Major changes in the kinetic mechanism of AMP inhibition and AMP cooperativity attend the mutation of Arg49 in fructose-1,6-bisphosphatase

Central cavity of fructose-1,6-bisphosphatase and the evolution of AMP/fructose 2,6-bisphosphate synergism in eukaryotic organisms

The effects of AMP and fructose 2,6-bisphosphate (Fru-2,6-P2) on porcine fructose-1,6-bisphosphatase (pFBPase) and Escherichia coli FBPase (eFBPase) differ in three respects. AMP/Fru-2,6-P2 synergism in pFBPase is absent in eFBPase. Fru-2,6-P2 induces a 13° subunit pair rotation in pFBPase but no rotation in eFBPase. Hydrophilic side chains in eFBPase occupy what otherwise would be a central aqueous cavity observed in pFBPase. Explored here is the linkage of AMP/Fru-2,6-P2 synergism to the central cavity and the evolution of synergism in FBPases. The single mutation Ser(45) → His substantially fills the central cavity of pFBPase, and the triple mutation Ser(45) → His, Thr(46) → Arg, and Leu(186) → Tyr replaces porcine with E. coli type side chains. Both single and triple mutations significantly reduce synergism while retaining other wild-type kinetic properties. Similar to the effect of Fru-2,6-P2 on eFBPase, the triple mutant of pFBPase with bound Fru-2,6-P2 exhibits only a 2° subunit pair rotation as opposed to the 13° rotation exhibited by the Fru-2,6-P2 complex of wild-type pFBPase. The side chain at position 45 is small in all available eukaryotic FBPases but large and hydrophilic in bacterial FBPases, similar to eFBPase. Sequence information indicates the likelihood of synergism in the FBPase from Leptospira interrogans (lFBPase), and indeed recombinant lFBPase exhibits AMP/Fru-2,6-P2 synergism. Unexpectedly, however, AMP also enhances Fru-6-P binding to lFBPase. Taken together, these observations suggest the evolution of AMP/Fru-2,6-P2 synergism in eukaryotic FBPases from an ancestral FBPase having a central aqueous cavity and exhibiting synergistic feedback inhibition by AMP and Fru-6-P.

Determination of enzyme activity inhibition by FTIR spectroscopy on the example of fructose bisphosphatase

A mid-infrared enzymatic assay for label-free monitoring of the enzymatic reaction of fructose-1,6-bisphosphatase with fructose 1,6-bisphosphate has been proposed. The whole procedure was done in an automated way operating in the stopped flow mode by incorporating a temperature-controlled flow cell in a sequential injection manifold. Fourier transform infrared difference spectra were evaluated for kinetic parameters, like the Michaelis-Menten constant (K(M)) of the enzyme and Vmax of the reaction. The obtained K(M) of the reaction was 14 +/- 3 g L(-1) (41 microM). Furthermore, inhibition by adenosine 5'-monophosphate (AMP) was evaluated, and the K(M)(App) value was determined to be 12 +/- 2 g L(-1) (35 microM) for 7.5 and 15 microM AMP, respectively, with Vmax decreasing from 0.1 +/- 0.03 to 0.05 +/- 0.01 g L(-1) min(-1). Therefore, AMP exerted a non-competitive inhibition.

Inhibition of fructose-1,6-bisphosphatase by a new class of allosteric effectors

A highly constrained pseudo-tetrapeptide (OC252-324) further defines a new allosteric binding site located near the center of fructose-1,6-bisphosphatase. In a crystal structure, pairs of inhibitory molecules bind to opposite faces of the enzyme tetramer. Each ligand molecule is in contact with three of four subunits of the tetramer, hydrogen bonding with the side chain of Asp187 and the backbone carbonyl of residue 71, and electrostatically interacting with the backbone carbonyl of residue 51. The ligated complex adopts a quaternary structure between the canonical R- and T-states of fructose-1,6-bisphosphatase, and yet a dynamic loop essential for catalysis (residues 52-72) is in a conformation identical to that of the T-state enzyme. Inhibition by the pseudo-tetrapeptide is cooperative (Hill coefficient of 2), synergistic with both AMP and fructose 2,6-bisphosphate, noncompetitive with respect to Mg2+, and uncompetitive with respect to fructose 1,6-bisphosphate. The ligand dramatically lowers the concentration at which substrate inhibition dominates the kinetics of fructose-1,6-bisphosphatase. Elevated substrate concentrations employed in kinetic screens may have facilitated the discovery of this uncompetitive inhibitor. Moreover, the inhibitor could mimic an unknown natural effector of fructose-1,6-bisphosphatase, as it interacts strongly with a conserved residue of undetermined functional significance.

