Intramolecular deuterium distributions reveal disequilibrium of chloroplast phosphoglucose isomerase

• Published in: Plant, cell and environment cell and environment, 1999-05, Vol.22 (5), p.525-533
• Main Authors: SCHLEUCHER, J., VANDERVEER, P., MARKLEY, J. L., SHARKEY, T. D.
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; C3 photosynthesis; Cell physiology; chloroplast; cytosol; deuterium; Economic plant physiology; enzyme kinetics; Fundamental and applied biological sciences. Psychology; General aspects, methods; Net assimilation, photosynthesis, carbon metabolism. Photorespiration, respiration, fermentation (anoxia, hypoxia); nuclear magnetic resonance; Nutrition. Photosynthesis. Respiration. Metabolism; phosphoglucose isomerase; Plant physiology and development; stable isotopes
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1046/j.1365-3040.1999.00440.x

D, deuterium
δD(NMR), chemical shift axis in a deuterium NMR spectrum
F6P, fructose‐6‐phosphate
G6P, glucose‐6‐phosphate
IRMS, isotope ratio mass spectrometry
NMR, nuclear magnetic resonance
PGI, phosphoglucose isomerase

Intramolecular deuterium distributions of the carbon‐bound hydrogens of glucose were measured using deuterium nuclear magnetic resonance. Glucose isolated from leaf starch of common bean (Phaseolus vulgaris cv. Linden) or spinach (Spinacia oleracea cv. Giant nobel) was depleted in deuterium in the C(2) position, compared with glucose isolated from leaf sucrose or bean endosperm starch. In beans, the depletion of C(2) was independent of the light intensity during growth (150 or 700 μmol photons s–1 m–2). The ratio of glucose‐6‐phosphate to fructose‐6‐phosphate ([G6P]/[F6P]) in bean chloroplasts was 0·9 in high light, indicating that the phosphoglucose isomerase reaction was not in equilibrium ([G6P]/[F6P]) ≈ 3). This implies that the kinetic isotope effect of phosphoglucose isomerase depleted deuterium in the C(2) position of G6P. Because the depletion was the same, the chloroplastic ([G6P]/[F6P]) ratio was in disequilibrium irrespective of the light intensity. If the ([G6P]/[F6P]) ratio was in equilibrium, a large chloroplastic pool of G6P would be unavailable for regeneration of ribulose‐1,5‐bisphospate. We argue that chloroplast phosphoglucose isomerase activity is regulated to avoid this. The deuterium depletion of C(2) explains the known low overall deuterium abundance of leaf starch. This example shows that measurements of intramolecular deuterium distributions can be essential to understand overall deuterium abundances of plant material.