In Silico Structure Modeling and Molecular Docking Analysis of Phosphoribosyl Pyrophosphate Amidotransferase (PPAT) with Antifolate Inhibitors.

BACKGROUND: Cancer remains one of the most serious disease worldwide. Robust metabolism is the hallmark of cancer. PPAT (phosphoribosyl pyrophosphate amidotransferase) catalyzes the first committed step of de novo purine biosynthesis. Hence PPAT, the key regulatory spot in De novo purine nucleotide biosynthesis, is an attractive and credible drug target for leukemia and other cancer therapeutics. OBJECTIVE: In the present study, detailed computational analysis has been performed for PPAT protein, the key enzyme in de novo purine biosynthesis which is inhibited by many folate derivatives, hence we aimed to investigate and gauge the inhibitory effect of antifolate derivatives; lomexterol (LTX) methotrexate (LTX), and pipretixin (PTX) with human PPAT to effectively capture and inhibit De novo purine biosynthesis pathway. METHODS: The sequence to structure computational approaches followed by molecular docking experiments was performed to gain insight into the inhibitory mode, binding orientation and binding affinities of selected antifolate derivatives against important structural features of PPAT. RESULTS: Results indicated a strong affinity of antifolate inhibitors for the conserved active site of PPAT molecule encompassing a number of hydrophobic, hydrogen bonding, Vander Waals and electrostatic interactions. CONCLUSION: Conclusively, the strong physical interaction of selected antifolate inhibitors with human PPAT suggests the selective inhibition of De novo purine biosynthesis pathway by antifolate derivatives towards cancer therapeutics.

In silico analysis of the amido phosphoribosyltransferase inhibition by PY873, PY899 and a derivative of isophthalic acid.

Selectively decreasing the availability of precursors for the de novo biosynthesis of purine nucleotides is a valid approach towards seeking a cure for leukaemia. Nucleotides and deoxynucleotides are required by living cells for syntheses of RNA, DNA, and cofactors such as NADP(+), FAD(+), coenzyme A and ATP. Nucleotides contain purine and pyrimidine bases, which can be synthesized through salvage pathway as well. Amido phosphoribosyltransferase (APRT), also known as glutamine phosphoribosylpyrophosphate amidotransferase (GPAT), is an enzyme that in humans is encoded by the PPAT (phosphoribosyl pyrophosphate amidotransferase) gene. APRT catalyzes the first committed step of the de novo pathway using its substrate, phosphoribosyl pyrophosphate (PRPP). As APRT is inhibited by many folate analogues, therefore, in this study we focused on the inhibitory effects of three folate analogues on APRT activity. This is extension of our previous wet lab work to analyze and dissect molecular interaction and inhibition mechanism using molecular modeling and docking tools in the current study. Comparative molecular docking studies were carried out for three diamino folate derivatives employing a model of the human enzyme that was built using the 3D structure of Bacillus subtilis APRT (PDB ID; 1GPH) as the template. Binding orientation of interactome indicates that all compounds having nominal cluster RMSD in same active site's deep narrow polar fissure. On the basis of comparative conformational analysis, electrostatic interaction, binding free energy and binding orientation of interactome, we support the possibility that these molecules could behave as APRT inhibitors and therefore may block purine de novo biosynthesis. Consequently, we suggest that PY899 is the most active biological compound that would be a more potent inhibitor for APRT inhibition than PY873 and DIA, which also confirms previous wet lab report.

Crystal structure of the ATPPase subunit and its substrate-dependent association with the GATase subunit: a novel regulatory mechanism for a two-subunit-type GMP synthetase from Pyrococcus horikoshii OT3.

Guanosine 5'-monophosphate synthetase(s) (GMPS) catalyzes the final step of the de novo synthetic pathway of purine nucleotides. GMPS consists of two functional units that are present as domains or subunits: glutamine amidotransferase (GATase) and ATP pyrophosphatase (ATPPase). GATase hydrolyzes glutamine to yield glutamate and ammonia, while ATPPase utilizes ammonia to convert adenyl xanthosine 5'-monophosphate (adenyl-XMP) into guanosine 5'-monophosphate. Here we report the crystal structure of PH-ATPPase (the ATPPase subunit of the two-subunit-type GMPS from the hyperthermophilic archaeon Pyrococcus horikoshii OT3). PH-ATPPase consists of two domains (N-domain and C-domain) and exists as a homodimer in the crystal and in solution. The N-domain contains an ATP-binding platform called P-loop, whereas the C-domain contains the xanthosine 5'-monophosphate (XMP)-binding site and also contributes to homodimerization. We have also demonstrated that PH-GATase (the glutamine amidotransferase subunit of the two-subunit-type GMPS from the hyperthermophilic archaeon P. horikoshii OT3) alone is inactive, and that all substrates of PH-ATPPase except for ammonia (Mg(2+), ATP and XMP) are required to stabilize the active complex of PH-ATPPase and PH-GATase subunits.

