2-Phosphoglycolate (chemical formula C2H2O6P3-; also known as phosphoglycolate, 2-PG, or PG) is a natural metabolic product of the oxygenase reaction mediated by the enzyme ribulose 1,5-bisphosphate carboxylase (RuBisCo). RuBisCo catalyzes the fixation of atmospheric carbon dioxide in the chloroplasts of plants. It uses ribulose 1,5-bisphosphate (RuBP) as substrate and facilitates carboxylation at the C2 carbon via an endiolate intermediate. The two three-carbon products (3-phosphoglycerate) are subsequently fed into the Calvin cycle. Atmospheric oxygen competes with this reaction. In a process called photorespiration RuBisCo can also catalyze addition of atmospheric oxygen to the C2 carbon of RuBP forming a high energy hydroperoxide intermediate that decomposes into 2-phosphoglycolate and 3-phosphoglycerate. Despite a higher energy barrier for the oxygenation reaction compared to carboxylation, photorespiration accounts for up to 25% of RuBisCo turnover in C3 plants. In plants, 2-phosphoglycolate has a potentially toxic effect as it inhibits a number of metabolic pathways. The activities of important enzymes in the central carbon metabolism of the chloroplast such as triose-phosphate isomerase, phosphofructokinase, or sedoheptulose 1,7-bisphosphate phosphatase show a significant decrease in the presence of 2-PG. Therefore, degradation of 2-PG during photorespiration is important for cellular homeostasis. Photorespiration is the main way of chloroplasts to rid themselves of 2-PG. However, this pathway comes at a decreased return on investment ratio as 2-PG is transformed to 3-phosphoglycerate in an elaborate salvage pathway at the cost of one equivalent of NADH and ATP, respectively. In addition, this salvage pathway loses 1⁄2 equivalent of previously fixed carbon dioxide and releases 1⁄2 equivalent of toxic ammonia per molecule of 2-PG. This leads to a net loss of carbon in photorespiration, making it much less efficient than the Calvin cycle.
Daniel Eduardo Hurtado Salinas
Hikari Ananda Infinity Yoshihara
Anders Meibom, Charlotte Madeleine Nicole Lekieffre, Emmanuelle Geslin