Structural Basis of Diverse Substrate Recognition by the Enzyme PMM/PGM from P. aeruginosa

A deeply divergent phosphoglucomutase (PGM) of Giardia lamblia has both PGM and phosphomannomutase activities

Giardia lamblia, which is an important parasitic cause of diarrhea, uses activated forms of glucose to make glycogen and activated forms of mannose to make glycophosphosphoinositol anchors. A necessary step for glucose activation is isomerization of glucose-6-phosphate to glucose-1-phosphate by a phosphoglucomutase (PGM). Similarly, a phosphomannomutase (PMM) converts mannose-6-phosphate to mannose-1-phosphate. While whole genome sequences of Giardia predict two PGM candidates, no PMM candidate is present. The hypothesis tested here is that at least one of the two Giardia PGM candidates has both PGM and PMM activity, as has been described for bacterial PGM orthologs. Nondenaturing gels showed that Giardia has two proteins with PGM activity, one of which also has PMM activity. Phylogenetic analyses showed that one of the two Giardia PGM candidates (Gl-PGM1) shares recent common ancestry with other eukaryotic PGMs, while the other Giardia PGM candidate (Gl-PGM2) is deeply divergent. Both Gl-PGM1 and Gl-PGM2 rescue a Saccharomyces cerevisiae pgm1Delta/pgm2Delta double deletion strain, while only Gl-PGM2 rescues a temperature-sensitive PMM mutant of S. cerevisiae (sec53-ts). Recombinant Gl-PGM1 has PGM activity only, whereas Gl-PGM2 has both PGM and PMM activities. We conclude that Gl-PGM1 behaves as a conventional eukaryotic PGM, while Gl-PGM2 is a novel eukaryotic PGM that also has PMM activity.

On the origin of life in the zinc world. 2. Validation of the hypothesis on the photosynthesizing zinc sulfide edifices as cradles of life on Earth

BACKGROUND

The accompanying article (A.Y. Mulkidjanian, Biology Direct 4:26) puts forward a detailed hypothesis on the role of zinc sulfide (ZnS) in the origin of life on Earth. The hypothesis suggests that life emerged within compartmentalized, photosynthesizing ZnS formations of hydrothermal origin (the Zn world), assembled in sub-aerial settings on the surface of the primeval Earth.

RESULTS

If life started within photosynthesizing ZnS compartments, it should have been able to evolve under the conditions of elevated levels of Zn2+ ions, byproducts of the ZnS-mediated photosynthesis. Therefore, the Zn world hypothesis leads to a set of testable predictions regarding the specific roles of Zn2+ ions in modern organisms, particularly in RNA and protein structures related to the procession of RNA and the "evolutionarily old" cellular functions. We checked these predictions using publicly available data and obtained evidence suggesting that the development of the primeval life forms up to the stage of the Last Universal Common Ancestor proceeded in zinc-rich settings. Testing of the hypothesis has revealed the possible supportive role of manganese sulfide in the primeval photosynthesis. In addition, we demonstrate the explanatory power of the Zn world concept by elucidating several points that so far remained without acceptable rationalization. In particular, this concept implies a new scenario for the separation of Bacteria and Archaea and the origin of Eukarya.

CONCLUSION

The ability of the Zn world hypothesis to generate non-trivial veritable predictions and explain previously obscure items gives credence to its key postulate that the development of the first life forms started within zinc-rich formations of hydrothermal origin and was driven by solar UV irradiation. This concept implies that the geochemical conditions conducive to the origin of life may have persisted only as long as the atmospheric CO2 pressure remained above ca. 10 bar. This work envisions the first Earth biotopes as photosynthesizing and habitable areas of porous ZnS and MnS precipitates around primeval hot springs. Further work will be needed to provide details on the life within these communities and to elucidate the primordial (bio)chemical reactions.

REVIEWERS

This article was reviewed by Arcady Mushegian, Eugene Koonin, and Patrick Forterre. For the full reviews, please go to the Reviewers' reports section.

Crystallization and initial crystallographic analysis of phosphoglucosamine mutase from Bacillus anthracis

The enzyme phosphoglucosamine mutase catalyzes the conversion of glucosamine 6-phosphate to glucosamine 1-phosphate, an early step in the formation of the nucleotide sugar UDP-N-acetylglucosamine, which is involved in peptidoglycan biosynthesis. These enzymes are part of the large alpha-D-phosphohexomutase enzyme superfamily, but no proteins from the phosphoglucosamine mutase subgroup have been structurally characterized to date. Here, the crystallization of phosphoglucosamine mutase from Bacillus anthracis in space group P3(2)21 by hanging-drop vapor diffusion is reported. The crystals diffracted to 2.7 A resolution under cryocooling conditions. Structure determination by molecular replacement was successful and refinement is under way. The crystal structure of B. anthracis phosphoglucosamine mutase should shed light on the substrate-specificity of these enzymes and will also serve as a template for inhibitor design.

