Conservation of primary structure at the proteinase-sensitive site of fructose 1,6-bisphosphatases

Glu 69 is essential for the high sensitivity of muscle fructose-1,6-bisphosphatase inhibition by calcium ions

Muscle fructose-1,6-bisphosphatase (FBPase) is highly sensitive toward inhibition by AMP and calcium ions. In allosteric inhibition by AMP, a loop 52-72 plays a decisive role. This loop is a highly conservative region in muscle and liver FBPases. It is feasible that the same region is involved in the inhibition by calcium ions. To test this hypothesis, chemical modification, limited proteolysis and site directed mutagenesis Glu(69)/Gln were employed. The chemical modification of Lys(71-72) and the proteolytic cleavage of the loop resulted in the significant decrease of the muscle FBPase sensitivity toward inhibition by calcium ions. The mutation of Glu(69)-->Gln resulted in a 500-fold increase of muscle isozyme I(0.5) vs. calcium ions. These results demonstrate the key role that the 52-72 amino acid loop plays in determining the sensitivity of FBPase to inhibition by AMP and calcium ions.

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.

AMP activation of snake muscle fructose 1,6-bisphosphatase at alkaline pH

AMP, an allosteric inhibitor at neutral pH, activates snakes muscle fructose 1,6-bisphosphatase at pH 9.2. The activation is virtually unique for the snake muscle enzyme: activation was not observed for the enzymes from either human and rabbit liver or porcine kidney. The activation is Mg(2+)-dependent but was not observed until the concentration of Mg2+ reaches 1 mM. It is known that subtilisin, trypsin, or lysosomal proteases hydrolyse the N-terminal loop of fructose-1,6-bisphosphatase in the vicinity of amino acid residue 60 generating a form of the enzyme with a pH optimum at 9.2. In the presence of AMP, the pH profile of the native snake muscle enzyme resembles that of the alkaline form and modification of the highly reactive sulfhydryl group abolishes AMP activation. The fact that AMP has a dual function at different pH levels suggests that pH might be an important factor in regulating the activity of the enzyme upon binding of AMP at the allosteric site. Indeed, the mode of AMP binding to the allosteric site may differ at neutral and alkaline pH levels. A residue that ionizes with a pKa of 8.9 might be involved in this process.

Conformation of 60-residue peptide fragment from N-terminal of porcine kidney fructose 1,6-bisphosphatase

Limited digestion of fructose 1, 6-bisphosphatase with subtilisin produces an S-peptide with an about 60-residue peptide fragment that is non-covalently associated with the enzyme. The 60-residue peptide fragment consists of the most part of allosteric site for AMP binding. It could be separated from S-protein by gel filtration with a Sephadex G-75 column equilibrated with 9% formic acid. According to X-ray diffraction results the S-peptide consists of two alpha-helices without beta-strand and the alpha-helix content is about 60% in the 60-residue-peptide fragment. When the enzyme is subjected to limited proteolysis with subtilisin, the secondary structure of the enzyme does not show a detectable change in CD spectrum. The CD spectra of the isolated S-peptide were measured under different concentrations. In the absence of GuHCl, S-peptide had 30% alpha-helix and 38.5% turn-like structure but had no beta-strand, suggesting that the N-terminal 60-residue fragment, which is synthesized initially by ribosome, would form a conformation spontaneously similar to that of the isolated 60-residue-peptide, i.e. about 30% alpha-helix and 30% turn-like structure. As the elongation of the peptide chain of the enzyme proceeds, the newly synthesized segment or the final entire enzyme, in turn, affects the conformation of prior peptide segment and adjusts its conformation to the final native state. The content of alpha-helix did not increase as perturbing the conformation of S-peptide by adding ethanol, cyclohexane or a small amount of SDS. On the contrary, the ordered structure was slightly decreased, indicating that the difference of conformations of S-peptide in the isolated form and in the associated protein was not an artifact produced by isolation process.

Kinetic properties of D-fructose-1,6-bisphosphate 1-phosphohydrolase isolated from human muscle

D-Fructose-1,6-bisphosphate 1-phosphohydrolase (EC 3.1.3.11) [Fru(1,6)Pase] was isolated from human muscle in an electrophoretically homogeneous form, free of aldolase contamination. The enzyme is inhibited by the substrate [fructose (1,6)-bisphosphate]. Km is 0.77 microM; Kis is 90 microM. The fructose-2,6-bisphosphate [Fru(2,6)P2], a regulator of gluconeogenesis, inhibits human muscle Fru(1,6)Pase with Ki = 0.13 microM. To determine Km, Kis and Ki the integrated method was used. AMP is an allosteric inhibitor of Fru(1,6)Pase. As with other mammalian isoenzymes, the human muscle enzyme is more strongly inhibited by AMP than is the liver isoenzyme [Dzugaj and Kochman (1980) Biochim. Biophys. Acta 614, 407-412]. Both of the inhibitors [AMP and Fru(2,6)P2] act synergistically on human muscle Fru(1,6)Pase. Ki for Fru(2,6)P2 determined in the presence of 0.4 microM AMP was 0.028 microM. The human muscle enzyme, like other mammalian Fru(1,6)Pases, requires Mg2+ for its activity. The Ka for magnesium was 232 microM, and h (Hill coefficient) = 2.0.

