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Author Hanson, K. R.
Source United States Department of Energy Office of Scientific and Technical Information
Content type Text
Language English
Subject Keyword SOLAR ENERGY ♦ BASIC BIOLOGICAL SCIENCES ♦ PHOTOSYNTHESIS ♦ CHEMISTRY ♦ CARBON 14 COMPOUNDS ♦ GLYCERIC ACID ♦ PHOTOCHEMISTRY ♦ PLANTS ♦ RESPIRATION ♦ TRACER TECHNIQUES ♦ TRITIUM COMPOUNDS ♦ CARBOXYLIC ACIDS ♦ CHEMICAL REACTIONS ♦ HYDROXY ACIDS ♦ ISOTOPE APPLICATIONS ♦ LABELLED COMPOUNDS ♦ ORGANIC ACIDS ♦ ORGANIC COMPOUNDS ♦ PHOTOCHEMICAL REACTIONS ♦ SYNTHESIS ♦ Solar Energy Conversion- Photochemical, Photobiological, & Thermochemical Conversion- (1980-) ♦ Biochemistry- Tracer Techniques
Abstract When (3R)-D-(3-3H1,3-14C)glyceric acid is supplied in tracer amounts to illuminated tobacco leaf discs, the acid penetrates to the chloroplasts without loss of 3H, and is phosphorylated there. Subsequent metabolism associated with the reductive photosynthetic cycle fully conserves 3H. Oxidation of ribulose bisphosphate (RuBP) by RuBP carboxylase-oxygenase (EC 4.1.1.39) results in the formation of (2R)-(2-3H1, 14C)glycolic acid which, on oxidation by glycolate oxidase (EC 1.1.3.1), releases 3H to water. Loss of 3H from the combined photosynthetic and photorespiratory systems is, therefore, associated with the oxidative photorespiratory loop. Assuming steady-state conditions and a basic metabolic model, the fraction of RuBP oxidized and the photorespiratory carbon flux relative to gross or net CO2 fixation can be calculated from the fraction of supplied 3H retained in the triose phosphates exported from the chloroplasts. This retention can be determined from the 3H:14C ratio for glucose obtained from isolated sucrose. The dependence of 3H retention upon O2 and CO2 concentrations can be deduced by assuming simple competitive kinetics for RuBP carboxylase-oxygenase. The experimental results confirmed the stereochemical assumptions made. Under conditions of negligible photorespiration 3H retention was essentially complete. The change in 3H retention with O2 and CO2 concentrations were investigated. For leaf discs (upper surface up) in normal air, it was estimated that 39% of the RuBP was oxidized, 32% of the fixed CO2 was photorespired, and the photorespiration rate was 46% of the net photosynthetic CO2 fixation rate. These are minimal estimates, as it is assumed that the only source of photorespired CO2 is glycine decarboxylation.
Educational Use Research
Learning Resource Type Article
Publisher Date 1984-07-01
Publisher Place United States
Journal Arch. Biochem. Biophys.
Volume Number 232
Issue Number 1
Organization Department of Biochemistry and Genetics, Connecticut Agricultural Experiment Station, New Haven


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