Phosphofructokinases from Escherichia coli

Introduction by site-directed mutagenesis of a tryptophan residue as a fluorescent probe for the folding of Escherichia coli phosphofructokinase

The leucine residue at position 178 in the major allosteric phosphofructokinase from Escherichia coli has been replaced by a tryptophan using site-directed mutagenesis. Transformation by the mutated gene of pfk- bacteria results into the expression of a pfk+ phenotype and the production of an active enzyme. The modified protein has been purified and its fluorescence properties show that it contains 2 tryptophan residues, the original Trp 311 and the new Trp 178. During unfolding of the protein by guanidine hydrochloride, the changes in the fluorescence of these 2 residues take place at different steps: Trp 311 becomes exposed to solvent when the dimeric form dissociates into monomers, while Trp 178 is exposed only when a folded chain loses its tertiary structure. The mutant enzyme is stabilized by its substrate fructose-6-phosphate against denaturation induced by heat or guanidine hydrochloride.

Role of the C-terminal region in the allosteric properties of Escherichia coli phosphofructokinase-1

In order to investigate the role of the carboxy-terminal segment in the catalytic, regulatory, and structural properties of the major allosteric phosphofructokinase (ATP:D-fructose-6-phosphate-1-phosphotransferase: EC 2.7.1.11) from Escherichia coli, the corresponding gene has been modified at either of two sites using oligonucleotide-directed mutagenesis: the codon at position 279 was changed from TAC (Tyr) into TAA (Ochre), and the codon at position 311 from TGG (Trp) into TAG (Amber). The gene mutated at position 279 is not expressed as an active enzyme, probably because a polypeptide chain lacking 41 C-terminal residues cannot fold and/or assemble under the intracellular conditions. The gene mutated at position 311 is expressed as an active enzyme which has been purified to homogeneity. The fluorescence of this protein shows that it has no tryptophan, which confirms that the last nine residues at the carboxy terminal are missing. This derivative has almost the same specific activity and affinities for the two substrates (fructose-6-phosphate and ATP) as intact phosphofructokinase; the saturation by fructose 6-phosphate is also very cooperative. The last nine residues are thus not important for substrate binding, homotropic cooperativity, and catalytic efficiency. The activity of the mutant enzyme is still sensitive to activation by GDP or inhibition by phosphoenolpyruvate, but its affinity for the allosteric effectors is reduced. The carboxy-terminal segment also appears to contribute to the stability of the interactions between subunits: the mutant protein is less stable than the wild type towards denaturation by heat or guanidinium hydrochloride.

Phosphofructokinase from Plasmodium berghei: a kinetic model of allosteric regulation

As in mammalian cells, phosphofructokinase (PFK) is of major regulatory importance in the glucose metabolism of Plasmodium berghei. The malarial enzyme shows allosteric properties similar to PFK from various sources; it is activated by fructose-6-phosphate and inhibited by ATP, but differs with respect to allosteric regulation. Enzyme activity is only marginally increased by AMP, a potent activator of many phosphofructokinases. Phosphoenolpyruvate, which is reported to inhibit PFK activity, efficiency activates the malarial enzyme. No activation by ADP was observed. Instead, ADP inhibits the enzyme non-allosterically and competitively to the substrate MgATP. Phosphate stimulates the catalytic activity of malarial PFK independently of the activation by F6P and PEP.

Phosphofructokinase from Plasmodium berghei. Influence of Mg2+, ATP and Mg2(+)-complexed ATP

The control enzyme phosphofructokinase is of regulatory significance in the metabolism of glucose by the malarial parasite Plasmodium berghei. (1) The enzyme was partially purified from erythrocytic stages of P. berghei by precipitation with poly(ethylene glycol) and chromatography on 2',5'-bisphosphoadenosine-Sepharose 4B. (2) Similarly to various other phosphofructokinases, the enzyme from P. berghei shows an allosteric behaviour. It is activated by fructose 6-phosphate and inhibited by ATP. (3) The effects of Mg2(+)-complexed ATP, free ATP and Mg2+ were studied by keeping constant the concentration of one of these and varying the concentrations of the other two. (4) The enzyme is shown to be allosterically inhibited by free ATP and by higher concentrations of Mg2+. Compared with phosphofructokinase of erythrocytes, inhibition by ATP is weaker by two orders of magnitude. Mg2(+)-complexed ATP has no effect on allosteric regulation. (5) The proposed kinetic model provides an adequate description of the data.

