2-Phosphoglycolate (chemical formula C₂H₂O₆P^3-; 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). Photorespiration eng

Synthesis

RuBisCo catalyzes the fixation of atmospheric

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

in the

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s of

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

s. 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 The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into ...

. 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 Hydroperoxides or peroxols are Chemical compound, compounds of the form ROOH, where R stands for any group, typically Organic compound, organic, which contain the hydroperoxy functional group (). Hydroperoxide also refers to the hydroperoxide anio ...

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.

Biological role

Plants

In plants, 2-phosphoglycolate has a potentially

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

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 In biology, homeostasis (British English, British also homoeostasis; ) is the state of steady internal physics, physical and chemistry, chemical conditions maintained by organism, living systems. This is the condition of optimal functioning fo ...

Photorespiration Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant physiology, plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. Th ...

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 Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an ade ...

and ATP, respectively. In addition, this salvage pathway loses ½ equivalent of previously fixed carbon dioxide and releases ½ equivalent of toxic

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

per molecule of 2-PG. This leads to a net loss of carbon in photorespiration, making it much less efficient than the Calvin cycle. However, this salvage pathway can also act as a cellular energy sink, preventing the chloroplastidal

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

from over reduction. It is believed that this pathway also plays a role in improving the

abiotic stress Abiotic stress is the negative impact of non-living factors on the living organisms in a specific environment. The non-living variable must influence the environment beyond its normal range of variation to adversely affect the population performan ...

response of plants.

Bacteria

2-PG is similarly a toxic product in bacteria. Bacteria remove this substance using a glycerate pathway. This shorter pathway branches out from photorespiration after the formation of glyoxylate, proceeding to use glycoxylate carboxylase and tartronic semialdehyde reductase to rejoin at the formation of glycerate. Some Cyanobacteria can use a combination of photorespiration and glycerate pathways. Transferring the shorter glycerate pathway into plant chloroplasts, combined with stopping chloroplastic export of glycolate, results in higher photosynthetic efficiency. In tobacco, the biomass increases by 13%, not as good a result as a designed pathway.

Animals

Although mainly produced in plants, 2-PG also plays a role in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

ian metabolism, though the source of 2-PG in mammals remains incompletely understood. It is thought that the processing of breaks in the DNA-strand produces small amounts of 2-PG, but other processes may yield 2-PG as well. The phosphatase subunit of bisphosphoglycerate mutase, an enzyme found in

red blood cell Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

s, shows an increase in activity by up three orders of magnitude in the presence of 2-PG, resulting in an increase of the oxygen affinity of

hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...

Agricultural significance

RuBisCo has been a potential target for bioengineers for

agricultural Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created f ...

purposes. A decrease in the oxygenation of RuBP may result in a boost in the efficiency of carbon assimilation in crops such as

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

and therefore increase their net

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

production. Attempts have been made to artificially alter the protein structure of RuBisCo to enhance its catalytic turnover rate.

Mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...

s in the L-subunit of the enzyme, for example, have been shown to increase both the catalytic turnover rate and RuBisCos affinity for carbon dioxide

References

{{DEFAULTSORT:Phosphoglycolate, 2- Organophosphates Organic acids