Computational studies of ammonia channel function in glutamine 5'-phosphoribosylpyrophosphate amidotransferase.

Glutamine 5'-phosphoribosylpyrophosphate amidotransferase (GPATase) catalyzes the synthesis of 5'-phosphoribosylamine in a reaction that involves the translocation of ammonia along an intramolecular tunnel linking the two active sites of the enzyme. We now report a locally enhanced sampling (LES) strategy for modeling ammonia transfer between the active sites of Escherichia coli GPATase in its active conformation. Our calculations demonstrate that the ammonia channel in GPATase is best regarded as a "pipe" through which ammonia travels in the absence of an external "driving" potential. This combined LES/PMF computational approach, which offers a straightforward alternative to steered molecular dynamics simulations in studies of substrate channeling, also provides new insights into the molecular basis of the reduced ammonia transfer efficiency exhibited by the L415A GPATase mutant.

Carrier proteins for fusion expression of antimicrobial peptides in Escherichia coli.

Antimicrobial peptides are an essential component of innate immunity and play an important role in host defence against microbial pathogens. They have received increasing attention recently as potential novel pharmaceutical agents. To meet the requirement for necessary basic science studies and clinical trials, large quantities of these peptides are needed. In general, isolation from natural sources and chemical synthesis are not cost-effective. The relatively low cost and easy scale-up of the recombinant approach renders it the most attractive means for large-scale production of antimicrobial peptides. Among the many systems available for protein expression, Escherichia coli remains the most widely used host. Antimicrobial peptides produced in E. coli are often expressed as fusion proteins, which effectively masks these peptides' potential lethal effect towards the bacterial host and protects the peptides from proteolytic degradation. Although some carriers confer peptide solubility, others promote the formation of inclusion bodies. The present minireview considers the most commonly used carrier proteins for fusion expression of antimicrobial peptides in E. coli. The favourable properties of SUMO (small ubiquitin-related modifier) as a novel fusion partner are also discussed.

A study in molecular contingency: glutamine phosphoribosylpyrophosphate amidotransferase is a promiscuous and evolvable phosphoribosylanthranilate isomerase.

The prevalence of paralogous enzymes implies that novel catalytic functions can evolve on preexisting protein scaffolds. The weak secondary activities of proteins, which reflect catalytic promiscuity and substrate ambiguity, are plausible starting points for this evolutionary process. In this study, we observed the emergence of a new enzyme from the ASKA (A Complete Set of E. coli K-12 ORF Archive) collection of Escherichia coli open reading frames. The overexpression of (His)(6)-tagged glutamine phosphoribosylpyrophosphate amidotransferase (PurF) unexpectedly rescued a Delta trpF E. coli strain from starvation on minimal media. The wild-type PurF and TrpF enzymes are unrelated in sequence, tertiary structure and catalytic mechanism. The promiscuous phosphoribosylanthranilate isomerase activity of the ASKA PurF variant apparently stems from a preexisting affinity for phosphoribosylated substrates. The relative fitness of the (His)(6)-PurF/Delta trpF strain was improved 4.8-fold to nearly wild-type levels by random mutagenesis of purF and genetic selection. The evolved and ancestral PurF proteins were purified and reacted with phosphoribosylanthranilate in vitro. The best evolvant (k(cat)/K(M)=0.3 s(-1) M(-1)) was approximately 25-fold more efficient than its ancestor but >10(7)-fold less efficient than the wild-type phosphoribosylanthranilate isomerase. These observations demonstrate in quantitative terms that the weak secondary activities of promiscuous enzymes can dramatically improve the fitness of contemporary organisms.

Metabolic engineering of the purine pathway for riboflavin production in Ashbya gossypii.

