The stereochemical course of phosphoryl transfer catalysed by Bacillus stearothermophilus and rabbit skeletal-muscle phosphofructokinase with a chiral [16O,17O,18O]phosphate ester

Oxygen chiral phosphate esters

During the past four years, methods have been reported for the syntheses and configurational analyses of virtually any phosphate mono- and diester chiral by virtue of oxygen isotope substitution, and these techniques have already been applied to an impressive number of enzymic and chemical reactions. At present, no experimental information is available that contradicts the simplest interpretations that have been applied to the results obtained for enzymic reactions: inversion of configuration indicating a direct displacement of the leaving group by the attacking group, and retention of configuration implying the formation of a phosphorylated or nucleotidylated enzyme intermediate. However, it does seem necessary to further investigate the mechanisms of at least some of the reactions discussed in this review to ensure that the simplest interpretation is correct. For example, the caveat we have raised about the interpretation of inversions of configurations in phosphohydrolase reactions is chemically reasonable, and these reactions should be reexamined to evaluate the importance of covalent catalysis by carboxylate groups. However, for the vast majority of the enzymic reactions that have been investigated, the stereochemical approach to ascertaining whether catalysis involves the formation of covalent intermediates remains the simplest and most direct method.

Acid-base catalytic mechanism and pH dependence of fructose 2,6-bisphosphate activation of the Ascaris suum phosphofructokinase

A form of phosphofructokinase (PFK) from Ascaris suum desensitized to hysteresis in the reaction time course and ATP allosteric inhibition has been used to study the activation by fructose 2,6-bisphosphate (F26P2) at varied pH in both reaction directions. In the direction of phosphorylation of F6P, V and V/KMgATP are constant over the pH range 6-9, while V/KF6P decreases at low pH, giving a pK value of 7.0, and at high pH, giving a pK of 8.9. V and V/KMgATP are insensitive to the presence of F26P2, but V/KF6P is increased by a constant amount in the presence of saturating F26P2 over the entire pH range studied. The concentration of F26P2 that gives half the change in V/KF6P, Kact, increases as the pH decreases, giving a pK of 7.4, reflecting an enzyme group that must be unprotonated for optimum binding of F26P2. In the direction of phosphorylation of MgADP, V and V/KMgADP are pH-independent, and both are insensitive to the presence of F26P2. V/KFBP decreases at high pH, giving a pK of about 7.3, and is increased by a constant amount in the presence of F26P2 over the entire pH range studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Slow ligand-induced transitions in the allosteric phosphofructokinase from Escherichia coli

The fluorescence of the unique tryptophan residue of the allosteric phosphofructokinase from Escherichia coli varies upon binding of any ligand, whether substrate or effector, suggesting that the protein undergoes a conformational change. This fluorescent probe has been exploited to determine the rates of the structural transitions that occur upon ligand binding and that are responsible for the remarkable allosteric behavior of this enzyme. The kinetics of fluorescence changes measured after rapidly mixing phosphofructokinase with one of its ligands show the presence of several allosteric transitions with widely different rates, ranging from a few hundred s-1 to less than 0.1 s-1. The rate of each conformational change increases with the concentration of the ligand used to trigger it, suggesting that ligands induce a conformational change and do not displace a pre-existing equilibrium. The hypothesis that each ligand stabilizes a different conformational state of the protein is confirmed by the kinetics of displacement of one ligand by another: for instance, the binary complexes between phosphofructokinase and either its substrate, fructose-6-phosphate, or its allosteric activator, ADP, have the same low fluorescence and should be in the same active state, but they show different rates of conformational transition upon binding the inhibitor phosphoenolpyruvate. It appears that phosphofructokinase can exist in more than two states. Some conformational changes between these multiple states are slow enough to play an important role in the kinetics of the reaction catalyzed by phosphofructokinase, and could even explain part of its allosteric behavior. These results show that steady-state measurements are not sufficient to analyze the regulatory properties of E. coli phosphofructokinase.

Interaction between the carboxyl groups of Asp127 and Asp129 in the active site of Escherichia coli phosphofructokinase

The pH dependence of the enzymic properties of the phosphofructokinase from Escherichia coli was compared to those of two mutants in which one carboxyl group of the active site has been removed from either Asp127 or Asp129. All measurements of activity were made in the presence of allosteric activator ADP or GDP to eliminate any cooperative process. Asp129 is a crucial residue for the activity of phosphofructokinase since its conversion to Ser decreases the catalytic activity by 2-3 orders of magnitude in both the forward and reverse reactions, but the ionization of Asp129 is not directly related the pH dependence of phosphofructokinase activity. This pH dependence is however modified by the Asp129----Ser mutation, which decreases the pK of another residue, Asp127, by as much as pH of 1.5. The side chain of Asp127 has the catalytic role proposed earlier: its deprotonated form acts as a base in the forward reaction, and its protonated form acts as an acid in the reverse reaction. The protonated form of Asp127 is also required for the binding of fructose 1,6-bisphosphate. The electrostatic interaction between the carboxyl groups of Asp127 and Asp129 seems different in free phosphofructokinase to that in enzyme/substrate complexes, suggesting that a conformational change occurs upon substrate binding. The pH dependence of phosphofructokinase activity involves one other ionizable group with a pK of approximately 6 which does not belong to the side chains of Asp127 or Asp129.

