TY - JOUR
T1 - Quantitative relationship between photosystem I electron transport and ATP formation
AU - Ort, Donald R.
N1 - Funding Information:
‘This work was supported by Grant GB37959X from the National Science Foundation to Drs. N. E. Good and S. Izawa. ZPresent address: Department of Biological Sciences, Purdue University, West Lafayette, IN 47905.
PY - 1975/2
Y1 - 1975/2
N2 - The Photosystem I-dependent transport of electrons from diaminodurene to methylviologen is linear with reaction time and supports a constant rate of phosphorylation. However, if the diaminodurene is not kept fully reduced by the presence of excess ascorbate, the oxidized diaminodurene accumulates and begins to compete with the methylviologen as the electron acceptor. Thus, although the rate of ATP formation remains unchanged, an increasing proportion of the electron transport becomes cyclic and hence unmeasured. This leads to a rapid increase in the apparent efficiency of phosphorylation which is misleading. In contrast, it is known that the oxidized form of 3,3′-diaminobenzidine polymerizes to form an insoluble substance which should not be available to serve as an electron acceptor. However, 3,3′-diaminobenzidine is not a satisfactory donor of electrons in Photosystem I reactions for two reasons: the rate of electron transport quickly falls with reaction time and the oxidized form of 3,3′-diaminobenzidine seems to be an exceptionally efficient electron acceptor near the beginning of the period of illumination when it is presumably not yet polymerized. Thus in the first 2-3 sec of illumination when the reaction is still rapid much of the electron transport is cyclic and therefore unmeasured, especially in the absence of excess ascorbate. This cycling of electrons, which leads to an inflated apparent efficiency ( P e2 > 2), is particularly pronounced at low donor concentrations. When cyclic electron transport is avoided by the use of ascorbate or by the selection of appropriate reaction times, both diaminodurene and 3,3′-diaminobenzidine support phosphorylation with an efficiency which is approximately half of the efficiency exhibited by the overall Hill reaction. The same is true when 2,5-diaminotoluene, tetrachlorohydroquinone, 4,5-dimethyl-o-phenylenediamine, and reduced 2,6-dichloroindophenol serve as electron donors. With these six substances, the phosphorylation efficiences were 0.57 ± 0.1 molecules of ATP formed for each pair of electrons transferred ( P e2). In the same chloroplasts preparations, the transport of electrons from water to methylviologen-supported phosphorylation with a P e2 of 1.2.
AB - The Photosystem I-dependent transport of electrons from diaminodurene to methylviologen is linear with reaction time and supports a constant rate of phosphorylation. However, if the diaminodurene is not kept fully reduced by the presence of excess ascorbate, the oxidized diaminodurene accumulates and begins to compete with the methylviologen as the electron acceptor. Thus, although the rate of ATP formation remains unchanged, an increasing proportion of the electron transport becomes cyclic and hence unmeasured. This leads to a rapid increase in the apparent efficiency of phosphorylation which is misleading. In contrast, it is known that the oxidized form of 3,3′-diaminobenzidine polymerizes to form an insoluble substance which should not be available to serve as an electron acceptor. However, 3,3′-diaminobenzidine is not a satisfactory donor of electrons in Photosystem I reactions for two reasons: the rate of electron transport quickly falls with reaction time and the oxidized form of 3,3′-diaminobenzidine seems to be an exceptionally efficient electron acceptor near the beginning of the period of illumination when it is presumably not yet polymerized. Thus in the first 2-3 sec of illumination when the reaction is still rapid much of the electron transport is cyclic and therefore unmeasured, especially in the absence of excess ascorbate. This cycling of electrons, which leads to an inflated apparent efficiency ( P e2 > 2), is particularly pronounced at low donor concentrations. When cyclic electron transport is avoided by the use of ascorbate or by the selection of appropriate reaction times, both diaminodurene and 3,3′-diaminobenzidine support phosphorylation with an efficiency which is approximately half of the efficiency exhibited by the overall Hill reaction. The same is true when 2,5-diaminotoluene, tetrachlorohydroquinone, 4,5-dimethyl-o-phenylenediamine, and reduced 2,6-dichloroindophenol serve as electron donors. With these six substances, the phosphorylation efficiences were 0.57 ± 0.1 molecules of ATP formed for each pair of electrons transferred ( P e2). In the same chloroplasts preparations, the transport of electrons from water to methylviologen-supported phosphorylation with a P e2 of 1.2.
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U2 - 10.1016/0003-9861(75)90429-4
DO - 10.1016/0003-9861(75)90429-4
M3 - Article
C2 - 1119813
AN - SCOPUS:0016637225
SN - 0003-9861
VL - 166
SP - 629
EP - 638
JO - Archives of Biochemistry and Biophysics
JF - Archives of Biochemistry and Biophysics
IS - 2
ER -