Substrate deactivation of phenylalanine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase by erythrose 4-phosphate.

3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS, EC 4.1.2.15) catalyzes the condensation of phosphoenolpyruvate (PEP) with erythrose 4-phosphate (E4P) to give DAH7P via an ordered sequential mechanism. In the absence of PEP (the first substrate to bind), E4P binds covalently to the phenylalanine-sensitive DAH7PS of Escherichia coli, DAH7PS(Phe), deactivating the enzyme. Activity is restored on addition of excess PEP but not if deactivation was carried out in the presence of sodium cyanoborohydride. Electrospray mass spectrometry indicates that a single E4P is bound to the protein. These data are consistent with a slow, reversible Schiff base reaction of the aldehydic functionality of E4P with a buried lysine. Molecular modeling indicates that Lys186, a residue at the base of the substrate-binding cavity involved in hydrogen bonding with PEP, is well placed to react with E4P forming an imine linkage that is substantially protected from solvent water.     

Altered substrate selectivity in a mutant of an intrahelical salt bridge in UhpT, the sugar phosphate carrier of Escherichia coli

Site-directed and second site suppressor mutagenesis identify an intrahelical salt bridge in the eleventh transmembrane segment of UhpT, the sugar phosphate carrier of Escherichia coli. Glucose 6-phosphate (G6P) transport by UhpT is inactivated if cysteine replaces either Asp388 or Lys391 but not if both are replaced. This suggests that Asp388 and Lys391 are involved in an intrahelical salt bridge and that neither is required for normal UhpT function. This interpretation is strengthened by the finding that mutations at Lys391 (K391N, K391Q, and K391T) are recovered as revertants of the inactive D388C variant. Further work shows that although the D388C variant is null for G6P transport, movement of 32Pi by homologous Pi/Pi exchange is unaffected. This raises the possibility that this derivative may have latent function, a possibility confirmed by showing that D388C is a gain-of-function mutation in which phosphoenolpyruvate (PEP) is the preferred substrate. Added study of the Pi/Pi exchange shows that in wild type UhpT this partial reaction is readily blocked by G6P but not PEP. By contrast, in the D388C variant, Pi/Pi exchange is unaffected by G6P but is inhibited by both PEP and 3-phosphoglycerate. These latter substrates are used by PgtP, a related Pi-linked antiporter, which lacks the Asp388-Lys391 salt bridge but has instead an uncompensated arginine at position 391. For this reason, we conclude that in both UhpT and PgtP position 391 can serve as a determinant of substrate selectivity by acting as a receptor for the anionic carboxyl brought into the translocation pathway by PEP.     

Characterization of the phosphoenolpyruvate carboxykinase gene from Corynebacterium glutamicum and significance of the enzyme for growth and amino acid production

Corynebacterium glutamicum possesses phosphoenolpyruvate (PEP) carboxykinase, oxaloacetate decarboxylase and malic enzyme, all three in principle being able to catalyze the first step in gluconeogenesis. To investigate the role of PEP carboxykinase for growth and amino acid production, the respective pck gene was isolated, characterized and used for construction and analysis of mutants and overexpressing strains. Sequence analysis of the pck gene predicts a polypeptide of 610 amino acids showing up to 64% identity with ITP-/GTP-dependent PEP carboxykinases from other organisms. C. glutamicum cells harbouring pck on plasmid showed about tenfold higher specific PEP carboxykinase activities than the wildtype. Inactivation of the chromosomal pck gene led to the absence of PEP carboxykinase activity and the inability to grow on acetate or lactate indicating that the enzyme is essential for growth on these carbon sources and thus, for gluconeogenesis. The growth on glucose was not affected. Examination of glutamate production by the recombinant C. glutamicum strains revealed that the PEP carboxykinase-deficient mutant showed about fourfold higher, the pck-overexpressing strain two- to threefold lower glutamate production than the parental strain. Inactivation and overexpression of pck in a lysine-producer of C. glutamicum led to an only 20% higher and lower lysine accumulation, respectively. The results show that PEP carboxykinase activity in C. glutamicum is counteractive to the production of glutamate and lysine and indicate that the enzyme is an important target in the development of strains producing amino acids derived from citric acid cycle intermediates.     

