Glycine decarboxylation and photorespiratory metabolism in C3 plants

In higher plants which carry out C3 photosynthesis, photosynthetic and photorespiratory metabolism is based on the action of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase and the regeneration of its substrate ribulose-1, 5-bisphosphate by the reductive pentose phosphate pathway (RPP) or Calvin cycle. These reactions occur in the chloroplast. The product of CO2 fixation by RuBP carboxylase is two molecules of 3-phosphoglycerate (a three-carbon compound; hence C3 photosynthesis) which is either exported from the chloroplast as triose phosphate for sucrose synthesis in the cytosol or metabolized to form starch within the chloroplast or used for regeneration of RuBP. Oxygen competes with CO2 for the active site of RuBP carboxylase/oxygenase leading to an oxygenase reaction which reduces the rate of CO2 assimilation. The products of the oxygenase reaction are 3-phosphogly cerate and phosphoglycolate (a two-carbon compound). The production of phosphoglycolate represents a drain of carbon away from the RPP and to recover this carbon the phosphoglycolate is metabolized through a series of reactions involving enzymes in the chloroplasts, peroxisomes, and mitochondria. In the course of this pathway two molecules of glycine (i.e. four carbon atoms) are metabolized to one molecule of serine, CO2 and NH3. Serine is metabolized further to 3-phosphoglycerate and so three out of four carbon atoms entering the pathway are returned to the RPP (Husic, Husic & Tolbert, 1987; Ogren, 1984). The light energy requirement of photosynthesis and photorespiration is for the synthesis of ATP and NADPH via the electron transport pathway of the chloroplast.