Phosphoglycerate kinase

Enzyme

Phosphoglycerate kinase (PGK 1) is an enzyme that catalyzes the reversible transfer of a phosphate group from 1,3 bisphosphoglycerate (1,3 BPG) to ADP producing 3 phosphoglycerate (3 PG) and ATP :

1,3 bisphosphoglycerate + ADP glycerate 3 phosphate + ATP

Like all kinases it is a transferase. PGK is a major enzyme used in glycolysis, in the first ATP generating step of the glycolytic pathway. In gluconeogenesis, the reaction catalyzed by PGK proceeds in the opposite direction, generating ADP and 1,3 BPG. In humans, two isozymes of PGK have been so far identified, PGK1 and PGK2. The isozymes have 87 88% identical amino acid sequence identity and though they are structurally and functionally similar, they have different localizations: PGK2, encoded by an autosomal gene, is unique to meiotic and postmeiotic spermatogenic cells, while PGK1, encoded on the X chromosome, is ubiquitously expressed in all cells. The human isozyme PGK2, which is only expressed during spermatogenesis, was shown to be essential for sperm function in mice.

PGK is found in all living organisms and its sequence has been highly conserved throughout evolution. The enzyme exists as a 415 residue monomer containing two nearly equal sized domains that correspond to the N and C termini of the protein. 3 phosphoglycerate (3 PG) binds to the N terminal, while the nucleotide substrates, MgATP or MgADP, bind to the C terminal domain of the enzyme. This extended two domain structure is associated with large scale 'hinge bending' conformational changes, similar to those found in hexokinase. The two domains of the protein are separated by a cleft and linked by two alpha helices. Though the binding of either substrate triggers a conformational change, only through the binding of both substrates does domain closure occur, leading to the transfer of the phosphate group. The bivalent metal assists the enzyme ligands in shielding the bound phosphate group's negative charges, allowing the nucleophilic attack to occur; this charge stabilization is a typical characteristic of phosphotransfer reaction. It is theorized that the ion may also encourage domain closure when PGK has bound both substrates. Then, in the case of the forward glycolytic reaction, the beta phosphate of ADP initiates a nucleophilic attack on the 1 phosphate of 1,3 BPG. The Lys219 on the enzyme guides the phosphate group to the substrate. PGK proceeds through a charge stabilized transition state that is favored over the arrangement of the bound substrate in the closed enzyme because in the transition state, all three phosphate oxygens are stabilized by ligands, as opposed to only two stabilized oxygens in the initial bound state. In the glycolytic pathway, 1,3 BPG is the phosphate donor and has a high phosphoryl transfer potential. The PGK catalyzed transfer of the phosphate group from 1,3 BPG to ADP to yield ATP can the carbon oxidation reaction of the previous glycolytic step (converting glyceraldehyde 3 phosphate to 3 phosphoglycerate).

The enzyme is activated by low concentrations of various multivalent anions, such as pyrophosphate, sulfate, phosphate, and citrate. High concentrations of MgATP and 3 PG activates PGK, while Mg2+ at high concentrations non competitively inhibits the enzyme. PGK exhibits a wide specificity toward nucleotide substrates. Its activity is inhibited by salicylates, which appear to mimic the enzyme's nucleotide substrate. Macromolecular crowding has been shown to increase PGK activity in both computer simulations and in vitro environments simulating a cell interior; as a result of crowding, the enzyme becomes more enzymatically active and more compact. depending on form – there exists a hemolytic form and a myopathic form. Since the trait is X linked, it is usually fully expressed in males, who have one X chromosome; affected females are typically asymptomatic. PGK is the only enzyme in the immediate glycolytic pathway encoded by an X linked gene. In the case of hemolytic anemia, PGK deficiency occurs in the erythrocytes. Currently, no definitive treatment exists for PGK deficiency. PGK1 overexpression has been associated with gastric cancer and has been found to increase the invasiveness of gastric cancer cells in vitro. The enzyme is secreted by tumor cells and participates in the angiogenic process, leading to the release of angiostatin and the inhibition of tumor blood vessel growth.