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Plant Physiology 53:491-495 (1974)
© 1974 American Society of Plant Biologists

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Articles

Photosynthesis in the Higher Plant, Vicia faba

III. Serine, a Precursor of the Tricarboxylic Acid Cycle 1

S. Sherrill Kent2, Frederick D. Pinkerton and Gary A. Strobel

a Department of Plant Pathology, Montana State University, Bozeman, Montana 59715

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.

2 To whom reprint requests should be addressed.

1 This investigation was supported by the National Science Foundation Grant GB 23918 and United States Department of Agriculture Cooperative States Research Service Grant 216-15-23.






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ASPB Publications PLANT PHYSIOLOGY® THE PLANT CELL
Copyright © 1974 by the American Society of Plant Biologists