Purification and characterization of a recombinant pea cytoplasmic fructose-1,6-bisphosphatase

Full-length cDNA encoding pea cytoplasmic fructose-1,6-bisphosphatase (cyFBPase) was cloned from a pea cDNA library. The cloned cDNA was introduced into the Escherichia coli expression vector pET-15b. The recombinant cyFBPase was expressed in E. coli BL21 (DE3) cells in a soluble form and purified to homogeneity by Ni(+)-NTA affinity chromatography. The identity of the recombinant cyFBPase was confirmed by SDS-PAGE and immunoblot analysis using a polyclonal anti-His tag antibody. The recombinant cyFBPase was active at neutral pH ranges (6.6-9.0) and thermostable as other cyFBPases. The activation energy (E(a)) and Arrhenius frequency factor were 17.4 kcal/mol and 2.6 x 10(12)/s, respectively. The K(M) and V(max) values of the recombinant enzyme were calculated as 10.47 microM and 109 micromol/min, respectively. In case of removal of histidine tag, the K(M) value was calculated as 5.03 microM. The recombinant enzyme was non-competitively and competitively inhibited by AMP and fructose-2,6-bisphosphate, respectively.

Hybrid tetramers of porcine liver fructose-1,6-bisphosphatase reveal multiple pathways of allosteric inhibition

Fructose-1,6-bisphosphatase is a square planar tetramer of identical subunits, which exhibits cooperative allosteric inhibition of catalysis by AMP. Protocols for in vitro subunit exchange provide three of five possible hybrid tetramers of fructose-1,6-bisphosphatase in high purity. The two hybrid types with different subunits in the top and bottom halves of the tetramer co-purify. Hybrid tetramers, formed from subunits unable to bind AMP and subunits with wild-type properties, differ from the wild-type enzyme only in regard to their properties of AMP inhibition. Hybrid tetramers exhibit cooperative, potent, and complete (100%) AMP inhibition if at least one functional AMP binding site exists in the top and bottom halves of the tetramer. Furthermore, titrations of hybrid tetramers with AMP, monitored by a tryptophan reporter group, reveal cooperativity and fluorescence changes consistent with an R- to T-state transition, provided that again at least one functional AMP site exists in the top and bottom halves of the tetramer. In contrast, hybrid tetramers, which have functional AMP binding sites in only one half (top/bottom), exhibit an R- to T-state transition and complete AMP inhibition, but without cooperativity. Evidently, two pathways of allosteric inhibition of fructose-1,6-bisphosphatase are possible, only one of which is cooperative.

Mutations in the hinge of a dynamic loop broadly influence functional properties of fructose-1,6-bisphosphatase

Loop 52-72 of porcine fructose-1,6-bisphosphatase may play a central role in the mechanism of catalysis and allosteric inhibition by AMP. The loop pivots between different conformational states about a hinge located at residues 50 and 51. The insertion of proline separately at positions 50 and 51 reduces k(cat) by up to 3-fold, with no effect on the K(m) for fructose 1,6-bisphosphate. The K(a) for Mg(2+) in the Lys(50) --> Pro mutant increases approximately 15-fold, whereas that for the Ala(51) --> Pro mutant is unchanged. Although these mutants retain wild-type binding affinity for AMP and the fluorescent AMP analog 2'(3')-O-(trinitrophenyl)adenosine 5'-monophosphate, the K(i) for AMP increases 8000- and 280-fold in the position 50 and 51 mutants, respectively. In fact, the mutation Lys(50) --> Pro changes the mechanism of AMP inhibition with respect to Mg(2+) from competitive to noncompetitive and abolishes K(+) activation. The K(i) for fructose 2,6-bisphosphate increases approximately 20- and 30-fold in the Lys(50) --> Pro and Ala(51) --> Pro mutants, respectively. Fluorescence from a tryptophan introduced by the mutation of Tyr(57) suggests an altered conformational state for Loop 52-72 due to the proline at position 50. Evidently, the Pro(50) mutant binds AMP with high affinity at the allosteric site, but the mechanism of allosteric regulation of catalysis has been disabled.

The C1-C2 interface residue lysine 50 of pig kidney fructose-1, 6-bisphosphatase has a crucial role in the cooperative signal transmission of the AMP inhibition

To understand the mechanism of signal propagation involved in the cooperative AMP inhibition of the homotetrameric enzyme pig-kidney fructose-1,6-bisphosphatase, Arg49 and Lys50 residues located at the C1-C2 interface of this enzyme were replaced using site-directed mutagenesis. The mutant enzymes Lys50Ala, Lys50Gln, Arg49Ala and Arg49Gln were expressed in Escherichia coli, purified to homogeneity and the initial rate kinetics were compared with the wild-type recombinant enzyme. The mutants exhibited kcat, Km and I50 values for fructose-2,6-bisphosphate that were similar to those of the wild-type enzyme. The kinetic mechanism of AMP inhibition with respect to Mg2+ was changed from competitive (wild-type) to noncompetitive in the mutant enzymes. The Lys50Ala and Lys50Gln mutants showed a biphasic behavior towards AMP, with total loss of cooperativity. In addition, in these mutants the mechanism of AMP inhibition with respect to fructose-1,6-bisphosphate changed from noncompetitive (wild-type) to uncompetitive. In contrast, AMP inhibition was strongly altered in Arg49Ala and Arg49Gln enzymes; the mutants had > 1000-fold lower AMP affinity relative to the wild-type enzyme and exhibited no AMP cooperativity. These studies strongly indicate that the C1-C2 interface is critical for propagation of the cooperative signal between the AMP sites on the different subunits and also in the mechanism of allosteric inhibition of the enzyme by AMP.