Cellular concentrations of enzymes and their substrates

The activity of crude and pure enzyme preparations as well as the molecular weight of these enzymes were obtained from the literature for several organisms. From these data enzyme concentrations were calculated and compared to the concentration(s) of their substrates in the same organism. The data are expressed as molar ratios of metabolite concentration to enzyme site concentration. Of the 140 ratios calculated, 88% were one or greater, indicating that in general substrates exceed their cognate enzyme concentrations. Of the 17 cases where enzyme exceeds metabolite concentration, 16 were in glycolysis. The data in general justify the use of enzyme kinetic mechanisms determined in vitro in the construction of dynamic models which simulate in vivo metabolism.

Active-site mutants altering the cooperativity of E. coli phosphofructokinase

Crystal structures of the high- and low-activity states of the allosteric enzyme phosphofructokinase implicate three arginines in substrate binding, catalysis and cooperativity. Arginines 162 and 243 reach into the active site from an adjacent subunit and interact with the cooperative substrate fructose 6-phosphate. Mutation of these arginines to serine results in mutant enzymes with reduced substrate binding and lowered cooperativity, but with little change in their catalytic ability (kcat). Arg 72 bridges the two substrates fructose 6-phosphate and ATP, and interacts with the 1-phosphate of the product fructose 1,6-biphosphate. Mutation of this residue to serine reduces the catalytic activity, cooperativity and binding of fructose 6-phosphate and fructose 1,6-bisphosphate. In the reverse reaction, the kinetics of wild-type and the Ser 72 mutant with respect to fructose 1,6-bisphosphate are hyperbolic, whereas those of the Ser 162 and Ser 243 mutants are sigmoidal. These results show that each of the three arginines contributes to cooperativity and to the transmission of allosteric signals between the four subunit of the enzyme.

Dissection of the effector-binding site and complementation studies of Escherichia coli phosphofructokinase using site-directed mutagenesis

A systematic study by site-directed mutagenesis has been conducted on the effector site of phosphofructokinase from Escherichia coli to delineate the role of side chains in binding the allosteric activator, GDP, and inhibitor, PEP, and to search for key residues in the allosteric transtion. Target residues were identified from the crystal structure of the enzyme-nucleoside diphosphate complex. It is found that both activator and inhibitor bind to the same set of amino acid side chains. Deletion of positively charged groups (Arg21, Arg25, Arg54, Arg154, and Lys213 mutated to alanine) weakens binding of both effectors by 2-3 kcal/mol, consistent with the disruption of charged hydrogen bonds. Residue Glu187, which is known from the crystal structure to bind the coordinated Mg2+ ion of GDP, is found to have a unique behavior on mutation and appears to be crucial in triggering the allosteric transition. All other residues mutated simply weaken binding of both PEP and GDP in a parallel manner. However, mutation of Glu----Ala187 reverses the roles of GDP and PEP, causing GDP to become an allosteric inhibitor and PEP an activator. Mutation of Glu----Gln187 has only a small effect on the binding of PEP, and both PEP and GDP are inhibitors. Studies are described in which mutations in different subunits of a tetrameric complex complement each other. The effector site is composed of residues from two subunits. In particular, Arg21 and Lys213 in each site are from different subunits. Mutations of either one of these residues abolishes activation by GDP of the homotetramer.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of cooperativity and allosteric regulation in proteins

AUosteric proteins control and coordinate chemical events in the living cell. When Monod conceived that idea he said that he had discovered the second secret of life. The first was the structure of DNA. The theory as published by Monodet al.(1963) was concerned chiefly with cooperativity and feedback inhibition of enzymes, such as the inhibition of threonine deaminase, the first enzyme in the pathway of the synthesis of isoleucine, by isoleucine, and its activation by valine. Two years later the theory was formalized by Monodet al.(1965).

