Photosynthesis in the Higher Plant, Vicia faba: III. Serine, a Precursor of the Tricarboxylic Acid Cycle 1

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Evidence is presented to support the hypothesis that serine, rather than 3-phosphoglycerate of the Calvin cycle, is a precursor of the tricarboxylic acid cycle during photosynthesis by the higher plant, Vicia faba. Identification of the serine intermediate is based upon a unique C1 > C2 > C3 isotope distribution for that metabolite following the fixation of 14CO2. This labeling pattern, while incompatible with an origin either in the Calvin cycle or the glycolate pathway, satisfies a critical criterion for the 3-carbon precursor of the anomalously labeled organic acids. The predominant carboxyl carbon atom labeling of serine reflects either a mixing of two pools of that metabolite, ie., C1 = C2 > C3 and C1 > C2 = C3, or a higher order of complexity in its synthesis. An anomalous C1 = C2 > C3 < C4 distribution for aspartate, however, suggests an origin by the carboxylation of a 3-carbon intermediate related to serine which has a C1 = C2 > C3 distribution. The latter distribution has been proposed for the serine intermediate of the postulated formate pathway. This pathway is described by the generalized metabolic sequence: CO2 → formate → serine → organic acids. Corresponding carbon atom distributions for citrate (C1 > C2), aspartate (C2 > C3), and serine (C2 > C3) belie a precursor-product relationship with alanine (C2 = C3), which is a molecular parameter of the Calvin cycle product, 3-phosphoglycerate.

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