Oligomerization and the sensitivity of phosphoenolpyruvate carboxylase to inactivation by proteinases

Phosphenolpyruvate (PEP) carboxylase from leaves of Crassula argentea displays varying levels of sensitivity to inactivation by various proteolytic enzymes. In general, the native enzyme is sensitive to proteinases known to attack at the carbonyl end of lysine or arginine (trypsin, papain, or bromelain). The ineffective proteolytic enzymes are those which have low specificity or which attack at the N-terminal end of hydrophobic amino acids, or which cannot attack lysine. The lack of an effect of endoproteinase arginine C, which is specific for arginine, probably indicates that lysine is the critical residue. When the native enzyme, which is comprised of an equilibrium of dimers with tetramers in approximately equal quantities, is treated by preincubation with 5 millimolar PEP, the enzyme becomes much more resistant to proteolytic inactivation. When the preincubation is with 5 millimolar malate rather than buffer alone, the effect is to slightly increase (ca. 15%) the sensitivity of the enzyme to inactivation by trypsin as measured by estimates of the pseudo-first order rate constant for inactivation. PEP carboxylase from corn leaves appears to be relatively susceptible to inactivation by trypsin, but is unaffected by preincubation with malate or PEP. The sensitivity of this C₄ enzyme to inhibition by malate is also unaffected by preincubation with these ligands.     

3-Deoxy-D-manno-octulosonate-8-phosphate synthase from Escherichia coli. Model of binding of phosphoenolpyruvate and D-arabinose-5-phosphate

The crystal structure of 3-deoxy-d-manno-octulosonate-8-phosphate synthase (KDOPS) from Escherichia coli was determined by molecular replacement using coordinates given to us by Radaev and co-workers prior to publication. The KDOPS crystals reported by Radaev et al. were grown in the presence of 1.4 M (NH(4))(2)SO(4) and 0.4 M (K/H)(3)PO(4). They are in the cubic space group I23 (a=228.6 A) with a tetramer in the asymmetric unit; the structure has been refined with data to 2.4 A. Our crystals of E. coli KDOPS, grown in 24 % (w/v) polyethylene glycol (PEG) 1500 in the presence of the substrates, 2-phosphoenolpyruvate (PEP) and d-arabinose-5-phosphate (A5P), are also in space group I23 (a=118.2 A), with one subunit in the asymmetric unit.The medium of crystallization, 1.8 M SO(4)/PO(4) versus 24 % PEG, does not significantly affect the conformation of KDOPS. The inter-monomer contacts in both structures are the same. The beta(8)/alpha(8) loop (residues 246 to 251) situated near the entrance to the active site is not seen in the 229 A structure but can be traced in the 118 A structure.Most significantly, Radaev et al. interpreted two SO(4)/PO(4) sites in the 229 A structure as marking the phosphate positions of the substrates, PEP and A5P, after the precedent of DAHPS. In the 118 A structure the inner of these two SO(4)/PO(4) peaks is present at the same position as in the 229 A structure of KDOPS. The outer phosphate peak in the 118 A KDOPS is 3.7 A from the outer SO(4)/PO(4) peak in the 229 A structure and is within hydrogen bonding distance of Arg63 of the same subunit and Arg120 of another subunit. Based on the precedent of the d-erythrose-4-phosphate (E4P) modeled in the active site of DAHPS, we have modeled PEP and A5P in KDOPS and compared the coordination of PEP and A5P in KDOPS with that of PEP and E4P in DAHPS.     

3-Deoxy-D-manno-octulosonate-8-phosphate synthase catalyzes the C-O bond cleavage of phosphoenolpyruvate

The mechanism of 3-deoxy-D-manno-octulosonate-8-phosphate (KDO8P) synthase was investigated. When [18O]-PEP specifically labeled in the enolic oxygen is a substrate for KDO8P synthase, the 18O is recovered in Pi. This indicates that the KDO8P synthase reaction proceeds with C-O bond cleavage of PEP similar to that observed in the 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase catalyzed condensation of PEP and erythrose-4-phosphate (1). No evidence for a covalent enzyme-PEP intermediate could be obtained. No [32P]-Pi exchange into PEP nor scrambling of bridge 18O to non-bridging positions in [18O]-PEP was observed in the presence or absence of arabinose-5-phosphate or its analog ribose-5-phosphate. Bromopyruvate inactivated KDO8P synthase in a time dependent process. It is likely that bromopyruvate reacts with a functional group at the PEP binding site since PEP, but not arabinose-5-phosphate, protects against inactivation.     