Ordered disruption of subunit interfaces during the stepwise reversible dissociation of Escherichia coli phosphofructokinase with KSCN

The reversible inactivation and dissociation of the allosteric phosphofructokinase from Escherichia coli has been studied in relatively mild conditions, i.e., in the presence of the chaotropic agent KSCN. At moderate KSCN concentration, the loss of enzymatic activity involves two separated phases: first, a rapid dissociation of part of the tetramer into dimers, second, a slower displacement of the dimer-tetramer equilibrium upon further dissociation of the dimer into monomers. These two reactions can no longer be distinguished above 0.3 M KSCN since complete inactivation occurs in a single reaction. Different changes are observed for the fluorescence and the activity of the enzyme in KSCN: the fluorescence is not affected by the dissociation into dimers which is responsible for inactivation. The decrease in fluorescence reflects the change in environment of the unique tryptophan residue, Trp 311, during the dimer to monomer dissociation. This residue belongs to the interface containing the regulatory site, and its native fluorescence indicates that this interface is still present in the dimer. The substrate fructose 6-phosphate protects phosphofructokinase from inactivation by binding to the tetramer and prevents its dissociation into dimers. The presence of phosphoenolpyruvate prevents the slow dissociation of the dimer into monomers, which shows the ability of the dimer to bind the inhibitor. Two successive processes can be observed during reassociation of the protein upon KSCN dilution. First, a fast reaction (k1 = 2 x 10(5) M-1.s-1) is accompanied by a fluorescence increase and results in the formation of the dimeric species.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystal structure of the complex of phosphofructokinase from Escherichia coli with its reaction products

The crystal structure of Escherichia coli phosphofructokinase complexed with its reaction products fructose 1,6-bisphosphate (Fru1,6P) and ADP/Mg2+, and the allosteric activator ADP/Mg2+, has been determined at 2.4 A resolution. The structure was solved by molecular replacement using the known structure of Bacillus stearothermophilus phosphofructokinase, and has been refined to a crystallographic R-factor of 0.165 for all data. The crystallization mixture contained the substrate fructose 6-phosphate, but the electron density maps showed clearly the presence of the product fructose 1,6-bisphosphate, presumably formed by the enzyme reaction with contaminating ATP. The crystal consists of tetrameric molecules with subunits in two different conformations despite their chemical identity. The magnesium ion in the "closed" subunit bridges the phosphate groups of the two products. In the "open" subunit, the products are about 1.5 A further apart, with the Mg2+ bound only to ADP. These two conformations probably represent two successive stages along the reaction pathway, in which the closure of the subunit is required to bring the substrates sufficiently close to react. This conformational change within the subunit is distinct from the quaternary structure change seen previously in the inactive T-state conformation. It is probably not involved in the co-operativity or allosteric control of the enzyme, since the co-operative product fructose 1,6-bisphosphate is not moved, nor are the subunit interfaces changed. The structure of the enzyme is similar to that of B. stearothermophilus phosphofructokinase, and confirms the location of the sites for the two reaction products (or substrates), and of the effector site binding the activator ADP/Mg2+. However, this structure gives a clearer picture of the active site, and of the interactions between the enzyme and its reaction products.

Reactivity of the thiol groups of Escherichia coli phosphofructo-1-kinase

Modification of Escherichia coli phosphofructo-1-kinase (6-phosphofructokinase; EC 2.7.1.11) with several thiol modifying reagents led to a pseudo-first-order loss of activity that was associated with the modification of a single cysteine residue, identified as the cysteine at position 119 in the protein sequence. This cysteine was protected from reaction with vinyl pyridine, bromopyruvate, and dithionitrobenzoic acid by the substrate, fructose-6-P. In the crystal structure of the highly homologous phosphofructokinase from Bacillus stearothermophilus, cysteine 119 is sufficiently distant from the catalytic site to exclude a direct steric inhibition of the binding of substrate as a mechanism of inactivation for the modification. Thus, the inhibition is unlikely to be a direct one but to be the result of interference with the conformational change that is associated with fructose-6-P binding. A second thiol, position 283, was shown to be protected from reaction when the enzyme was in its native conformation. In contrast to the previously published sequence for the E. coli enzyme six cysteines as opposed to seven have been found both in enzyme from strain LE392 and in enzyme produced by a plasmid that was derived from pLC 16-4. The position in question, 75, was identified as phenylalanine.

