Pseudocyclic electron flow

Electrons from ferredoxin may also reduce O2, which yields H2O2 and eventually H20 (o2 + 2e + 2H+ -> H202 - H20 +102). (The light-de-pendent consumption of O2, as occurs when electrons from ferredoxin or from one of the iron-plus-sulfur-containing intermediates of Photosystem I move to O2, is termed the Mehler reaction.) Because equal amounts of O2 are evolved at Photosystem II and then consumed using reduced ferredoxin in a separate reaction, such electron flow is termed pseudocyclic (see Figs. 5-18 and 6-4). No net O2 change accompanies pseudocyclic electron flow,... 

While NADP is the preferred electron acceptor in photosynthesis, oxygen can also accept electrons from toe photosynthetic electron transport chain (Mehler, 1951 Mehler and Brown, 1952 Allen, 1975, 1992 Egneus et al., 1975 Marsho et al., 1979). Molecular oxygen contains two unpaired electrons with parallel spins. As a consequence Oj is most easily reduced by single electron additions because divalent reduction of Oj (to HjOj) requires a spin inversion (Cadenas, 1989). The addition of an electron to oxygen by the photosynthetic electron transport chain produces superoxide (Asada et al, 1974). The reduction of molecular oxygen by the photosynthetic electron transport system is called the Mehler reaction (Mehler, 1951 Mehler and Brown, 1952) and electron transport from water to molecular is called pseudocyclic electron flow (Allen, 1975, 1977). Reported values for concentrations required to half-saturate the rate of the Mehler reaction are between 2 and 60 juM (Robinson, 1988) which corresponds to 0.17 to 5% Oj in air at 25 °C. 

Figure 5-18. The three electron flow pathways in chloroplasts, showing the pivotal role played by ferredoxin (FD) in noncyclic (solid line), cyclic (dashed line), and pseudocyclic (dotted line) election flow. See also Figure 6-4, where the three types of electron flow are presented based on redox potentials.

D. The uncoupling of ATP formation from electron flow is analogous to disengaging a clutch, which allows a motor to run faster the O2 evolution therefore initially speeds up. After awhile, the lack of ATP formation causes no CO2 to be fixed the electron acceptors therefore stay reduced and electron flow to them is curtailed. Moreover, electron flow may eventually switch over to the pseudocyclic type, which involves no net O2 evolution. 

Fig. 2. the Z-scheme model of photosynthetic electron flow showing non-cyclic, cyclic and pseudocyclic paths of electron transport. 

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