Plant fructose-1,6-bisphosphatases: characteristics and properties

In this minireview the properties and characteristics of plant fructose-1,6-bisphosphatases (D-fructose-1,6-bisphosphatase 1-phosphohydrolase, EC 3.1.3.11) are discussed. The properties and characteristics of the chloroplastic and cytoplasmic forms of the enzyme are reviewed. For purposes of comparison some reference is made to fructose-1,6-bisphosphatases from other species.

Crystal structure of the neutral form of fructose-1,6-bisphosphatase complexed with the product fructose 6-phosphate at 2.1-A resolution

The crystal structure of fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with the product fructose 6-phosphate (F6P) has been refined at 2.1-A resolution to an R factor of 0.177 with root-mean-square deviations of 0.014 A and 2.9 degrees from the ideal geometries of bond lengths and bond angles, respectively. The secondary structures but not the trace of the unligated enzyme have been slightly revised in the F6P complex at this higher resolution. Helix H4 in the unligated structure has been refined to a helix-like coil, and two very short 3(10) helices have been found, one in H4 and one in H5. F6P at 10 mM concentration in the absence of divalent metals in our study shows major binding at the active site and minor binding at the AMP site. The major site has almost equal full occupancy in the C1 and C2 chains of the crystallographic asymmetric unit, while the minor site shows occupancy only in the C1 chain at about 50%. The electron density in both (2Fo - Fc) and (Fo - Fc) maps calculated by omitting F6P slightly favors the beta anomer of D-F6P over the alpha anomer. Possible functions of the active-site residues are discussed, and candidates are suggested for site-directed mutagenesis.

The form II fructose 1,6-bisphosphatase and phosphoribulokinase genes form part of a large operon in Rhodobacter sphaeroides: primary structure and insertional mutagenesis analysis

Fructose 1,6-bisphosphatase (FBPase) and phosphoribulokinase (PRK) are two key enzymes of the reductive pentose phosphate pathway or Calvin cycle of photosynthetic carbon dioxide assimilation. Early studies had indicated that the properties of enzymes isolated from photosynthetic bacteria were clearly distinct from those of enzymes obtained from the chloroplasts of higher plants [for a review, see Tabita (1988)]. The eucaryotic enzymes, which are light activated by the thioredoxin/ferredoxin system (Buchanan, 1980), were each shown to contain a putative regulatory amino acid sequence (Marcus et al., 1988; Porter et al., 1988). The enzymes from photosynthetic bacteria are not controlled by the thioredoxin/ferredoxin system but exhibit complex kinetic properties and, in the case of PRK, there is an absolute requirement of NADH for activity. In the photosynthetic bacterium Rhodobacter sphaeroides, the structural genes of the Calvin cycle, including the genes that encode FBPase (fbp) and PRK (prk), are found in two distinct clusters, and the fbp and prk genes are closely associated in each cluster. In the present investigation, we have determined the nucleotide sequence of the fbpB and prkB genes of the form II cluster and have compared the deduced amino acid sequences to previously determined sequences of light-activated enzymes from higher plants and from other eucaryotic and procaryotic sources. In the case of FBPase, there are several regions that are conserved in the R. sphaeroides enzymes, including a protease-sensitive area located in a region equivalent to residues 51-71 of mammalian FBPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystal structure of fructose-1,6-bisphosphatase complexed with fructose 6-phosphate, AMP, and magnesium

The crystal structure of fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with fructose 6-phosphate, AMP, and Mg2+ has been solved by the molecular replacement method and refined at 2.5-A resolution to a R factor of 0.215, with root-mean-square deviations of 0.013 A and 3.5 degrees for bond lengths and bond angles, respectively. No solvent molecules have been included in the refinement. This structure shows large quaternary and tertiary conformational changes from the structures of the unligated enzyme or its fructose 2,6-bisphosphate complex, but the secondary structures remain essentially the same. Dimer C3-C4 of the enzyme-fructose 6-phosphate-AMP-Mg2+ complex twists about 19 degrees relative to the same dimer of the enzyme-fructose 2,6-bisphosphate complex if their C1-C2 dimers are superimposed on one another. Nevertheless, many interfacial interactions between dimers of C1-C2 and C3-C4 are conserved after quaternary structure changes occur. Residues of the AMP domain (residues 6-200) show large migrations of C alpha atoms relative to barely significant positional changes of the FBP domain (residues 201-335).