Inorganic pyrophosphate: fructose-6-phosphate 1-phosphotransferase of the potato tuber is related to the major ATP-dependent phosphofructokinase of E. coli

A procedure was developed for the purification of inorganic pyrophosphate: fructose-6-phosphate 1-phospho-transferase (PPi-PFK) from potato tubers. The enzyme has the structure alpha 4 beta 4 with a subunit of 68 kDa and a beta subunit of 60 kDa. The structural relationship of this enzyme to other PFKs and to fructose bisphosphatase was examined by immunoprecipitation and immunoblotting. Antibodies to the plant enzyme did not react with E. coli PFK. No cross-reaction was seen among the following enzymes or their antibodies: yeast fructose bisphosphatase; rabbit PFKs A, B, or the enzyme from brain; and the two subunits of the potato PPi-PFK. On the other hand, antibody to E. coli PFK-1 strongly cross-reacts with the 60 kDa polypeptide but not 68 kDa peptide.

Conversion of allosteric inhibition to activation in phosphofructokinase by protein engineering

Many enzymes are subject to allosteric control, often with inhibitors and activators binding to the same effector site. Phosphofructokinase in Escherichia coli is such an enzyme, being inhibited by phosphoenolpyruvate (PEP) and activated by ADP and GDP. How do individual interactions with effectors affect the balance between activation and inhibition, especially when both ligands share aspects of the same binding site? We find that mutation of a single residue in the effector site, Glu----Ala 187, leads to PEP being an activator rather than an inhibitor. With low concentrations of the substrate fructose-6-phosphate, the mutant enzyme is more than one hundred times more active than wild-type enzyme at millimolar concentrations of PEP. The classical Monod-Wyman-Changeux two-state model is too simple to account for the properties of the mutant enzyme.

High-level expression of Bacillus stearothermophilus 6-phosphofructo-1-kinase in Escherichia coli

The 6-phosphofructo-1-kinase (PFK) gene from Bacillus stearothermophilus has been expressed at high levels in Escherichia coli. This expression has been demonstrated by complementation studies, sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and PFK assays of cell extracts. A level of B. stearothermophilus PFK expression corresponding to 20% of the total extracted protein was calculated from densitometric scans of an SDS-polyacrylamide gel. The high level of recombinant gene expression will enable this laboratory to determine structure-function relationships in B. stearothermophilus PFK by the method of oligodeoxynucleotide-directed mutagenesis.

Adipose-tissue phosphofructokinase. Rapid purification and regulation by phosphorylation in vitro

A new procedure for the purification of phosphofructokinase using Blue Dextran-Sepharose is described. This allowed an approx. 1000-fold purification of phosphofructokinase from rat white and brown adipose tissue to be achieved in essentially a single step. The purified enzymes from both tissues were found to exhibit hyperbolic kinetics with fructose 6-phosphate, to be inhibited by ATP and citrate, and to be activated by 5'-AMP, phosphate and fructose 2,6-bisphosphate. The enzymes were phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase, and phosphorylation was found to be associated with increases in activity when the enzymes were assayed under appropriate sub-optimal conditions. In particular, the phosphorylated enzymes exhibited less inhibition by ATP and the white-adipose-tissue enzyme was more sensitive to activation by fructose 2,6-bisphosphate. It is suggested that an increase in the cytoplasmic concentration of cyclic AMP in tissues other than liver may result in an increase in glycolysis through the phosphorylation of phosphofructokinase by cyclic AMP-dependent protein kinase.

Nucleotide sequence and high-level expression of the major Escherichia coli phosphofructokinase

The gene for the major phosphofructokinase enzyme in Escherichia coli, pfkA, has been sequenced. Comparison of the amino acid sequence with other phosphofructokinases showed that this enzyme is related to the Bacillus stearothermophilus and rabbit muscle enzymes, but is different from the second, minor phosphofructokinase found in E. coli. The region which has been sequenced comprises the complete pfkA--tpi interval on the E. coli genetic map. Two other genes have been identified from the nucleotide sequence: a gene for a periplasmic sulphate-binding protein, sbp, and for a membrane-bound enzyme, CDP-diglyceride hydrolase, cdh. This establishes the complete gene arrangement in this region as pfkA-sbp-cdh-tpi. The pfkA gene has been subcloned into a high-copy-number plasmid under the control of a strong, chimaeric promoter which arose as an artefact in the construction of the plasmid gene bank from which the original pfkA recombinant was isolated. A specialised recombinant has been constructed which carries a 1.4 X 10(3)-nucleotide insert containing just the pfkA gene flanked by two HindIII recognition sites providing a simple system for the recloning of this gene into different vectors. This recombinant expresses the enzyme at high levels (40-50% of total cell protein is active, soluble phosphofructokinase). This expression system is now being used to study the enzyme using 'reverse genetics'.