Purine nucleotides are essential precursors for living organisms because they are involved in many important processes, such as nucleic acid synthesis, energy supply, and the biosynthesis of several amino acids and vitamins such as riboflavin. GTP is the immediate precursor for riboflavin biosynthesis, and its formation through the purine pathway is subject to several regulatory mechanisms in different steps. Extracellular purines repress the transcription of most genes required for de novo ATP and GTP synthesis. Additionally, three enzymes of the pathway, phosphoribosyl pyrophosphate (PRPP) amidotransferase, adenylosuccinate synthetase, and IMP dehydrogenase, are subject to feedback inhibition by their end products. Here we report the characterization and manipulation of the committed step in the purine pathway of the riboflavin overproducer Ashbya gossypii. We report that phosphoribosylamine biosynthesis in A. gossypii is negatively regulated at the transcriptional level by extracellular adenine. Furthermore, we show that ATP and GTP exert a strong inhibitory effect on the PRPP amidotransferase from A. gossypii. We constitutively overexpressed the AgADE4 gene encoding PRPP amidotransferase in A. gossypii, thereby abolishing the adenine-mediated transcriptional repression. In addition, we replaced the corresponding residues (aspartic acid310, lysine333, and alanine417) that have been described to be important for PRPP amidotransferase feedback inhibition in other organisms by site-directed mutagenesis. With these manipulations, we managed to enhance metabolic flow through the purine pathway and to increase the production of riboflavin in the triple mutant strain 10-fold (228 mg/liter).

Channeling of substrates and intermediates in enzyme-catalyzed reactions.

The three-dimensional structures of tryptophan synthase, carbamoyl phosphate synthetase, glutamine phosphoribosylpyrophosphate amidotransferase, and asparagine synthetase have revealed the relative locations of multiple active sites within these proteins. In all of these polyfunctional enzymes, a product formed from the catalytic reaction at one active site is a substrate for an enzymatic reaction at a distal active site. Reaction intermediates are translocated from one active site to the next through the participation of an intermolecular tunnel. The tunnel in tryptophan synthase is approximately 25 A in length, whereas the tunnel in carbamoyl phosphate synthetase is nearly 100 A long. Kinetic studies have demonstrated that the individual reactions are coordinated through allosteric coupling of one active site with another. The participation of these molecular tunnels is thought to protect reactive intermediates from coming in contact with the external medium.

Structural comparison of Ntn-hydrolases.

The Ntn-hydrolases (N-terminal nucleophile) are a superfamily of diverse enzymes that has recently been characterized. All of the proteins in this family are activated autocatalytically; they contain an N-terminally located catalytic nucleophile, and they cleave an amide bond. In the present study, the structures of four enzymes of this superfamily are compared in more detail. Although the amino acid sequence homology is almost completely absent, the enzymes share a similar alphabeta betaalpha-core structure. The central beta-sheets in the core were found to have different packing angles, ranging from 5 to 35 degrees. In the Ntn-hydrolases under study, eight totally conserved secondary structure units were found (region C). Five of them were observed to contain the greatest number of conserved and functionally important residues and are therefore crucial for the structure and function of Ntn-hydrolases. Two additional regions, consisting of secondary structure units (regions A and B), were found to be in structurally similar locations, but in different orders in the polypeptide chain. The catalytic machinery is located in the structures in a similar manner, and thus the catalytic mechanisms of all of the enzymes are probably similar. However, the substrate binding and the oxyanion hole differed partially.

Interdomain signaling in glutamine phosphoribosylpyrophosphate amidotransferase.

The glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase-catalyzed synthesis of phosphoribosylamine from PRPP and glutamine is the sum of two half-reactions at separated catalytic sites in different domains. Binding of PRPP to a C-terminal phosphoribosyltransferase domain is required to activate the reaction at the N-terminal glutaminase domain. Interdomain signaling was monitored by intrinsic tryptophan fluorescence and by measurements of glutamine binding and glutamine site catalysis. Enzymes were engineered to contain a single tryptophan fluorescence reporter in key positions in the glutaminase domain. Trp(83) in the glutamine loop (residues 73-84) and Trp(482) in the C-terminal helix (residues 471-492) reported fluorescence changes in the glutaminase domain upon binding of PRPP and glutamine. The fluorescence changes were perturbed by Ile(335) and Tyr(74) mutations that disrupt interdomain signaling. Fluoresence titrations of PRPP and glutamine binding indicated that signaling defects increased the K(d) for glutamine but had little or no effect on PRPP binding. It was concluded that the contact between Ile(335) in the phosphoribosyltransferase domain and Tyr(74) in the glutamine site is a primary molecular interaction for interdomain signaling. Analysis of enzymes with mutations in the glutaminase domain C-terminal helix and a 404-420 peptide point to additional signaling interactions that activate the glutamine site when PRPP binds.

Three-dimensional structure of Escherichia coli asparagine synthetase B: a short journey from substrate to product.

