PGA Full Form

Many biological and chemical processes happening around us, involve organic compounds in the form of acids or bases. One such organic acid is Phosphoglyceric acid. It is a monophosphoglyceric acid having the phosphor group at 3-position and is an extremely weak compound. It is present in all living organisms ranging from microorganisms like bacteria to extremely complex species like humans.

What is PGA?

PGA stands for Phosphoglyceric Acid, a 3-carbon organic acid which is an important metabolic intermediate in the processes- glycolysis and the Calvin-Benson cycle. It is the derivative of a tetronic acid and is actually the conjugate acid of 3-phosphoglycerate. Its chemical formula is C3H7O7P and is also known as 3-phosphoglycerate.Its molar mass is 186.06 and is naturally found in organisms such as mycoplasma gallisepticum, trypanosoma brucei.

The IUPAC name of this compound is (2R)-2-hydroxy-3-phosphonooxypropanoic acid and the carboxylation of ribulose-1, 5-bisphosphate which is a five-carbon compound helps in the formation of 2-carboxy-3-keto-D-arabinitol 1, 5-bisphosphate that in turn helps in the formation of PGA. 

PGA in Calvin Cycle

American biochemist Melvin Calvin conducted an experiment by allowing photosynthesis of green plants in the presence of radioactive carbon dioxide under various conditions. It includes fixation, reduction and utilization of carbon by green plants to form sugar phosphates. This was the Calvin cycle, commonly known as Calvin-Benson Cycle. The products of the carbon fixation experiment included PGA, Phosphoglyceric Acid as the first stable principal radioactive compound formed.  The pathway is known as C3 pathway. Whereas, photorespiration is a wasteful pathway completing the Calvin cycle which is referred as C2 pathway. During the process of photosynthesis, the phosphorus containing organic compound- Phosphoglyceric Acid is formed.

Carbon dioxide is first attached to a 5-carbon atom compound called 

 ribulose-1-5-diphosphate (RudP) from an intermediate 6-carbon compound. Each molecule of RudP then splits to form two molecules of PGA (Phosphoglyceric Acid). Radioactive carbon dioxide (14CO2) used in the experiment contributed one carbon atom of the two PGA molecules formed, thus, only one has radioactive 

Carbon. Therefore, only one free molecule of PGA is formed per molecule of CO2 entering the cycle. But in the process, 6 molecules of RudP and 6 molecules of  CO2  react to produce 12 molecules of PGA. The catalysis of carboxylation is done by ribulose bisphosphate carboxylase or oxygenase, also known as Rubisco. It can constitute up to 50% of the soluble protein present in the leaf and is probably the most abundant protein on Earth. The 3-phosphoglycerate formed, further transforms into the components of cell wall, which is, cellulose, sucrose, starch and a variety of polysaccharides.

PGA in Glycolysis

Glycolysis is the primary stage of cellular respiration. It is a 10 step process that converts glucose into pyruvate and releases energy. The formation of PGA (Phosphoglyceric Acid) from BPGA (Biphosphoglyceric acid) is an energy yielding reaction of glycolysis. In the process, 1, 3-bisphophoglycerate forms the 3-phosphoglyceric acid through the process of dephosphorylation in the glucotic pathway. The enzyme phosphoglyceramomutase catalyzes the internal transfer of the phosphate group from C-3 to C-2, which means, phosphoglyceromutase carries out the conversion of 3-phosphoglycerate into 2-phosphoclycerate in glycolysis. Glycerate 3-phosphate is also a precursor for serine that can create other products in biochemical processes. 

So, it is also involved in the biosynthesis of amino acids. Amino acid synthesis is the set of biochemical processes through which amino acids are produced. Humans cannot synthesize all 20 of the amino acids but amino acids- arginine, cysteine, glycine, glutamine, histidine, proline, serine and tyrosine are quite essential. 

The enzyme phosphoglycerate dehydrogenase helps in the conversion of phosphohydroxyl- pyruvate from 3-phosphoglycerate. Phosphoglycerate dehydrogenase catalyses the first step of the phosphorylated serine biosynthesis pathway. Glycine and cysteine will be regulated after the serine is concentrated in the cell.

Hence, because of its role in both Calvin cycle and glycolysis, phosphoglyceric acid acts as an algal metabolite as well as a fundamental metabolite.

Conclusion

PGA- phosphoglyceric Acid belongs to a class of organic compounds called sugar acids and derivatives. It is an important carbon compound generated in two essential processes- glycolysis and Calvin-Benson Cycle. In Calvin cycle, there is carbon fixation in the presence of catalysts that produce Phosphoglyceric acid. So, it is referred to as PGA when speaking about the Calvin Cycle. In the process of glycolysis it is an extremely weak carbon compound (basically neutral), yet it is very essential to science and society. It is also an important component that contributes in the biosynthesis of amino acids as well. Hence, PGA is a commonly used term in biology which stands for Phosphoglyceric Acid.