3-Phosphoglyceric acid (3PG, 3-PGA, or PGA) is the conjugate acid of 3-phosphoglycerate or glycerate 3-phosphate (GP or G3P).
This glycerate is a
biochemically significant metabolic intermediate in both
glycolysis and the
Calvin-Benson cycle. The anion is often termed as PGA when referring to the Calvin-Benson cycle. In the Calvin-Benson cycle, 3-phosphoglycerate is typically the product of the spontaneous scission of an unstable 6-carbon intermediate formed upon CO
2 fixation. Thus, two equivalents of 3-phosphoglycerate are produced for each molecule of CO
2 that is fixed.
In glycolysis, 3-phosphoglycerate is an intermediate following the
dephosphorylation (
reduction) of
1,3-bisphosphoglycerate.
Glycolysis
In the glycolytic pathway, 1,3-bisphosphoglycerate is dephosphorylated to form 3-phosphoglyceric acid in a coupled reaction producing two
ATP via
substrate-level phosphorylation
Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP supported by the energy released from another high-energy bond that leads to phosphorylation of ADP or GDP to ATP or GTP (note that the rea ...
.
The single phosphate group left on the 3-PGA molecule then moves from an end carbon to a central carbon, producing 2-phosphoglycerate.
This phosphate group relocation is catalyzed by
phosphoglycerate mutase, an enzyme that also catalyzes the reverse reaction.
Calvin-Benson cycle
In the
light-independent reactions (also known as the Calvin-Benson cycle), two 3-phosphoglycerate molecules are synthesized.
RuBP, a 5-carbon sugar, undergoes
carbon fixation
Biological carbon fixation, or сarbon assimilation, is the Biological process, process by which living organisms convert Total inorganic carbon, inorganic carbon (particularly carbon dioxide, ) to Organic compound, organic compounds. These o ...
, catalyzed by the
rubisco enzyme, to become an unstable 6-carbon intermediate. This intermediate is then cleaved into two, separate 3-carbon molecules of 3-PGA.
One of the resultant 3-PGA molecules continues through the Calvin-Benson cycle to be regenerated into RuBP while the other is reduced to form one molecule of
glyceraldehyde 3-phosphate (G3P) in two steps: the
phosphorylation
In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols:
:
This equation can be writ ...
of 3-PGA into
1,3-bisphosphoglyceric acid via the enzyme
phosphoglycerate kinase (the reverse of the reaction seen in glycolysis) and the subsequent catalysis by
glyceraldehyde 3-phosphate dehydrogenase
Glyceraldehyde 3-phosphate dehydrogenase (abbreviated GAPDH) () is an enzyme of about 37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules. In addition to this long establis ...
into G3P.
G3P eventually reacts to form the sugars such as
glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
or
fructose
Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...
or more complex
starches.
Amino acid synthesis
Glycerate 3-phosphate (formed from 3-phosphoglycerate) is also a precursor for
serine, which, in turn, can create
cysteine and
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...
through the
homocysteine cycle.
Measurement
3-phosphoglycerate can be separated and measured using
paper chromatography as well as with
column chromatography and other chromatographic separation methods.
It can be identified using both
gas-chromatography and
liquid-chromatography mass spectrometry and has been optimized for evaluation using
tandem MS techniques.
See also
*
2-Phosphoglyceric acid
*
Calvin-Benson cycle
*
Photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
*
Ribulose 1,5-bisphosphate
References
{{DEFAULTSORT:Phosphoglyceric Acid, 3-
Carboxylate anions
Organophosphates
Photosynthesis
Glycolysis
Metabolic intermediates
Biomolecules