Cooperative pathway of O 2 reduction to H 2 O 2 in chloroplast thylakoid membrane: new insight into the Mehler reaction.

Oxygen reduction in chloroplasts in the light was discovered by (Mehler Arch Biochem Biophys 33:65-77, 1951) as production of hydrogen peroxide. Later, it was shown that the primary product of the oxygen reduction is superoxide radical produced in thylakoids by one-electron transfer from reduced components of photosynthetic electron transport chain to O 2 molecule. For a long time, the formation of hydrogen peroxide was considered to be a result of disproportionation of superoxide radicals in chloroplast stroma. Here, we overview a growing number of evidence indicating on another one, additional to disproportionation, pathway of hydrogen peroxide formation in chloroplasts, namely its formation in thylakoid membrane due to reaction of superoxide radical generated in the membrane with the reduced plastoquinone molecule, plastohydroquinone. Since various components of photosynthetic electron transport chain (primarily photosystem I) can supply superoxide radicals to this reaction, we refer this two-step O 2 photoreduction to H 2 O 2 as a cooperative process. The significance of hydrogen peroxide production via this pathway for redox signaling and scavenging of reactive oxygen species is discussed.