Differential Inhibition of Cytosolic PEPCK by Substrate Analogues. Kinetic and Structural Characterization of Inhibitor Recognition ‡ (original) (raw)
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Proceedings of the National Academy of Sciences of the United States of America, 1999
The crystal structure of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) has been determined by x-ray diffraction methods at 2.8-Å resolution by using Escherichia coli PEPC complexed with l-aspartate, an allosteric inhibitor of all known PEPCs. The four subunits are arranged in a “dimer-of-dimers” form with respect to subunit contact, resulting in an overall square arrangement. The contents of α-helices and β-strands are 65% and 5%, respectively. All of the eight β-strands, which are widely dispersed in the primary structure, participate in the formation of a single β-barrel. Replacement of a conserved Arg residue (Arg-438) in this linkage with Cys increased the tendency of the enzyme to dissociate into dimers. The location of the catalytic site is likely to be near the C-terminal side of the β-barrel. The binding site for l-aspartate is located about 20 Å away from the catalytic site, and four residues (Lys-773, Arg-832, Arg-587, and Asn-881) are involved in effector binding. The participation of Arg-587 is unexpected, because it is known to be catalytically essential. Because this residue is in a highly conserved glycine-rich loop, which is characteristic of PEPC, l-aspartate seemingly causes inhibition by removing this glycine-rich loop from the catalytic site. There is another mobile loop from Lys-702 to Gly-708 that is missing in the crystal structure. The importance of this loop in catalytic activity was also shown. Thus, the crystal-structure determination of PEPC revealed two mobile loops bearing the enzymatic functions and accompanying allosteric inhibition by l-aspartate.
Inorganic Chemistry, 1994
The interactions of small inorganic anions such as C1-, N3-, and NCO-with carboxypeptidase A (EC 3.4.17.1, CPA) have been the object of extensive research in the last few years.'-3 Most of these studies take advantage of the spectral and magnetic properties of the cobalt-substituted derivative, which displays an activity toward peptide substrate hydrolysis even higher than the native zinc enzymes4 These studies have shown that the CPA active site becomes accessible to anions at low pH5 or after chemical modification of the residue Glu-270: which forms a strong hydrogen bond with the zinc-bound water molecule in the native enzyme.' These findings suggested that the breaking of the Glu-270-Wat-571 hydrogen bond was responsible for the activation of the zinc-bound water toward anion replacement. Indeed the crystal structures of the binary complexes of CPA with the D-phenylalanine and D-tyrosines inhibitors and of the ternary complex with azide and ~-phenylalanineg have provided the structural evidence for the above hypothesis. In both structures a strong hydrogen bond is formed between Glu-270 and the amino group of the amino acid inhibitor with consequent breaking of the former hydrogen bond with Wat-57 1, allowing, in the ternary complex, its replacement by an azide anion.
Biochemistry, 2001
The crystal structures of 3-deoxy-D-manno-2-octulosonate-8-phosphate synthase (KDOPS) from Escherichia coli complexed with the substrate phosphoenolpyruvate (PEP) and with a mechanism-based inhibitor (K d ) 0.4 µM) were determined by molecular replacement using X-ray diffraction data to 2.8 and 2.3 Å resolution, respectively. Both the KDOPS‚PEP and KDOPS‚inhibitor complexes crystallize in the cubic space group I23 with cell constants a ) b ) c ) 117.9 and 117.6 Å, respectively, and one subunit per asymmetric unit. The two structures are nearly identical, and superposition of their CR atoms indicates an rms difference of 0.41 Å. The PEP in the KDOPS‚PEP complex is anchored to the enzyme in a conformation that blocks its si face and leaves its re face largely devoid of contacts. This results from KDOPS's selective choice of a PEP conformer in which the phosphate group of PEP is extended toward the si face. Furthermore, the structure reveals that the bridging (P-O-C) oxygen atom and the carboxylate group of PEP are not strongly hydrogen-bonded to the enzyme. The resulting high degree of negative charge on the carboxylate group of PEP would then suggest that the condensation step between PEP and D-arabinose-5-phosphate (A5P) should proceed in a stepwise fashion through the intermediacy of a transient oxocarbenium ion at C2 of PEP. The molecular structural results are discussed in light of the chemically similar but mechanistically distinct reaction that is catalyzed by the enzyme 3-deoxy-Darabino-2-heptulosonate-7-phosphate synthase and in light of the preferred enzyme-bound states of the substrate A5P.
