title: A redox study of the electron transport pathway responsible for generation of the slow electrochromic phase in chloroplasts.
abstract: The amplitude of the slow phase of the electrochromic bandshift and the dark redox state of cytochrome b6, as well as its flash-induced turnover, have been measured as a function of ambient redox potential between +200 and -200 mV. Formation of a quinol-like donor with an Em,7 = +100 +/- 10 mV is required for generation of the slow phase. 80-100% of the amplitude of this signal with a t 1/2 = 3-4 ms is observed at -200 mV where cytochrome b6 was almost fully reduced (Em,7 of dark and flash-induced photoreduction was -30 mV and -75 mV, respectively). The change in the photoreduction of cytochrome b6 above 0 mV had an Em,7 of +50 mV, about 50 mV more negative than the midpoint at this pH for the onset of the slow electrochromic change. At potentials below -140 mV the amplitude of b6 photoreduction becomes small or negligible. The nature of the cytochrome b6 photoresponse is changed at potentials below -140 mV from a net photoreduction with a t1/2 = approximately less than 1 ms to a photooxidation with a t1/2 = 15-20 ms that is substantially slower than the electrochromic band-shift with a t1/2 = 3-4 ms. It is concluded that the slow electrochromic phase probably does not arise from a mechanism involving a turnover of cytochrome b6. From consideration of the possible flash-induced electron-transfer steps and alternative mechanisms for generation of the slow phase, it is suggested that it may arise from a redox-linked H+ pump involving the high potential iron-sulfur protein.
mesh_terms: D002736:Chloroplasts / Q000378:metabolism*; D003573:Cytochrome b Group / Q000378:metabolism; D045346:Cytochrome b6f Complex; D004563:Electrochemistry; D004579:Electron Transport; D006863:Hydrogen-Ion Concentration; D006873:Hydroquinones / Q000378:metabolism; D010084:Oxidation-Reduction; D010777:Photochemistry; D010788:Photosynthesis; D010944:Plants; D013053:Spectrophotometry
chemical_list: D003573:Cytochrome b Group; D006873:Hydroquinones; D045346:Cytochrome b6f Complex