Coupling of Carbon Dioxide Fixation to the Oxyhydrogen Reaction in the Isolated Chloroplast of Chlamydomonas reinhardtii.
The oxyhydrogen reaction (the reduction of O(2) to water by H(2)) in the presence of CO(2) was studied in the isolated Chlamydomonas reinhardtii chloroplast by monitoring the rate of (14)CO(2) incorporation into acid-stable products in the dark. The endogenous rate of CO(2) uptake (50-125 nmol/mg chlorophyll per h) was increased about 3- to 4-fold by ATP and additionally when combined with glucose, ribose-5-phosphate, and glycerate-3-phosphate. The rate was diminished 50 to 75%, respectively, when H(2) was replaced by N(2) or by air. Decrease in CO(2) uptake by dl-glyceraldehyde was taken to indicate that the regenerative phase and complete Calvin cycle turnover were involved. Diminution of CO(2) incorporation by rotenone, antimycin A, and 2,5-dibromo-3-methyl-6-isopropanol-p-benzoquinone was attributed to an inhibition of the oxyhydrogen reaction, resulting in an elevated NADPH/NADP ratio. If so, then the diminished CO(2) uptake could have been by "product inhibition" of the carbon metabolic network. Our data are consistent with the proposal (H. Gaffron [1942] J Gen Physiol 26: 241-267) that CO(2) fixation coupled to the oxyhydrogen reaction is dependent to some extent on exchloroplastic metabolism. This support is primarily ATP provided by mitochondrial respiration.
Chen
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Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02254.
Gibbs
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