Biochemistry, 2018
Protein engineering to alter recognition underlying ligand binding and activity has enormous potential. Here, ligand binding for Escherichia coli phosphoenolpyruvate carboxykinase (PEPCK), which converts oxaloacetate into CO 2 and phosphoenolpyruvate as the first committed step in gluconeogenesis, was engineered to accommodate alternative ligands as an exemplary system with structural information. From our identification of bicarbonate binding in the PEPCK active site at the supposed CO 2 binding site, we probed binding of nonnative ligands with three oxygen atoms arranged to resemble the bicarbonate geometry. Crystal structures of PEPCK and point mutants with bound nonnative ligands thiosulfate and methanesulfonate along with strained ATP and reoriented oxaloacetate intermediates and unexpected bicarbonate were determined and analyzed. The mutations successfully altered the bound ligand position and orientation and its specificity: mutated PEPCKs bound either thiosulfate or methanesulfonate but never both. Computational calculations predicted a methanesulfonate binding mutant and revealed that release of the active site ordered solvent exerts a strong influence on ligand binding. Besides nonnative ligand binding, one mutant altered the Mn 2+ coordination sphere: instead of the canonical octahedral ligand arrangement, the mutant in question had an only five-coordinate arrangement. From this work, *
Phosphoenolpyruvate carboxylase: three-dimensional structure and molecular mechanisms
Archives of Biochemistry and Biophysics, 2003
Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) catalyzes the irreversible carboxylation of phosphoenolpyruvate (PEP) to form oxaloacetate and Pi using Mg 2þ or Mn 2þ as a cofactor. PEPC plays a key role in photosynthesis by C4 and Crassulacean acid metabolism plants, in addition to its many anaplerotic functions. Recently, three-dimensional structures of PEPC from Escherichia coli and the C4 plant maize (Zea mays) were elucidated by X-ray crystallographic analysis. These structures reveal an overall square arrangement of the four identical subunits, making up a ''dimer-of-dimers'' and an eight-stranded b barrel structure. At the Cterminal region of the b barrel, the Mn 2þ and a PEP analog interact with catalytically essential residues, confirmed by site-directed mutagenesis studies. At about 20 A A from the b barrel, an allosteric inhibitor (aspartate) was found to be tightly bound to downregulate the activity of the E. coli enzyme. In the case of maize C4-PEPC, the putative binding site for an allosteric activator (glucose 6-phosphate) was also revealed. Detailed comparison of the various structures of E. coli PEPC in its inactive state with maize PEPC in its active state shows that the relative orientations of the two subunits in the basal ''dimer'' are different, implicating an allosteric transition. Dynamic movements were observed for several loops due to the binding of either an allosteric inhibitor, a metal cofactor, a PEP analog, or a sulfate anion, indicating the functional significance of these mobile loops in catalysis and regulation. Information derived from these three-dimensional structures, combined with related biochemical studies, has established models for the reaction mechanism and allosteric regulation of this important C-fixing enzyme.
Journal of Molecular Biology, 2007
Scytalidoglutamic peptidase (SGP) from Scytalidium lignicolum is the founding member of the newly discovered\ family of peptidases, G1, so far found exclusively in fungi. The crystal structure of SGP revealed a previously undescribed fold for peptidases and a unique catalytic dyad of residues Gln53 and Glu136. Surprisingly, the β-sandwich structure of SGP is strikingly similar to members of the carbohydrate-binding concanavalin A-like lectins/glucanases superfamily. By analogy with the active sites of aspartic peptidases, a mechanism employing nucleophillic attack by a water molecule activated by the general base functionality of Glu136 has been proposed. Here, we report the first crystal structures of SGP in complex with two transition state peptide analogs designed to mimic the tetrahedral intermediate of the proteolytic reaction. Of these two analogs, the one containing a central S-hydroxyl group is a potent sub-nanomolar inhibitor of SGP. The inhibitor binds non-covalently to the concave surface of the upper β-sheet and enables delineation of the S4 to S3′ substrate specificity pockets of the enzyme. Structural differences in these pockets account for the unique substrate preferences of SGP among peptidases having an acidic pH optimum. Inhibitor binding is accompanied by a structuring of the region comprising residues Tyr71-Gly80 from being mostly disordered in the apoenzyme and leading to positioning of crucial active site residues for establishing enzyme-inhibitor contacts. In addition, conformational rearrangements are seen in a disulfide bridged surface loop (Cys141-Cys148), which moves inwards, partially closing the open substrate binding cleft of the native enzyme. The non-hydrolysable scissile bond analog of the inhibitor is located in the active site forming close contacts with Gln53 and Glu136. The nucleophilic water molecule is displaced and a unique mode of binding is observed with the S-OH of the inhibitor occupying the oxyanion binding site of the proposed tetrahedral intermediate. Details of the enzyme-inhibitor interactions and mechanistic interpretations are discussed.
Journal of Protein Chemistry, 2000
Molecular mechanics calculations have been employed to obtain models of the complexes between Saccharomyces cerevisiae phosphoenolpyruvate (PEP) kinase and the ATP analogs pyridoxal 5′-diphosphoadenosine (PLP-AMP) and pyridoxal 5′-triphosphoadenosine (PLP-ADP), using the crystalline coordinates of the ATP-pyruvate-Mn2+-Mg2+ complex of Escherichia coli PEP carboxykinase [Tari et al. (1997), Nature Struct. Biol. 4, 990–994]. In these models, the preferred conformation of the pyridoxyl moiety of PLP-ADP and PLP-AMP was established through rotational barrier and simulated annealing procedures. Distances from the carbonyl-C of each analog to ε-N of active-site lysyl residues were calculated for the most stable enzyme-analog complex conformation, and it was found that the closest ε-N is that from Lys290, thus predicting Schiff base formation between the corresponding carbonyl and amino groups. This prediction was experimentally verified through chemical modification of S. cerevisiae PEP carboxykinase with PLP-ADP and PLP-AMP. The results here described demonstrate the use of molecular modeling procedures when planning chemical modification of enzyme-active sites.