7 - Phosphoglycolate formed by the oxygenase activity of RubisCO is recycled in the photorespiratory pathway

Abstract

In the photorespiratory pathway, the by-product 2-phosphoglycolate is recycled to ribulose 1,5-bisphosphate. The term photorespiration indicates that it involves oxygen consumption in the light, which is accompanied by the release of CO2. Whereas in mitochondrial respiration, the oxidation of substrates to CO2 serves the purpose of producing ATP, and in the course of photorespiration ATP is consumed. The oxygenation of two molecules of ribulose 1,5-bisphosphate yields two molecules of 3-phosphoglycerate and two molecules of 2-phosphoglycolate. The latter are recycled to yield another molecule of 3-phosphoglycerate. This recycling begins with the hydrolytic release of phosphate by glycolate phosphate phosphatase present in the chloroplast stroma. The resultant glycolate leaves the chloroplasts via a specific translocator located in the inner envelope membrane and enters the peroxisomes via pores in the peroxisomal boundary membrane, probably facilitated by a porin. In the peroxisomes, the alcoholic group of glycolate is oxidized to a carbonyl group in an irreversible reaction catalyzed by glycolate oxidase, resulting in the synthesis of glyoxylate. The reducing equivalents are transferred to molecular oxygen to produce H2O2. Like other H2O2 forming oxidases, the glycolate oxidase contains a flavin mononucleotide cofactor as redox mediator between glycolate and oxygen. H2O2 is then converted to water and oxygen by the enzyme catalase, which is present in the peroxisomes. Thus, in total, 0.5 mol of O2 is consumed for the oxidation of one mole of glycolate to glyoxylate.