pH dependence of the reverse reaction catalyzed by phosphofructokinase I from Escherichia coli: implications for the role of Asp 127

The kinetics of the reverse reaction catalyzed by Escherichia coli phosphofructokinase, i.e., the synthesis of ATP and fructose-6-phosphate from ADP and fructose-1,6-bisphosphate, have been studied at different pH values, from pH 6 to pH 9.2. Hyperbolic saturations of the enzyme are observed for both substrates. The affinity for fructose-1,6-bisphosphate decreases with pH following the ionization of a group with a pK of 6.6, whereas the catalytic rate constant and perhaps the affinity for ADP are controlled by the ionization of a group with a pK of 6. Several arguments show that the pK of 6.6 is probably that of the carboxyl group of Asp 127, whereas the pK of 6 is tentatively attributed to the carboxyl group of Asp 103. The pK of 6.6 is assigned to the carboxyl group of Asp 127 in the free enzyme, and a simple model suggests that the same group would have an abnormally high pK, above 9.6, in the complex between phosphofructokinase and fructose-1,6-bisphosphate. It is proposed that the large pK shift of more than 3 pH units upon binding of fructose-1,6-bisphosphate is due to an electrostatic repulsion that could exist between the 1-phosphate group and the carboxyl group of Asp 127, which are close to each other in the crystal structure of phosphofructokinase (Shirakihara, Y. & Evans, P.R., 1988, J. Mol. Biol. 204, 973-994). The same interpretation would also explain the much higher affinity of the enzyme for fructose-1,6-bisphosphate when Asp 127 is protonated.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and its kinase domain in Escherichia coli

The rat liver bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (ATP:D-fructose-6-phosphate 2-phosphotransferase/D-fructose-2,6-bisphosphate 2-phosphohydrolase, EC 2.7.1.105/EC 3.1.3.46) and its separate kinase domain were expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The bifunctional enzyme (470 residues per subunit) was efficiently expressed as a protein that starts with the initiator methionine residue and ends at the carboxyl-terminal tyrosine residue. The expressed protein was purified to homogeneity by anion exchange and Blue Sepharose chromatography and had kinetic and physical properties similar to the purified rat liver enzyme, including its behavior as a dimer during gel filtration, activation of the kinase by phosphate and inhibition by alpha-glycerol phosphate, and mediation of the bisphosphatase reaction by a phosphoenzyme intermediate. The expressed 6-phosphofructo-2-kinase also started with the initiator methionine but ended at residue 257. The partially purified kinase domain was catalytically active, had reduced affinities for ATP and fructose 6-phosphate compared with the kinase of the bifunctional enzyme, and had no fructose-2,6-bisphosphatase activity. The kinase domain also behaved as an oligomeric protein during gel filtration. The expression of an active kinase domain and the previous demonstration of an actively expressed bisphosphatase domain provide strong support for the hypothesis that the hepatic enzyme consists of two independent catalytic domains encoded by a fused gene.

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).

Mutations in the active site of Escherichia coli phosphofructokinase

The enzyme-catalysed transfer of a phosphoryl group from ATP is an important reaction in a wide variety of biological processes. We demonstrate here the essential function of an aspartate group in the catalysis of phosphoryl transfer by Escherichia coli phosphofructokinase, and the minor role of an arginine residue. We have used oligonucleotide-directed mutagenesis to replace two amino-acid residues which X-ray analysis has shown to be close to the transferred phosphoryl group and we have analysed the forward and back reactions of the mutant enzymes by steady-state kinetics. Changing Asp 127 to Ser reduced the turnover number by a factor of 18,000 in the forward direction and 3,100 in the back reaction, and the Michaelis constant for fructose 1,6-bisphosphate in the reverse reaction by a factor of 45. This shows that this aspartate is a key residue in the rate enhancement by the enzyme, probably acting as a base in the reaction mechanism, and that it also destabilizes the product complex. Changing Arg 171 to Ser reduced the turnover numbers by about 3.4, showing that this arginine has only a minor effect on the catalysis.

Unambiguous stereochemical course of rabbit liver fructose bisphosphatase hydrolysis

The stereochemical course of rabbit liver fructose bisphosphatase (EC 3.1.3.11) was determined by hydrolyzing the substrate analogue (Sp)-[1-18O]fructose 1-phosphorothioate 6-phosphate in H(2)17O, incorporating the chiral, inorganic phosphorothioate product into adenosine 5'-O-(2-thiotriphosphate) (ATP beta S), and analyzing the isotopic distribution of 18O in ATP beta S by 31P NMR. The result indicates that the 1-phosphoryl group is transferred with inversion of configuration. A series of single-turnover experiments ruled out an acyl phosphate intermediate in the hydrolysis. Consequently, fructose bisphosphatase catalyzes the hydrolysis of fructose 1,6-bisphosphate via a direct transfer of the phosphoryl moiety to water.