The testis-specific phosphoglycerate kinase gene pgk-2 is a recruited retroposon.

In both humans and mice, two genes encode phosphoglycerate kinase, a key enzyme in the glycolytic pathway. The pgk-1 gene is expressed in all somatic cells, is located on the X chromosome, and contains 10 introns. The pgk-2 gene is expressed only in sperm cells, is located on an autosome, and has no introns. The nucleotide sequence of the pgk-2 gene suggests that it arose from pgk-1 more than 100 million years ago by RNA-mediated gene duplication. The pgk-2 gene may, then, be a transcribed retroposon. Thus, gene duplication by retroposition may have been used as a mechanism for evolutionary diversification.     

Secretion of phosphoglycerate kinase from tumour cells is controlled by oxygen-sensing hydroxylases

Solid tumour cells employ glycolytic enzymes including phosphoglycerate kinase (PGK) to make ATP when their supply of oxygen is limiting. PGK is also secreted by tumour cells and facilitates cleavage of disulfide bonds in plasmin, which triggers proteolytic release of the angiogenesis inhibitor, angiostatin. Although PGK production by tumour cells was enhanced by hypoxia, its secretion was inhibited. Inhibition of secretion correlated with decrease in angiostatin formation by the tumour cells. In contrast, hypoxia did not inhibit the secretion of the angiogenesis activator, vascular endothelial cell growth factor (VEGF). PGK secretion was reversed by normoxia and was under control of the oxygen-sensing protein hydroxylases, as inhibitors of this class of enzymes mimicked the effect of hypoxia on PGK secretion. Direct hydroxylation of PGK was not the mechanism by which the protein hydroxylases controlled its secretion. These findings show that production and secretion of PGK are regulated separately and indicate that oxygen and the protein hydroxylases can control not only gene expression but also protein secretion.     

Multiple forms of human phosphoglycerate kinase.

Phosphoglycerate kinase was isolated by affinity chromatography from human skeletal muscle and erythrocytes. As in the tissue extracts, the purified enzyme showed in Cellogel electrophoresis one major and two minor bands with phosphoglycerate kinase activity. The multiple forms were separated by chromatography on CM-Sepharose. From the three separated forms, A, B, and C, the latter was not detectable in electrophoresis of tissue extracts or in the purified unresolved phosphoglycerate kinase. The faintest, most anodically migrating form observed in the tissue extracts could not be isolated in pure form by chromatography on CM-Sepharose. The electrophoretic mobility of the phosphoglycerate kinase forms depended strongly on the buffer systems used. The different forms had identical molecular weight, substrate affinity, and heat stability and were inhibited to the same extent by antibody. They could also not be separated by column affinity chromatography. Small differences were found in thiol group content and in the specific activity, the latter being a consequence of diminished free sulfhydryl residues. Exposure to either reductive or oxidative conditions changed the specific activity, but did not result in interconversion among the pure forms. The multiple forms probably arise as a result of epigenetic factors occurring after the primary polypeptide chain has been synthesized.     

Anion binding to yeast phosphoglycerate kinase.

The single thiol of yeast phosphoglycerate kinase was labelled with the chromophoric sulfhydryl reagent, 2-chloromercuri-4-nitrophenol. Sequential additions of individual anions to this modified enzyme brought about a decrease in absorbance at 410 nm that reflected the degree of saturation of the enzyme with anion. The binding curves were analyzed to determine the dissociation constants of a number of anions with charges varying from--1 to--4.1. A linear relationship was found between the charge of the anion and the negative logarithm of the dissociation constant for the labelled enzyme-anion complex. The highly charged anions, such as ATP, bound more tightly than did anions with less charge, such as Cl-. The average number of binding sites for those anions for which accurate results could be obtained was 1.06 mol per 47000 g of enzyme. Several lines of evidence suggested that titration of the active center was not being monitored. Anions bound to phosphoglycerate kinase decreased the rate of reaction between the enzyme thiol and 5,5'-dithiobis(2-nitrobenzoic acid). The relationship between the degree of saturation of the anion binding site and the reaction rate constant was used to calculate the dissociation constant between anion and enzyme. Dissociation constants determined in this manner were in good agreement with those determined by titration of the enzyme-mercurial complex.     

Pressure-temperature-induced denaturation of yeast phosphoglycerate kinase, a double-domain protein.

Pressure-induced denaturation of yeast phosphoglycerate kinase was studied at various temperatures, as a model double-domain protein, using intrinsic fluorescence, 4th derivative absorbance, CD, and DSC. A thermodynamic transition intermediate was observed in the pressure-denaturation, as was reported for the cold denaturation. From the different response of Trp and Tyr residues, as monitored by fluorescence and 4th derivative absorbance changes, the C-terminal domain carrying all the Trp residues seemed to exert structural changes at relatively lower pressure. A further structural change involving both domains was observed at higher pressures. The two-step changes occurred almost simultaneously during heat denaturation.     

