Photosynthetic cyclic electron transport : measurement and variably proton-coupled mechanism

Abstract

In plant photosynthesis, linear electron transport (LET) from H2O to NADP+ and to CO2 further on is coupled with transmembrane H+ transport supporting ATP synthesis. The stoichiometry of 3ATP/2NADPH = 12H+/4e- is the absolute requirement for CO2 reduction. For a half of a century it has been a common belief that violations of this stoichiometry are corrected by cyclic electron transport (CET) during which some electrons are cycling around photosystem I (PSI) in an H+ coupled way. Though CET was detected in plant leaves, its rate was never exactly quantified, as well the amount of CET-related ATP synthesis was not compared with the actual ATP needs. In this work CET rate was measured and the assumed ATP synthesis quantified in leaves under experimental conditions where the metabolic ATP consumption was reduced to minimum. Differently from the present common belief, CET rate is at maximum when ATP is at maximum and its additional synthesis is unnecessary. The conclusion is that CET is variably coupled with transmembrane H+ translocation. Therefore the new model of CET is developed to explain this situation.

Taimede fotosünteesis on lineaarne elektrontransport (LET) H2O-lt NADPH+-le ja sealt edasi CO2-le seotud transmembraanse prootonite transpordiga, mis omakorda võimaldab ATP sünteesi. Stöhhiomeetria 3ATP/2NADPH = 12H+/4e- on ainuõige ATP sünteesi jaoks. Pool sajandit on kestnud üldine arusaam, et kõrvalekalded sellest stöhhiomeetriast korrigeeritakse tsüklilise elektrontranspordi (CET) poolt. CET kujutab endast elektrontransportahelat, kus mõned elektronid liiguvad ümber fotosüsteem I (PSI) selliselt, et võimaldavad ka prootonite transporti. Kuigi CETi on taimedes korduvalt mõõdetud, ei ole need mõõtmised olnud piisavalt täpsed ning neid ei ole võrreldud reaalse ATP tarbimisega elusas lehes. Antud töös mõõdeti CETi tingimustes, kus ATP tarve oli väga väike. Vastupidiselt oodatule oli CETi kiirus väga suur olukorras, kus ATPd sisuliselt ei tarbitudki. Seetõttu järeldati, et CET siiski ei transpordi prootoneid ning töötati välja uus mudel, mis kirjeldab saadud katsetulemusi nii, et ei minda vastuollu LETi funktsioneerimisega.