Mechanistic insight into 3-deoxy-D-manno-octulosonate-8-phosphate synthase and 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase utilizing phosphorylated monosaccharide analogues

Escherichia coli 3-deoxy-D-manno-octulosonate 8-phosphate (KDO8-P) synthase is able to utilize the five-carbon phosphorylated monosaccharide, 2-deoxyribose 5-phosphate (2dR5P), as an alternate substrate, but not D-ribose 5-phosphate (R5P) nor the four carbon analogue D-erythrose 4-phosphate (E4P). However, E. coli KDO8-P synthase in the presence of either R5P or E4P catalyzes the rapid consumption of approximately 1 mol of PEP per active site, after which consumption of PEP slows to a negligible but measurable rate. The mechanism of this abortive utilization of PEP was investigated using [2,3-(13)C(2)]-PEP and [3-F]-PEP, and the reaction products were determined by (13)C, (31)P, and (19)F NMR to be pyruvate, phosphate, and 2-phosphoglyceric acid (2-PGA). The formation of pyruvate and 2-PGA suggests that the reaction catalyzed by KDO8-P synthase may be initiated via a nucleophilic attack to PEP by a water molecule. In experiments in which the homologous enzyme, 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAH7-P) synthase was incubated with D,L-glyceraldehyde 3-phosphate (G3P) and [2,3-(13)C(2)]-PEP, pyruvate and phosphate were the predominant species formed, suggesting that the reaction catalyzed by DAH7-P synthase starts with a nucleophilic attack by water onto PEP as observed in E. coli KDO8-P synthase.     

核盘菌5-烯醇丙酮酰莽草酸-3-磷酸合酶的酶学性质

核盘菌5-烯醇丙酮酰莽草酸-3-磷酸合酶（EPSP合酶）是AROM多功能酶的活性之一.该酶催化莽草酸磷酸（S3P）和磷酸烯醇式丙酮酸（PEP）产生5-烯醇丙酮酰莽草酸-3-磷酸和无机磷酸的可逆反应,受除草剂草甘膦（N-（膦羧甲基）甘氨酸）抑制.纯化了核盘菌AROM蛋白并对EPSP合酶进行了酶学特征研究.结果显示,该酶反应的最适pH值为7.2,最适温度为30℃.热失活反应活化能是69.62 kJ/mol.底物S3P和PEP浓度分别高于1 mmol/L和2 mmol/L时,对EPSP合酶反应产生抑制作用.用双底物反应恒态动力学Dalziel方程求得的Km(PEP)为140.98 μmol/L,K _m(S3P)为139.58 μmol/L.酶动力学模型遵循顺序反应机制.草甘膦是该酶反应底物PEP的竞争性抑制剂（Ki为0.32 μmol/L）和S3P的非竞争性抑制剂.正向反应受K⁺激活.当［K⁺］增加时,K _m(PEP)随之降低,Km(S3P)不规律变化,而K _i(PEP)随［K⁺］增加而提高.     

Effect of Pyruvate Kinase Deficiency on l-Lysine Productivities of Mutants with Feedback-resistant Aspartokinases

The cultural conditions were investigated for a Brevibacterium flavum mutant, No. 2–190, with a low level of citrate synthase (CS) and with feedback-resistant phosphoenoipyruvate (PEP) carboxylase and aspartokinase (AK). The productivity was increased from 28 to 38 g/1 (as the HC1 salt) with a medium containing 10% glucose. From this strain, pyruvate kinase (PK)-defective mutants were derived and selected as to the inability to grow on ribose. Among them, strain Kl-18 showed higher lysine productivity than the parent under all cultural conditions tested, and produced 43 g/1 of lysine, at maximum. A lysine-producing mutant, No. 536–4, with a feedback- resistant AK was derived from PK-defective strain KH-21 which had low CS activity and a feedback-resistant PEP carboxylase. The mutant was isolated by a new selection method, that is, on the basis of resistance to α-amino-ß-hydroxyvaleric acid, a threonine analogue plus lysine. In this strain, HD had been altered so as to become feedback-resistant at the same time, resulting in the byproduction of threonine and isoleucine. The total amount of these aspartate family amino acids was higher on molar basis than that of lysine produced by strain No. 2–190.     