Sequence and structure of yeast phosphoglycerate kinase.

The structure of yeast phosphoglycerate kinase has been determined with data obtained from amino acid sequence, nucleotide sequence, and X-ray crystallographic studies. The substrate binding sites, as deduced from electron density maps, are compatible with known substrate specificity and the stereochemical requirements for the enzymic reaction. A carboxyl-imidazole interaction appears to be involved in controlling the transition between the open and closed forms of the enzyme.     

Characterisation of yeast phosphoglycerate kinase modified by mutagenesis at residue 21.

Site-directed mutagenesis has been used to produce mutant forms of yeast phosphoglycerate kinase in which the conserved active-site residue, Arg21, has been replaced by a methionine or a lysine. Kinetic results obtained using these mutant enzymes show that their Km for both 3-phospho-D-glycerate and ATP are significantly different from those recorded for the wild-type enzyme. The Vmax for the lysine mutant is reduced by a factor of two from that of the wild-type enzyme whereas the Vmax for the methionine mutant is reduced more than sevenfold. A very clean electron-density-difference map shows little, if any, evidence of a structural change associated with the C-terminal domain, although resonances in the NMR spectra associated with the ATP-binding site (C-terminal domain) are also affected by the mutation as one might expect from the kinetic results. The NMR data show that binding at both the 3-phospho-D-glycerate and the non-productive ATP-binding site (associated with the N-terminal domain) are affected in the mutant in a way which is different to that associated with the wild-type enzyme. These results, taken together with the X-ray and kinetic data, indicate that the non-productive ATP-binding site and the activating anion-binding site are both associated with the basic patch region of yeast phosphoglycerate kinase.     

The slow-refolding step of phosphoglycerate kinase as monitored by pulse proteolysis.

The kinetics of refolding of yeast phosphoglycerate kinase were studied by following the variation in circular dichroism at 218 nm, the recovery of enzyme activity, and the susceptibility to proteolysis by trypsin and V8-protease. A very rapid phase followed by a slower one was detected by circular dichroism, which revealed the formation of secondary structures. The slower phase, with a macroscopic rate constant of 0.35 min-1, was also detected by the susceptibility of the enzyme to both proteases. It was shown that cleavage sites located in the hinge region, in a part of the C-domain and, to a lesser extent, in a region of the N-domain, which are accessible in the intermediate state, became inaccessible during the slow-refolding step of the molecule. These results demonstrate, on the one hand, the role of domains as folding intermediates, and, on the other hand, the locking of the domain structure and the domain pairing that occurs during the slow-refolding step with a rate constant of 0.35 min-1. The return of the enzyme activity occurred in a slower last step upon conformational readjustments induced by domain interactions.     

Yeast phosphoglycerate kinase: investigation of catalytic function by site-directed mutagenesis.

A salt link buried in the domain interface of phosphoglycerate kinase has been implicated as being important in controlling the conformational transition from the open, or substrate-binding, to the closed, or catalytically competent, form of the enzyme. The residues contributing to the salt link are remote from the active site, but are connected to the substrate-binding sites through strands of beta-sheet. It has been suggested that these residues may also mediate sulphate and anion activation. These assumptions have been tested by examining the properties of a site-directed mutant (histidine-388----glutamine-388). The expression and overall structural integrity of the mutant, produced in yeast from a multicopy plasmid, remains essentially unaltered from the wild-type enzyme. However, the mutant enzyme has a kcat. reduced by 5-fold. The Km for ATP is lowered by 3-fold, and the Km for 3-phosphoglycerate is unaffected. The effects of sulphate on activity over a wide range of substrate concentrations appear to be the same for both the mutant and wild-type enzymes. These results lead to a reappraisal of the mechanistic role of the inter-domain histidine-glutamate interaction, as well as a refinement of the kinetic model of the enzyme.     

Human granulocyte phosphoglycerate kinase. Purification by double affinity elution and immunological study.

Human phosphoglycerate kinase has been totally purified from leukemic granulocytes by double 'affinity elution' with ATP and 3-phosphoglycerate. This purification procedure allowed to obtain 19 mg of protein, specific activity of which was 400 IU/mg i.e. a 105-fold purification and an overall yield of 47%. Purified enzyme was homogenous when tested by acrylamide sodium dodecyl sulfate electrophoresis and immunodiffusion. Specific antibodies raised in rabbits totally inactivated phosphoglycerate kinase of crude extracts as well as of the purified preparation. The molecular specific activity (i.e. the ratio enzyme activity/immunological reactivity) was identical in leukocytes, platelets, erythrocytes and was identical in these cells to the value found for the totally purified phosphoglycerate kinase.