Asparagine synthetase B catalyzes the assembly of asparagine from aspartate, Mg(2+)ATP, and glutamine. Here, we describe the three-dimensional structure of the enzyme from Escherichia colidetermined and refined to 2.0 A resolution. Protein employed for this study was that of a site-directed mutant protein, Cys1Ala. Large crystals were grown in the presence of both glutamine and AMP. Each subunit of the dimeric protein folds into two distinct domains. The N-terminal region contains two layers of antiparallel beta-sheet with each layer containing six strands. Wedged between these layers of sheet is the active site responsible for the hydrolysis of glutamine. Key side chains employed for positioning the glutamine substrate within the binding pocket include Arg 49, Asn 74, Glu 76, and Asp 98. The C-terminal domain, responsible for the binding of both Mg(2+)ATP and aspartate, is dominated by a five-stranded parallel beta-sheet flanked on either side by alpha-helices. The AMP moiety is anchored to the protein via hydrogen bonds with O(gamma) of Ser 346 and the backbone carbonyl and amide groups of Val 272, Leu 232, and Gly 347. As observed for other amidotransferases, the two active sites are connected by a tunnel lined primarily with backbone atoms and hydrophobic and nonpolar amino acid residues. Strikingly, the three-dimensional architecture of the N-terminal domain of asparagine synthetase B is similar to that observed for glutamine phosphoribosylpyrophosphate amidotransferase while the molecular motif of the C-domain is reminiscent to that observed for GMP synthetase.

Tryptophan fluorescence monitors multiple conformational changes required for glutamine phosphoribosylpyrophosphate amidotransferase interdomain signaling and catalysis.

Single tryptophan residues were incorporated into each of three peptide segments that play key roles in the structural transition of ligand-free, inactive glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase to the active enzyme-substrate complex. Intrinsic tryptophan fluorescence and fluorescence quenching were used to monitor changes in a phosphoribosyltransferase (PRTase) "flexible loop", a "glutamine loop", and a C-terminal helix. Steady state fluorescence changes resulting from substrate binding were used to calculate binding constants and to detect the structural rearrangements that coordinate reactions at active sites for glutamine hydrolysis and PRTase catalysis. Pre-steady state kinetics of enzyme.PRPP and enzyme.PRPP.glutamine complex formation were determined from stopped-flow fluorescence measurements. The kinetics of the formation of the enzyme.PRPP complex were consistent with a model with two or more steps in which rapid equilibrium binding of PRPP is followed by a slow enzyme isomerization. This isomerization is ascribed to the closing of the PRTase flexible loop and is likely the rate-limiting step in the reaction of PRPP with NH(3). The pre-steady state kinetics for binding glutamine to the binary enzyme. PRPP complex could also be fit to a model involving rapid equilibrium binding of glutamine followed by an enzyme isomerization step. The changes monitored by fluorescence account for the interconversions between "end state" structures determined previously by X-ray crystallography and define an intermediate enzyme.PRPP conformer.

Purine biosynthesis de novo in bovine retina: purification and characterization of amidophosphoribosyl transferase and phosphoribosyl pyrophosphate synthetase.

The ability of bovine retina to synthesize purines de novo is shown for the first time. Amidophosphoribosyl transferase (EC 2.4.2.14), the enzyme controlling the rate of the process, and phosphoribosyl pyrophosphate synthetase (EC 2.7.6.1), the enzyme regulating the intracellular contents of phosphoribosyl pyrophosphate (PRPP), were purified and characterized. The molecular masses of the enzyme subunits are similar to those of the purified enzyme from the liver. The molecular masses of amidophosphoribosyl transferase, PRPP synthetase catalytic subunit, and two PRPP synthetase-associated proteins are 50, 34, 39, and 41 kD, respectively. The apparent Km values of the enzymes and coenzymes are similar to those of the purified enzymes from the liver. For amidophosphoribosyl transferase, the apparent Km for Gln and PRPP are 0.75 +/- 0.05 and 0.66 +/- 0.09 mM, respectively (the corresponding Vmax values are 59 +/- 3 and 136 +/- 12 nmoles PPi/min per mg protein). For PRPP synthetase, the apparent Km for ribose-5-phosphate and ATP are 37.9 +/- 0.5 and 53 +/- 7 microM, respectively (the corresponding Vmax values are 61 +/- 4 and 52 +/- 3 nmoles PRPP/min per mg protein). The sensitivity of the retinal PRPP synthetase to inhibition by ADP and AMP was significantly lower than that of the enzyme from the liver.

The amidotransferase family of enzymes: molecular machines for the production and delivery of ammonia.