Structure of 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase from Escherichia coli: comparison of the Mn(2+)*2-phosphoglycolate and the Pb(2+)*2-phosphoenolpyruvate complexes and implications for catalysis

The crystal structure of the phenylalanine-regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS) from Escherichia coli in complex with Mn(2+) and the substrate analog, 2-phosphoglycolate (PGL), was determined by molecular replacement using X-ray diffraction data to 2.0 A resolution. DAHPS*Mn*PGL crystallizes in space group C2 (a=210.4 A, b=53.2 A, c=149.4 A, beta=116.1 degrees ) with its four (beta/alpha)(8) barrel subunits related by non-crystallographic 222 symmetry. The refinement was carried out without non-crystallographic symmetry restraints and yielded agreement factors of R=20.9 % and R(free)=23.9 %. Mn(2+), the most efficient metal activator, is coordinated by the same four side-chains (Cys61, His268, Glu302 and Asp326) as is the poorly activating Pb(2+). A fifth ligand is a well-defined water molecule, which is within hydrogen bonding distance to an essential lysine residue (Lys97). The distorted octahedral coordination sphere of the metal is completed by PGL, which replaces the substrate, 2-phosphoenolpyruvate (PEP), in the active site. However, unlike PEP in the Pb*PEP complex, PGL binds the Mn(2+) via one of its carboxylate oxygen atoms. A model of the active site is discussed in which PEP binds in the same orientation as does PGL in the DAHPS*Mn*PGL structure and the phosphate of E4P is tethered at the site of a bound sulfate anion. The re face of E4P can be positioned to interact with the si face of PEP with only small movement of the protein.     

Function of His185 in Aquifex aeolicus 3-deoxy-D-manno-octulosonate 8-phosphate synthase

Aquifex aeolicus 3-deoxy-D-manno-octulosonate 8-phosphate synthase (KDO8PS) catalyzes the condensation of arabinose 5-phosphate (A5P) and phosphoenolpyruvate (PEP) by favoring the activation of a water molecule coordinated to the active-site metal ion. Cys11, His185, Glu222 and Asp233 are the other metal ligands. Wild-type KDO8PS is purified with Zn(2+) or Fe(2+) in the active site, but maximal activity in vitro is achieved when the endogenous metal is replaced with Cd(2+). The H185G enzyme retains 8% of the wild-type activity. ICP mass spectrometry analysis indicates that loss of His185 decreases the enzyme affinity for Fe(2+), but not for Zn(2+). However, maximal activity is again achieved by substitution of the endogenous metal with Cd(2+). We have determined the X-ray structures of the Cd(2+) H185G enzyme in its substrate-free form, and in complex with PEP, and PEP plus A5P. These structures show a normal amount of Cd(2+) bound, suggesting that coordination by His185 is not essential to retain Cd(2+) in the active site. Nonetheless, there are significant changes in the coordination sphere of Cd(2+) with respect to the wild-type enzyme, as the carboxylate moiety of PEP binds directly to the metal ion and replaces water and His185 as ligands. These observations indicate that the primary function of His185 in A.aeolicus KDO8PS is to orient PEP in the active site of the enzyme in such a way that a water molecule on the sinister (si) side of PEP can be activated by direct coordination to the metal ion.     

Structure and mechanism of 3-deoxy-D-manno-octulosonate 8-phosphate synthase

3-deoxy-D-manno-octulosonate 8-phosphate (KDO8P) synthase catalyzes the condensation of phosphoenolpyruvate (PEP) with arabinose 5-phosphate (A5P) to form KDO8P and inorganic phosphate. KDO8P is the phosphorylated precursor of 3-deoxy-D-manno-octulosonate, an essential sugar of the lipopolysaccharide of Gram-negative bacteria. The crystal structure of the Escherichia coli KDO8P synthase has been determined by multiple wavelength anomalous diffraction and the model has been refined to 2.4 A (R-factor, 19.9%; R-free, 23.9%). KDO8P synthase is a homotetramer in which each monomer has the fold of a (beta/alpha)(8) barrel. On the basis of the features of the active site, PEP and A5P are predicted to bind with their phosphate moieties 13 A apart such that KDO8P synthesis would proceed via a linear intermediate. A reaction similar to KDO8P synthesis, the condensation of phosphoenolpyruvate, and erythrose 4-phosphate to form 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAH7P), is catalyzed by DAH7P synthase. In the active site of DAH7P synthase the two substrates PEP and erythrose 4-phosphate appear to bind in a configuration similar to that proposed for PEP and A5P in the active site of KDO8P synthase. This observation suggests that KDO8P synthase and DAH7P synthase evolved from a common ancestor and that they adopt the same catalytic strategy.     

