High and low potential states of the chloroplast cytochromeb-559 and thermodynamic control of non-cyclic electron transport |
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Authors: | W A Cramer H N Fan H Böhme |
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Institution: | (1) Department of Biological Sciences, Purdue University, 47907 Lafayette, Indiana |
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Abstract: | Most of the chloroplastb-559 is high potential at neutral pH as defined by hydroquinone reducibility. FCCP* (20 M) and antimycin A (50 M) convert high potentialb-559 to a low potential state which can be reduced by ascorbate but not hydroquinone. The low and high potential states of cytochromeb-559 are different forms of the same cytochrome.Three lines of evidence indicate that the cytb-559 oxidized by photosystem I is low potential: (1) theb-559 photooxidized by far-red light in the presence of FCCP (3 M) is low potentialb-559; (2) the amplitude of theb-559 oxidation by far-red light and the amount of low potentialb-559 present in the dark have the same general dependence on pH; (3) inhibitor studies show that plastoquinone mediates the oxidation of cytb-559 by PS I.The well-known stimulation ofb-559 oxidation by far-red light in the presence of FCCP is attributed to FCCP-facilitated photoconversion of high potentialb-559 to a low potential form.It is concluded that if cytb-559 is oxidized by system I light, then it is a low potential form (E
m7 +80 mV) which is oxidized. It is not proven, however, that a significant amount of cytb-559 is oxidized by PS I under coupled or physiological conditions.Possible thermodynamic regulation of non-cyclic electron flow involving the distribution between high and low potential forms of cytb-559 is discussed. |
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