of photosystem II acivity. The most important outcome, however, is recognition of two different reaction cycles with different transition probabili- II electron ties and, consequently, different oxygen yield patterns in a In photosystem (PSII), light activates transfer flash series. The relative contributions of the two cycles from the primary donor (P680) to the primary quinone depend on the initial conditions. This gives rise to an intrin- acceptor (QA) and then to the secondary quinone acceptor sic, kinetic heterogeneity of PSII activity, which may con- (QB). QA is a one-electron carrier, whereas the secondary tribute to the large number of heterogeneities derived from quinone acceptor QB can accept two electrons. Plastoquinol phenomenological analysis (e.g., ref. 13), only afew ofwhich (QBH2), generated after two turnovers ofPSII, can exchange have been shown to have a structural foundation (e.g., refs. with an oxidized molecule of the plastoquinone pool and the 14 and 15). acceptor quinone complex returns to the initial state (QAQB) with both quinones in the oxidized state (e.g., ref. 1). The "two-electron gate" character of QB, the kinetic stability of RESULTS AND DISCUSSION the QB semiquinone, and the exchange ofneutral forms ofthe From Kok Unicycle to Bicyde. In principle, the behavior of quinone result in flash number-dependent binary oscillations PSII RCs can be described by considering the states gener- of the semiquinones (e.g., ref. 2). ated by an ordered sequence of the electron-transferring Activation of PSII by single turnover actinic flashes leads components on the donor and acceptor sides: to oxygen evolution with periodicity offour (reviewed in ref. 3). This periodicity was originally explained by introducing S Yz P I QA QB, [ll the S states (S.) ofthe oxygen-evolving complex, where each S state has a different number (n = 0, 1, 2, 3, 4) of oxidizing where, in addition to QA and QB, S is the oxygen-evolving equivalents (4). The dominant dark-stable state was Si, complex, which can accumulate four oxidizing equivalents; resulting in maximal oxygen yield after three flashes. Later, Yz is the tyrosine [TyrDl-161 (16, 17)] electron donor to Joliot and Kok (3) considered four photoactive states includ- P680+; P is the primary electron donor, P680; and I is the ing the PSII reaction center (RC), YzP680Q, and the 02- electron acceptor pheophytin. evolving enzyme as components. Since then, however, many After each flash, the photogenerated hole is transferred authors have returned to the original interpretation, where from P680+ to Yz and then to the Mn-containing oxygen- the accumulated on the donor side are consid- evolving complex (reviewed in refs. 5-9). Because the flash- only charges induced transitions occur in the nanosecond-microsecond ered (for reviews, see refs. 5-9). relevant Damping of the period four oscillations was explained time range, we can assume that after each flash the empirically by misses and double hits (4), the nature ofwhich has never been made fully explicit. The double hit parameter Abbreviations: PSII, photosystem II; RC, reaction center. *Onleave from Biophysics Section, Department ofBiology, Moscow State University, Moscow, Russia. The publication costs of this article were defrayed in part by page charge tTo whom reprint requests should be addressed at: Department of payment. This article must therefore be hereby marked "advertisement" Plant Biology, University of Illinois at Urbana-Champaign, 190 in accordance with 18 U.S.C. §1734 solely to indicate this fact. PABL, 1201 West Gregory Drive, Urbana, IL 61801-3838. 1834 Downloaded by guest on September 25, 2021 Biophysics: Shinkarev and Wmight Proc. Natl. Acad. Sci. USA 90 (1993) 1835 reactions reach quasi-equilibrium on the time scale of milli- In the well-known Kok model for S-state transitions ofthe seconds and seconds. The flash-induced transitions on the oxygen-evolving complex donor side RC can then be represented as Kyp0 Koy KypI Kly SOi S1 S2 S3-Y S4 [81 p + -.A. + -.b. + soyp+,-.-- soy P,.,- Slyp --+ SI P , Sly P .W_ s2yp fast t hv I h. 02 all transition probabilities, y, have the same value. It has been common to define the S-state cycle in terms of misses (a), |S3YP+ = S3Y+P =S4YP IT S2YP+ 3PI-2+ which can be defined as (1 - y) in the limitofzero double hits. Kyp K3y K40 Ky K2Y Ifcycles V and W all have equal transition probabilities then they degenerate to a single Kok type cycle. However, the transition probabilities are necessarily different for cycle V Here K is the equilibrium constant ofthe transition: SnYP' and cycle W and they do not converge to a single kinetic =SnY+P (n = 0, 1, 2, 3); Kny is the equilibrium constant of cycle. transition: SnY+P = Sn+ YP; and K40 is equilibrium constant Dependence of Misses on Flash Number. Schemes 4 and 5 for the reaction: S4YP = SOYP. consider only the case when flashes induce sequential tran- For the time being, we do not consider the slow transitions sitions of the RC. Kok's model also considers transitions Of YD [TyrD2-160 (18, 19)], an alternate donor, in the between RC states differing by two oxidizing equivalents on second-minute time domain, which can reduce the oxygen- the donor side of the RC (double hits). These are attributed evolving complex in some S states (reviewed in refs. 5-8). to a double turnover of the RC, induced during the tail ofthe Each flash also activates the complex ofquinone acceptors according to the following simplified scheme: actinic flash, and are enhanced by oxidation ofthe iron atom (Q400; see ref. 20) of the quinone acceptor complex. In MAB MAB LAB principle, they can be minimized (21) or even eliminated (22), and for clarity we will not consider them here. However, they | QAQBH2= QAQB 3QAQB QAQ 1 can easily be incorporated in cycles V and W. Focusing on misses, therefore, we first consider cycle W: Pheophytin, I, may also be incorporated in this scheme but 'YO is omitted for clarity. DA--->D+A- --*A-lD2D2+A-D3+A-A2'b Light-induced electron transfer occurs from the donor side [9] to the acceptor side of the RC and necessitates parallel (1) (2) (3) (4) consideration of the transitions on both sides. The fast equilibrating states in Schemes 2 and 3 can be designated by 02 the letter D for the donor side and by the letter A for the The transition probabilities y' (k = 0, 1, 2, 3) are propor- acceptor side. If we consider D as having four stable oxida- tional to the probability that the RC is in the state with reduced tion states (DO, D+, D2+, D3+) and A with two states (A, A-), primary donor and oxidized primary plastoquinone acceptor: then two different schemes arise by combining the transitions of donor and acceptor sides of the RC: Vk c [P680 QA]before flash [10] hi', -hp hi' hi' D+A-h+ D2+A-I D3+A-n-.'DA- (Cycle V) [4] This can be determined from the equilibrium within the t 02 X quasi-states, such as 6 and 7.