A study of the in-vitro regulation of phosphoenolpyruvate carboxylase from the epidermis of Commelina communis by malate and glucose-6-phosphate

Phosphoenolpyruvate (PEP) carboxylase activity in epidermal extracts of Commelina communis has been compared in the presence of malate and glucose-6-phosphate. The activity of PEP carboxylase was inhibited by increasing malate concentrations at several substrate (PEP) concentrations and changes in both the apparent K _m (PEP) and V _max values in the presence of malate suggested the occurence of mixed-type inhibiton. In the presence of glucose-6-phosphate no increase in enzyme activity was observed, although there was a slight decrease in the K _m (PEP). However, glucose-6-phosphate appeared to alleviate the inhibition caused by malate. The possible implications of these properties in the control of malate production in guard cells is discussed.Abbreviations PEP phosphoenolpyruvate - Glc6P glucose-6-phosphate     

Yield regulation of lysine biosynthesis in Brevibacterium flavum

A scheme for lysine biosynthesis using variants of the Brevibacterium flavum intermediary metabolite synthesis is discussed. The main precursor of lysine that we are concerned with here is oxalacetate, which can be synthesized through the TCA or glyoxylate cycles or by carboxylation of PEP. Material energy balances for the main pathways of lysine biosynthesis from glucose and acetate have been formulated. Energy consumption, in the from of ATP – P_ATP (number of mol ATP consumed/1 mol lysine synthesized), was calculated for the main pathways of lysine biosynthesis. Theoretical conversion yields Y_p^max (g product/g substrate) were estimated. Experimental data were presented concerning the increase of Y_p by means of metabolism regulation: (a) by TCA-and glyoxylate-cycle enzyme induction; (b) by maintaining PEP carboxylase activity; (c) by eliminating by-product synthesis.     

Identification of the catalytic residues of AroA (Enolpyruvylshikimate 3-phosphate synthase) using partitioning analysis

AroA (EPSP synthase) catalyzes carboxyvinyl transfer through addition of shikimate 3-phosphate (S3P) to phosphoenolpyruvate (PEP) to form a tetrahedral intermediate (THI), followed by phosphate elimination to give enolpyruvylshikimate 3-phosphate (EPSP). A novel approach, partitioning analysis, was used to elucidate the roles of catalytic residues in each step of the reaction. Partitioning analysis involved trapping and purifying [1-(14)C]THI, degrading it with AroA, and quantitating the products. Wild-type AroA gave a partitioning factor, f(PEP) = 0.25 +/- 0.02 at pH 7.5, where f(PEP) = [[1-(14)C]PEP]/([[1-(14)C]PEP] + [[1-(14)C]EPSP]). Eighteen mutations were made to 14 amino acids to discover which residues preferentially catalyzed either the addition or the elimination step. Mutating a residue catalyzing one step (e.g., addition) should change f(PEP) to favor the opposite step (e.g., elimination). No mutants caused large changes in f(PEP), with experimental values from 0.07 to 0.41. This implied that there are no side chains that catalyze only addition or elimination, which further implied that the same residues are general acid/base catalysts in both forward and reverse THI breakdown. Only Lys22 (protonating S3P hydroxyl or phosphate) and Glu341 (deprotonating C3 of PEP) are correctly situated in the active site. In the overall reaction, Lys22 would act as a general base during addition, while Glu341 would act as a general acid. Almost half of the mutations (eight of 18) caused a >1000-fold decrease in specific activity, demonstrating that a large number of residues are important for transition state stabilization, "ensemble catalysis", in contrast to some enzymes where a single amino acid can be responsible for up to 10(8)-fold catalytic enhancement.     