Suppression of kinetic AMP cooperativity of fructose-1,6-bisphosphatase by carbamoylation of lysine 50

Selective treatment of pig kidney fructose 1,6-bisphosphatase with cyanate leads to the formation of an active carbamoylated derivative that shows no cooperative interaction between the AMP-binding sites, but completely retains the sensitivity to the inhibitor. By an exhaustive carbamoylation of the enzyme a derivative is formed that has a complete loss of cooperativity and a decrease of sensitivity to AMP. It was proposed that the observed changes of allosteric properties were due to the chemical modification of two lysine residues per enzyme subunit [Slebe et al. (1983), J. Protein Chem. 2, 437-443]. Studies of the temperature dependence of AMP sensitivity and the interaction with Cibacron Blue Sepharose of carbamoylated fructose 1,6-bisphosphatase derivatives indicate that the lysine residue involved in AMP sensitivity is located at the allosteric AMP site, while the lysine residue involved in AMP cooperativity is at a distinct location. Using [14C]cyanate, we identified both lysine residues in the primary structure of the enzyme; Lys50 is essential for AMP cooperativity and Lys112 appears to be the reactive residue involved in the AMP sensitivity. According to the fructose 1,6-bisphosphatase crystal structure, Lys50 is strategically positioned at the C1-C2 interface, near the molecular center of the tetramer, and Lys112 is in the AMP-binding site. The results reported here, combined with the structural data of the enzyme, strongly suggest that the C1-C2 interface is critical for the propagation of the allosteric signal among the AMP sites on different subunits.

Directed mutations in the poorly defined region of porcine liver fructose-1,6-bisphosphatase significantly affect catalysis and the mechanism of AMP inhibition

Asn64, Asp68, Lys71, Lys72, and Asp74 of porcine liver fructose-1, 6-bisphosphatase (FBPase) are conserved residues and part of a loop for which no electron density has been observed in crystal structures. Yet mutations of the above dramatically affect catalytic rates and/or AMP inhibition. The Asp74 --> Ala and Asp74 --> Asn mutant enzymes exhibited 50,000- and 2,000-fold reductions, respectively, in kcat relative to wild-type FBPase. The pH optimum for the catalytic activity of the Asp74 --> Glu, Asp68 --> Glu, Asn64 --> Gln, and Asn64 --> Ala mutant enzymes shifted from pH 7.0 (wild-type enzyme) to pH 8.5, whereas the Lys71 --> Ala mutant and Lys71,72 --> Met double mutant had optimum activity at pH 7.5. Mg2+ cooperativity, Km for fructose 1,6-bisphosphate, and Ki for fructose 2,6-bisphosphate were comparable for the mutant and wild-type enzymes. Nevertheless, for the Asp74 --> Glu, Asp68 --> Glu, Asn64 --> Gln, and Asn64 --> Ala mutants, the binding affinity for Mg2+ decreased by 40-125-fold relative to the wild-type enzyme. In addition, the Asp74 --> Glu and Asn64 --> Ala mutants exhibited no AMP cooperativity, and the kinetic mechanism of AMP inhibition with respect to Mg2+ was changed from competitive to noncompetitive. The double mutation Lys71,72 --> Met increased Ki for AMP by 175-fold and increased Mg2+ affinity by 2-fold relative to wild-type FBPase. The results reported here strongly suggest that loop 51-72 is important for catalytic activity and the mechanism of allosteric inhibition of FBPase by AMP.

Uncompetitive inhibition by adenine of the RNA-N-glycosidase activity of ribosome-inactivating proteins

Ricin is a member of the ribosome-inactivating protein (RIP) family with RNA-N-glycosidase activity which inactivates eukaryotic ribosomes by specifically removing adenine from the first adenosine of a highly conserved GAGA loop present in 28S rRNA. Free adenine protects ribosomes in cell-free systems from inactivation by ricin. Protection by adenine is highly specific, since AMP, adenosine and modified adenines (1-methyladenine and ethenoadenine) were completely ineffective. Kinetic analysis of the behaviour of adenine as inhibitor of the RNA-N-glycosidase reaction catalysed by ricin, Shiga-like toxin I and momordin, two other members of the RIP family, established that inhibition was of the uncompetitive type, the inhibitor binding to the enzyme-substrate complex. Adenine did not protect ribosomes from alpha-sarcin, an RNAase that inactivates ribosomes by cleaving the phosphodiester bond located in the GAGA loop at one nucleotide distance from the adenosine depurinated by the RNA-N-glycosidases. Adenine at the concentration of 1 mM lowered 1.5-fold the toxicity of ricin and 3.7-fold that of Shiga-like toxin I on Vero cells in culture. The same concentration of adenine decreased 2.4-fold the inactivation of isolated ribosomes by ricin, 2.8-fold the inactivation by Shiga-like toxin I and 20-fold that by momordin.