Pseudocyclic electron transport

A special electrochemical cell was often employed to detect the photosynthetic activity of immobilized thylakoids. " " With this electrochemical micro-cell, it was possible to detect a photocurrent of free or immobilized thylakoid membranes in the absence of exogenous artificial electron acceptor. The photosynthetic origin of the photocurrent was demonstrated by an inhibition in the presence of the photosynthetic inhibitor diuron. The inhibition of the photocurrent by catalase, an enzyme that degrades hydrogen peroxide, demonstrated that dissolved oxygen was involved. " The ability of oxygen to act as an electron acceptor for the photosynthesis membrane was initially demonstrated by Mehler. This process, known as pseudocyclic electron transport or Mehler... 

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

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. 

Applying the criteria of Gimmler (1977) for the characterization of the three types of photophosphorylation it is rather difficult to determine the type of the phosphorylation observed. The cyclic phosphorylation can be excluded on account of the fully reduced state of the algae as a consequence of the long anaerobiosis, the applied high light intensity and its insensitivity towards complete inhibition of an electron transport from PS II by DCMU. DCMU prevents photosynthetic O2 evolution and the non-cyclic electron transport. As the observed ATP increase is not affected by DCMU both non-cyclic and pseudocyclic phosphorylation should be ruled out. 

---