Substrate and metal complexes of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Saccharomyces cerevisiae provide new insights into the catalytic mechanism

3-Deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthases are metal-dependent enzymes that catalyse the first committed step in the biosynthesis of aromatic amino acids in microorganisms and plants, the condensation of 2-phophoenolpyruvate (PEP) and d-erythrose 4-phosphate (E4P) to DAHP. The DAHP synthases are possible targets for fungicides and represent a model system for feedback regulation in metabolic pathways. To gain further insight into the role of the metal ion and the catalytic mechanism in general, the crystal structures of several complexes between the tyrosine-regulated form of DAHP synthase from Saccharomyces cerevisiae and different metal ions and ligands have been determined. The crystal structures provide evidence that the simultaneous presence of a metal ion and PEP result in an ordering of the protein into a conformation that is prepared for binding the second substrate E4P. The site and binding mode of E4P was derived from the 1.5A resolution crystal structure of DAHP synthase in complex with PEP, Co2+, and the E4P analogue glyceraldehyde 3-phosphate. Our data suggest that the oxygen atom of the reactive carbonyl group of E4P replaces a water molecule coordinated to the metal ion, strongly favouring a reaction mechanism where the initial step is a nucleophilic attack of the double bond of PEP on the metal-activated carbonyl group of E4P. Mutagenesis experiments substituting specific amino acids coordinating PEP, the divalent metal ion or the second substrate E4P, result in stable but inactive Aro4p-derivatives and show the importance of these residues for the catalytic mechanism.     

Lysine 22 in UDP-N-acetylglucosamine enolpyruvyl transferase from Enterobacter cloacae is crucial for enzymatic activity and the formation of covalent adducts with the substrate phosphoenolpyruvate and the antibiotic fosfomycin.

UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) catalyzes the first committed step in the biosynthesis of the bacterial cell wall component peptidoglycan. The enzyme is the target of the antibiotic fosfomycin. A lysine residue (K22), strictly conserved in MurAs and the structurally and mechanistically related 5-enolpyruvylshikimate 3-phosphate synthases (EPSPS), is located near the active center of the enzyme. This residue is thought to be involved directly in the binding of the substrate phosphoenolpyruvate (PEP) and also to participate in the conformational change leading to the formation of the catalytically competent enzyme complex. Using site-directed mutagenesis, we have replaced this lysine with arginine (K22R), valine (K22V), and glutamate (K22E). These mutant proteins were expressed, purified, and characterized in comparison to wild-type MurA and a previously described inactive C115S mutant protein. It was found that all three K22 mutant proteins had less than 0.5% of the wild-type activity. Using isothermal titration calorimetry, it could be shown that the binding parameters for the UDP-sugar nucleotide substrate are not affected by the mutations, except for the K22E mutant protein. Similarly, binding of PEP was found to be unaffected in the K22 mutant proteins as demonstrated by tryptophan fluorescence quench titrations. On the other hand, the level of formation of a covalent adduct with either PEP or fosfomycin with the thiol group of cysteine 115 was diminished. The propensity to form an adduct with PEP decreased in the following order: wild type > K22R > K22V > K22E. A comparable effect was found on the formation of the inhibitory covalent adduct of MurA and the antibiotic fosfomycin. These results are discussed in terms of an involvement of lysine 22 in a conformational change of MurA.     

Substrate and metal complexes of 3-deoxy-D-manno-octulosonate-8-phosphate synthase from Aquifex aeolicus at 1.9-A resolution. Implications for the condensation mechanism