The amidotransferase family of enzymes utilizes the ammonia derived from the hydrolysis of glutamine for a subsequent chemical reaction catalyzed by the same enzyme. The ammonia intermediate does not dissociate into solution during the chemical transformations. A well-characterized example of the structure and mechanism displayed by this class of enzymes is provided by carbamoyl phosphate synthetase (CPS). Carbamoyl phosphate synthetase is isolated from Escherichia coli as a heterodimeric protein. The smaller of the two subunits catalyzes the hydrolysis of glutamine to glutamate and ammonia. The larger subunit catalyzes the formation of carbamoyl phosphate using 2 mol of ATP, bicarbonate, and ammonia. Kinetic investigations have led to a proposed chemical mechanism for this enzyme that requires carboxy phosphate, ammonia, and carbamate as kinetically competent reaction intermediates. The three-dimensional X-ray crystal structure of CPS has localized the positions of three active sites. The nucleotide binding site within the N-terminal half of the large subunit is required for the phosphorylation of bicarbonate and subsequent formation of carbamate. The nucleotide binding site within the C-terminal domain of the large subunit catalyzes the phosphorylation of carbamate to the final product, carbamoyl phosphate. The three active sites within the heterodimeric protein are separated from one another by about 45 A. The ammonia produced within the active site of the small subunit is the substrate for reaction with the carboxy phosphate intermediate that is formed in the active site found within the N-terminal half of the large subunit of CPS. Since the ammonia does not dissociate from the protein prior to its reaction with carboxy phosphate, this intermediate must therefore diffuse through a molecular tunnel that connects these two sites with one another. Similarly, the carbamate intermediate, initially formed at the active site within the N-terminal half of the large subunit, is the substrate for phosphorylation by the ATP bound to the active site located in the C-terminal half of the large subunit. A molecular passageway has been identified by crystallographic methods that apparently facilitates diffusion between these two active sites within the large subunit of CPS. Synchronization of the chemical transformations is controlled by structural perturbations among the three active sites. Molecular tunnels between distant active sites have also been identified in tryptophan synthase and glutamine phosphoribosyl pyrophosphate amidotransferase and are likely architectural features in an expanding list of enzymes.

Mutational analysis of Bacillus subtilis glutamine phosphoribosylpyrophosphate amidotransferase propeptide processing.

Glutamine phosphoribosylpyrophosphate amidotransferase from Bacillus subtilis is a member of an N-terminal nucleophile hydrolase enzyme superfamily, several of which undergo autocatalytic propeptide processing to generate the mature active enzyme. A series of mutations was analyzed to determine whether amino acid residues required for catalysis are also used for propeptide processing. Propeptide cleavage was strongly inhibited by replacement of the cysteine nucleophile and two residues of an oxyanion hole that are required for glutaminase function. However, significant propeptide processing was retained in a deletion mutant with multiple defects in catalysis that was devoid of enzyme activity. Intermolecular processing of noncleaved mutant enzyme subunits by active wild-type enzyme subunits was not detected in hetero-oligomers obtained from a coexpression experiment. While direct in vitro evidence for autocatalytic propeptide cleavage was not obtained, the results indicate that some but not all of the amino acid residues that have a role in catalysis are also needed for propeptide processing.

Crystal structure of glutamine phosphoribosylpyrophosphate amidotransferase from Escherichia coli.

Crystal structures of glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase from Escherichia coli have been determined to 2.0-A resolution in the absence of ligands, and to 2.5-A resolution with the feedback inhibitor AMP bound to the PRPP catalytic site. Glutamine PRPP amidotransferase (GPATase) employs separate catalytic domains to abstract nitrogen from the amide of glutamine and to transfer nitrogen to the acceptor substrate PRPP. The unliganded and AMP-bound structures, which are essentially identical, are interpreted as the inhibited form of the enzyme because the two active sites are disconnected and the PRPP active site is solvent exposed. The structures were compared with a previously reported 3.0-A structure of the homologous Bacillus subtilis enzyme (Smith JL et al., 1994, Science 264:1427-1433). The comparison indicates a pattern of conservation of peptide structures involved with catalysis and variability in enzyme regulatory functions. Control of glutaminase activity, communication between the active sites, and regulation by feedback inhibitors are addressed differently by E. coli and B. subtilis GPATases. The E. coli enzyme is a prototype for the metal-free GPATases, whereas the B. subtilis enzyme represents the metal-containing enzymes. The structure of the E. coli enzyme suggests that a common ancestor of the two enzyme subfamilies may have included an Fe-S cluster.

Glutamine PRPP amidotransferase: snapshots of an enzyme in action.

Recent studies of glutamine PRPP amidotransferase have provided a new understanding of the function and mechanism of this rather complicated enzyme that may be a paradigm for other complex enzymes. New insights have been gained into the mechanisms of catalysis in the active sites of the two half-reactions, catalytic coupling, allosteric control by feedback inhibitors and the channeling of reaction and metabolic intermediates.