Human platelet 6-phosphofructokinase. Purification, kinetic parameters and the influence of sulphate ions on enzyme activity

Ethyl isopropyl amiloride decreases oxidative phosphorylation and increases mitochondrial fusion in clonal untransformed and cancer cells

Many cancer cells, regardless of their tissue origin or genetic landscape, have increased expression or activity of the plasma membrane Na-H exchanger NHE1 and a higher intracellular pH (pHi) compared with untransformed cells. A current perspective that remains to be validated is that increased NHE1 activity and pHi enable a Warburg-like metabolic reprogramming of increased glycolysis and decreased mitochondrial oxidative phosphorylation. We tested this perspective and find it is not accurate for clonal pancreatic and breast cancer cells. Using the pharmacological reagent ethyl isopropyl amiloride (EIPA) to inhibit NHE1 activity and decrease pHi, we observe no change in glycolysis, as indicated by secreted lactate and intracellular pyruvate, despite confirming increased activity of the glycolytic enzyme phosphofructokinase-1 at higher pH. Also, in contrast to predictions, we find a significant decrease in oxidative phosphorylation with EIPA, as indicated by oxygen consumption rate (OCR). Decreased OCR with EIPA is not associated with changes in pathways that fuel oxidative phosphorylation or with mitochondrial membrane potential but occurs with a change in mitochondrial dynamics that includes a significant increase in elongated mitochondrial networks, suggesting increased fusion. These findings conflict with current paradigms on increased pHi inhibiting oxidative phosphorylation and increased oxidative phosphorylation being associated with mitochondrial fusion. Moreover, these findings raise questions on the suggested use of EIPA-like compounds to limit metabolic reprogramming in cancer cells.

Phosphofructokinase-1 and fructose bisphosphatase-1 in canine liver and kidney

In healthy individuals, plasma glucose levels are maintained within a normal range. During fasting, endogenous glucose is released either through glycogenolysis or gluconeogenesis. Gluconeogenesis involves the formation of glucose-6-phosphate from a variety of precursors followed by its subsequent hydrolysis to glucose. Gluconeogenesis occurs in the liver and the kidney. In order to compare gluconeogenesis in canine liver and kidney, the activity and expression of the rate limiting enzymes that catalyze the fructose-6-phosphate and fructose 1,6-bisphosphate steps, namely, phosphofructokinase-1 (PFK-1) (glycolysis) and fructose bisphosphatase-1 (FBP-1) (gluconeogenesis), were examined. Healthy male and female beagle dogs aged 1–2 years were euthanized humanely, and samples of their liver and kidney were obtained for analysis. The levels of PFK-1 and FBP-1 in canine liver and kidney were assessed by enzymatic assays, Western blotting, and RT-qPCR. Enzyme assays showed that, in dogs, the kidney had higher specific activity of PFK-1 and FBP-1 than the liver. Western blotting and RT-qPCR data demonstrated that of the three different subunits (PFK-M, PFK-L, and PFK-P) the PFK-1 in canine liver mainly comprised PFK-L, whereas the PFK-1 in the canine kidney comprised all three subunits. As a result of these differences in the subunit composition of PFK-1, glucose metabolism might be regulated differently in the liver and kidney.

Crystal structure of human platelet phosphofructokinase-1 locked in an activated conformation

Phosphofructokinase-1 (Pfk) acts as the main control point of flux through glycolysis. It is involved in complex allosteric regulation and Pfk mutations have been linked to cancer development. Whereas the 3D structure and structural basis of allosteric regulation of prokaryotic Pfk has been studied in great detail, our knowledge about the molecular basis of the allosteric behaviour of the more complex mammalian Pfk is still very limited. To characterize the structural basis of allosteric regulation, the subunit interfaces and the functional consequences of modifications in Tarui's disease and cancer, we analysed the physiological homotetramer of human platelet Pfk at up to 2.67 Å resolution in two crystal forms. The crystallized enzyme is permanently activated by a deletion of the 22 C-terminal residues. Complex structures with ADP and fructose-6-phosphate (F6P) and with ATP suggest a role of three aspartates in the deprotonation of the OH-nucleophile of F6P and in the co-ordination of the catalytic magnesium ion. Changes at the dimer interface, including an asymmetry observed in both crystal forms, are the primary mechanism of allosteric regulation of Pfk by influencing the F6P-binding site. Whereas the nature of this conformational switch appears to be largely conserved in bacterial, yeast and mammalian Pfk, initiation of these changes differs significantly in eukaryotic Pfk.