Structure refinement of fructose-1,6-bisphosphatase and its fructose 2,6-bisphosphate complex at 2.8 A resolution

The structures of the native fructose-1,6-bisphosphatase (Fru-1,6-Pase), from pig kidney cortex, and its fructose 2,6-bisphosphate (Fru-2,6-P2) complexes have been refined to 2.8 A resolution to R-factors of 0.194 and 0.188, respectively. The root-mean-square deviations from the standard geometry are 0.021 A and 0.016 A for the bond length, and 4.4 degrees and 3.8 degrees for the bond angle. Four sites for Fru-2,6-P2 binding per tetramer have been identified by difference Fourier techniques. The Fru-2,6-P2 site has the shape of an oval cave about 10 A deep, and with other dimensions about 18 A by 12 A. The two Fru-2,6-P2 binding caves of the dimer in the crystallographically asymmetric unit sit next to one another and open in opposite directions. These two binding sites mutually exchange their Arg243 side-chains, indicating the potential for communication between the two sites. The beta, D-fructose 2,6-bisphosphate has been built into the density and refined well. The oxygen atoms of the 6-phosphate group of Fru-2,6-P2 interact with Arg243 from the adjacent monomer and the residues of Lys274, Asn212, Tyr264, Tyr215 and Tyr244 in the same monomer. The sugar ring primarily contacts with the backbone atoms from Gly246 to Met248, as well as the side-chain atoms, Asp121, Glu280 and Lys274. The 2-phosphate group interacts with the side-chain atoms of Ser124 and Lys274. A negatively charged pocket near the 2-phosphate group includes Asp118, Asp121 and Glu280, as well as Glu97 and Glu98. The 2-phosphate group showed a disordered binding perhaps because of the disturbance from the negatively charged pocket. In addition, Asn125 and Lys269 are located within a 5 A radius of Fru-2,6-P2. We argue that Fru-2,6-P2 binds to the active site of the enzyme on the basis of the following observations: (1) the structure similarity between Fru-2,6-P2 and the substrate; (2) sequence conservation of the residues directly interacting with Fru-2,6-P2 or located at the negatively charged pocket; (3) a divalent metal site next to the 2-phosphate group of Fru-2,6-P2; and (4) identification of some active site residues in our structure, e.g. tyrosine and Lys274, consistent with the results of the ultraviolet spectra and the chemical modification. The structures are described in detail including interactions of interchain surfaces, and the chemically modifiable residues are discussed on the basis of the refined structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification of mRNA coding for rat-liver fructose-1,6-bisphosphatase by polysome immunoabsorption

The mRNA coding for rat liver fructose-1,6-bisphosphatase, which represents approx. 0.46% of total hepatic mRNA, has been purified to near homogeneity. Polysomes from rat liver were allowed to react with antibodies to rabbit anti-fructose-1,6-bisphosphatase purified by affinity chromatography. The complex was immobilized on a protein A-Sepharose column. After the removal of unabsorbed polysomes, the specific mRNA was eluted and chromatographed on an oligo(dT)-cellulose column. This method gave a 183-fold enrichment of the fructose-1,6-bisphosphatase mRNA to greater than 80% homogeneity as determined by electrophoreses of immunoprecipitated in vitro translation products on polyacrylamide slab gels in the presence of sodium dodecyl sulphate.

Amino acid sequence homology among fructose-1,6-bisphosphatases

The hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate is a key reaction of carbohydrate metabolism. The enzyme that catalyzes this reaction, fructose-1,6-bisphosphatase, appears to be present in all forms of living organisms. Regulation of the enzyme activity, however, occurs by a variety of distinct mechanisms. These include AMP inhibition (most sources), cyclic AMP-dependent phosphorylation (yeast), and light-dependent activation (chloroplast). In the present studies, we have made a comparison of the primary structure of mammalian fructose-1,6-bisphosphatase with the sequence of peptides isolated from the yeast Saccharomyces cerevisiae, Escherichia coli, and spinach chloroplast enzymes. Our results demonstrate a high degree of sequence homology, suggesting a common evolutionary origin for all fructose-1,6-bisphosphatases.

Limited proteolysis of glucose dehydrogenase from Bacillus megaterium by proteinase K

Glucose dehydrogenase from B. megaterium is subjected to proteolysis with proteinase K. Upon proteolysis the enzyme is inactivated and the polypeptide chain is cleaved into two distinct fragments. These components designated as K-protein and K-peptide have molecular masses of 26 000 and 3 000 Da, respectively. Under native conditions the K-protein and K-peptide remain associated and the tetrameric structure of the proteolytically modified enzyme is preserved. The K-protein and K-peptide were isolated and characterised. The cleavage occurs in the C-terminal region of the polypeptide chain. -Leu Ala decreases Ser-Ser-Glu is proposed as the cleavage site.