Identification of the elusive pyruvate reductase of Chlamydomonas reinhardtii chloroplasts
2016
Burgess, S.J. ((Imperial College London, London (UK). Sir Ernst Chain Building-Wolfson Laboratories, Department of Life Sciences), (University of Cambridge, Cambridge (UK). Department of Plant Sciences)) | Taha, H. | Yeoman, J.A. | Iamshanova, O. | Chan, K.X. | Boehm, M. | Behrends, V. | Bundy, J.G. | Bialek, W. | Murray, J.W. | Nixon, P.J.
Under anoxic conditions the green alga Chlamydomonas reinhardtii activates various fermentation pathways leading to the creation of formate, acetate, ethanol and small amounts of other metabolites including D-lactate and hydrogen. Progress has been made in identifying the enzymes involved in these pathways and their subcellular locations; however, the identity of the enzyme involved in reducing pyruvate to D-lactate has remained unclear. Based on sequence comparisons, enzyme activity measurements, X-ray crystallography, biochemical fractionation and analysis of knock-down mutants, we conclude that pyruvate reduction in the chloroplast is catalyzed by a tetrameric NADsup(+)-dependent D-lactate dehydrogenase encoded by Cre07.g324550. Its expression during aerobic growth supports a possible function as a 'lactate valve' for the export of lactate to the mitochondrion for oxidation by cytochrome-dependent D-lactate dehydrogenases and by glycolate dehydrogenase. We also present a revised spatial model of fermentation based on our immunochemical detection of the likely pyruvate decarboxylase, PDC3, in the cytoplasm.
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