3-Deoxy-D-manno-octulosonate-8-phosphate synthase (KDO8PS) from the hyperthermophilic bacterium Aquifex aeolicus differs from its Escherichia coli counterpart in the requirement of a divalent metal for activity (Duewel, H. S., and Woodard, R. W. (2000) J. Biol. Chem. 275, 22824-22831). Here we report the crystal structure of the A. aeolicus enzyme, which was determined by molecular replacement using E. coli KDO8PS as a model. The structures of the metal-free and Cd(2+) forms of the enzyme were determined in the uncomplexed state and in complex with various combinations of phosphoenolpyruvate (PEP), arabinose 5-phosphate (A5P), and erythrose 4-phosphate (E4P). Like the E. coli enzyme, A. aeolicus KDO8PS is a homotetramer containing four distinct active sites at the interface between subunits. The active site cavity is open in the substrate-free enzyme or when either A5P alone or PEP alone binds, and becomes isolated from the aqueous phase when both PEP and A5P (or E4P) bind together. In the presence of metal, the enzyme is asymmetric and appears to alternate catalysis between the active sites located on one face of the tetramer and those located on the other face. In the absence of metal, the asymmetry is lost. Details of the active site that may be important for catalysis are visible at the high resolution achieved in these structures. Most notably, the shape of the PEP-binding pocket forces PEP to assume a distorted geometry at C-2, which might anticipate the conversion from sp(2) to sp(3) hybridization occurring during intermediate formation and which may modulate PEP reactivity toward A5P. Two water molecules are located in van der Waals contact with the si and re sides of C-2(PEP), respectively. Abstraction of a proton from either of these water molecules by a protein group is expected to elicit a nucleophilic attack of the resulting hydroxide ion on the nearby C-2(PEP), thus triggering the beginning of the catalytic cycle.     

Characterization of Streptococcus pneumoniae 5-enolpyruvylshikimate 3-phosphate synthase and its activation by univalent cations.

The aroA gene (Escherichia coli nomenclature) encoding 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase from the gram-positive pathogen Streptococcus pneumoniae has been identified, cloned and overexpressed in E. coli, and the enzyme purified to homogeneity. It was shown to catalyze a reversible conversion of shikimate 3-phosphate (S3P) and phosphoenolpyruvate (PEP) to EPSP and inorganic phosphate. Activation by univalent cations was observed in the forward reaction, with NH+4, Rb+ and K+ exerting the greatest effects. Km(PEP) was lowered by increasing [NH+4] and [K+], whereas Km(S3P) rose with increasing [K+], but fell with increasing [NH+4]. Increasing [NH+4] and [K+] resulted in an overall increase in kcat. Glyphosate (GLP) was found to be a competitive inhibitor with PEP, but the potency of inhibition was profoundly affected by [NH+4] and [K+]. For example, increasing [NH+4] and [K+] reduced Ki(GLP versus PEP) up to 600-fold. In the reverse reaction, the enzyme catalysis was less sensitive to univalent cations. Our analysis included univalent cation concentrations comparable with those found in bacterial cells. Therefore, the observed effects of these metal ions are more likely to reflect the physiological behavior of EPSP synthase and also add to our understanding of how to inhibit this enzyme in the host organism. As there is a much evidence to suggest that EPSP synthase is essential for bacterial survival, its discovery in the serious gram-positive pathogen S. pneumoniae and its inhibition by GLP indicate its potential as a broad-spectrum antibacterial target.     

The high-resolution structure of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase reveals a twist in the plane of bound phosphoenolpyruvate

3-Deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS), the first enzyme of the aromatic biosynthetic pathway in microorganisms and plants, catalyzes the aldol-like condensation of phosphoenolpyruvate (PEP) and D-erythrose 4-phosphate (E4P) with the formation of DAHP. The native and the selenomethionine-substituted forms of the phenylalanine-regulated isozyme [DAHPS(Phe)] from Escherichia coli were crystallized in complex with PEP and a metal cofactor, Mn(2+), but the crystals displayed disorder in their unit cells, preventing satisfactory refinement. However, the crystal structure of the E24Q mutant form of DAHPS(Phe) in complex with PEP and Mn(2+) has been determined at 1.75 A resolution. Unlike the tetrameric wild-type enzyme, the E24Q enzyme is dimeric in solution, as a result of the mutational perturbation of four intersubunit salt bridges that are critical for tetramer formation. The protein chain conformation and subunit arrangement in the crystals of E24Q and wild-type DAHPS are very similar. However, the interaction of Mn(2+) and PEP in the enzymatically active E24Q mutant complex differs from the Pb(2+)-PEP and Mn(2+)-phosphoglycolate interactions in two enzymatically inactive wild-type complexes whose structures have been determined previously. The geometry of PEP bound in the active site of the E24Q enzyme deviates from planarity due to a 30 degrees twist of the carboxylate plane relative to the enol plane. In addition, seven water molecules are within contact distance of PEP, two of which are close enough to its C2 atom to serve as the nucleophile required in the reaction.