Structures of human phosphofructokinase-1 and atomic basis of cancer-associated mutations

Phosphofructokinase-1 (PFK1), the “gatekeeper” of glycolysis, catalyses the committed step of the glycolytic pathway by converting fructose 6-phosphate (F6P) to fructose 1,6-bisphosphate. Allosteric activation and inhibition of PFK1 by over 10 metabolites and in response to hormonal signaling fine-tune glycolytic flux to meet energy requirements¹. Mutations inhibiting PFK1 activity cause glycogen storage disease type VII, also known as Tarui disease², and mice deficient in muscle PFK1 have decreased fat stores³. Additionally, PFK1 is suggested to have important roles in metabolic reprograming in cancer^4,5. Despite its critical role in glucose flux, the biologically relevant crystal structure of the mammalian PFK1 tetramer has not been determined. We report here the first structures of the mammalian PFK1 tetramer, for the human platelet isoform (PFKP), in complex with ATP-Mg²⁺ and ADP at 3.1 and 3.4 Å, respectively. The structures reveal substantial conformational changes in the enzyme upon nucleotide hydrolysis as well as a unique tetramer interface. Mutations of residues in this interface can affect tetramer formation, enzyme catalysis and regulation, indicating the functional importance of the tetramer. With altered glycolytic flux being a hallmark of cancers⁶, these new structures allow a molecular understanding of the functional consequences of somatic PFK1 mutations identified in human cancers. We characterized three of these mutations and show they have distinct effects on allosteric regulation of PFKP activity and lactate production. The PFKP structural blueprint for somatic mutations as well as the catalytic site can guide therapeutic targeting of PFK1 activity to control dysregulated glycolysis in disease.

pH sensitivity of the thrombin-induced rise in fructose 2,6-bisphosphate content of human platelets

The stimulation of human platelets with thrombin results in a rapid and sustained increase in the fructose 2,6-bisphosphate content which may play an important role in the potentiation of glycolytic flux induced by the agonist. The investigation of the effect of pH on thrombin-induced rise in platelet fructose 2,6-bisphosphate content is reported here. The results indicate that the early intracellular alkalinization which follows platelet stimulation may contribute to mediate the positive effect of thrombin on the regulatory metabolite.

Regional assignment of the human gene for platelet-type phosphofructokinase (PFKP) to chromosome 10p: novel use of polyspecific rodent antisera to localize human enzyme genes

Human phosphofructokinase (PFK; EC 2.7.1.11) is under the control of three structural loci which encode muscle-type (M), liver-type (L), and platelet or fibroblast-type (P) subunits; human diploid fibroblasts and leukocytes express all three loci. In order to assign the human PFKP locus to a specific human chromosome, in this study, we have examined ten human X rodent somatic cell hybrids for the expression of human P subunits using a mouse anti-human P subunit-specific antiserum in an active-enzyme-immunoprecipitation technique. In nine of ten hybrids studied, the expression of the PFKP locus segregated concordantly with chromosome 10 and none other, indicating that PFKP is located on chromosome 10; the discordancy rates for all the other chromosomes were 0.2 or greater. In the one discordant hybrid, only the long arm of chromosome 10 was retained and PFKP was not expressed. Human fibroblasts from a patient with duplication of the short arm of chromosome 10 consistently exhibited PFK activity values 180% of normal. These data indicate that human PFKP is located on the short arm of chromosome 10, and that a gene dosage effect is demonstrable in fibroblasts with a duplication of 10p. The use of rodent antihuman antibody combined with immunoprecipitation aided by staphylococci-bearing protein A may find general application in mapping human enzyme genes, when human and rodent gene-products are not distinguishable by other means.

Effect of albumin on binding and recovery of enzymes in affinity chromatography on Cibacron Blue

Bovine serum albumin appears to improve the specificity of Cibacron Blue F3GA in affinity chromatography of enzymes which interact with nucleotides. The action of bovine serum albumin may rest in its ability to selectively mask affinity sites in the dye, which are not specific for the nucleotide-binding region of the enzyme, while not seriously impairing binding nor its elution by nucleotides. Thus, the elution of Chlorella nitrate reductase from a Blue Sepharose chromatographic column by its coenzyme, NADH, fails, unless the column is first treated with bovine serum albumin. Such treatment also improves the recovery of some other nucleotide-binding enzymes tested.

Protein purification using immobilised triazine dyes

This review attempts to identify proteins which selectively interact with immobilised triazine dyes such as Cibacron blue F3GA and Procion red HE 3B. Different support matrices are compared by examining the capacities of these dyes for proteins. Various approaches to the immobilisation of triazine dyes are considered together with the use of spacers. Some theories of the mechanism of protein retardation by immobilised dyes are discussed. A number of methods are suggested for the measurement of dye concentrations and for the modification of the binding of proteins to dye columns. The variety of elution methods is compared with a view to optimizing purifications. The scope of applications is reviewed as well as the choice of dye. Some advantages of triazine dyes over other affinity ligands are given. It is concluded that although no satisfactory mechanism for the binding of triazine dyes to proteins has yet been proposed, these dyes possess considerable potential for protein purification, particularly when applied on the large scale.

Human platelet 6-phosphofructokinase. Relation between inhibition by Mg-ATP2-and cooperativity towards fructose 6-phosphate and investigations on the formation of a ternary complex

Human platelet 6-phosphofructokinase (EC 2.7.1.11) shows cooperativity towards Fru-6-P and is allosterically inhibited by high Mg-ATP2- concentrations. No relation could be demonstrated between the cooperativity towards Fru-6-P and the inhibition by Mg-ATP2-. Increasing the concentrations of Mg-ATP2- only raised the apparent Km values for Fru-6-P, but did not change the Hill constants. A possible formation of a Mg-ATP2--enzyme-Fru-6-P complex during catalysis was investigated. Our calculations suggest that such a ternary complex is indeed formed during the reaction.