Phosphomannose isomerase and phosphomannomutase gene disruptions in Streptomyces nodosus: impact on amphotericin biosynthesis and implications for glycosylation engineering

Streptomycetes synthesise several bioactive natural products that are modified with sugar residues derived from GDP-mannose. These include the antifungal polyenes, the antibacterial antibiotics hygromycin A and mannopeptimycins, and the anticancer agent bleomycin. Three enzymes function in biosynthesis of GDP-mannose from the glycolytic intermediate fructose 6-phosphate: phosphomannose isomerase (PMI), phosphomannomutase (PMM) and GDP-mannose pyrophosphorylase (GMPP). Synthesis of GDP-mannose from exogenous mannose requires hexokinase or phosphotransferase enzymes together with PMM and GMPP. In this study, a region containing genes for PMI, PMM and GMPP was cloned from Streptomyces nodosus, producer of the polyenes amphotericins A and B. Inactivation of the manA gene for PMI resulted in production of amphotericins and their aglycones, 8-deoxyamphoteronolides. A double mutant lacking the PMI and PMM genes produced 8-deoxyamphoteronolides in good yields along with trace levels of glycosylated amphotericins. With further genetic engineering these mutants may activate alternative hexoses as GDP-sugars for transfer to aglycones in vivo.

Convergent evolution of enzyme active sites is not a rare phenomenon

Since convergent evolution of enzyme active sites was first identified in serine proteases, other individual instances of this phenomenon have been documented. However, a systematic analysis assessing the frequency of this phenomenon across enzyme space is still lacking. This work uses the Query3d structural comparison algorithm to integrate for the first time detailed knowledge about catalytic residues, available through the Catalytic Site Atlas (CSA), with the evolutionary information provided by the Structural Classification of Proteins (SCOP) database. This study considers two modes of convergent evolution: (i) mechanistic analogues which are enzymes that use the same mechanism to perform related, but possibly different, reactions (considered here as sharing the first three digits of the EC number); and (ii) transformational analogues which catalyse exactly the same reaction (identical EC numbers), but may use different mechanisms. Mechanistic analogues were identified in 15% (26 out of 169) of the three-digit EC groups considered, showing that this phenomenon is not rare. Furthermore 11 of these groups also contain transformational analogues. The catalytic triad is the most widespread active site; the results of the structural comparison show that this mechanism, or variations thereof, is present in 23 superfamilies. Transformational analogues were identified for 45 of the 951 four-digit EC numbers present within the CSA and about half of these were also mechanistic analogues exhibiting convergence of their active sites. This analysis has also been extended to the whole Protein Data Bank to provide a complete and manually curated list of the all the transformational analogues whose structure is classified in SCOP. The results of this work show that the phenomenon of convergent evolution is not rare, especially when considering large enzymatic families.

Complexes of the enzyme phosphomannomutase/phosphoglucomutase with a slow substrate and an inhibitor

Two complexes of the enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa with a slow substrate and with an inhibitor have been characterized by X-ray crystallography. Both ligands induce an interdomain rearrangement in the enzyme that creates a highly buried active site. Comparisons with enzyme-substrate complexes show that the inhibitor xylose 1-phosphate utilizes many of the previously observed enzyme-ligand interactions. In contrast, analysis of the ribose 1-phosphate complex reveals a combination of new and conserved enzyme-ligand interactions for binding. The ability of PMM/PGM to accommodate these two pentose phosphosugars in its active site may be relevant for future efforts towards inhibitor design.

Conformational Cycling in β-Phosphoglucomutase Catalysis: Reorientation of the β-d-Glucose 1,6-(Bis)phosphate Intermediate

Activated Lactococcus lactis beta-phosphoglucomutase (betaPGM) catalyzes the conversion of beta-d-glucose 1-phosphate (betaG1P) derived from maltose to beta-d-glucose 6-phosphate (G6P). Activation requires Mg(2+) binding and phosphorylation of the active site residue Asp8. Initial velocity techniques were used to define the steady-state kinetic constants k(cat) = 177 +/- 9 s(-)(1), K(m) = 49 +/- 4 microM for the substrate betaG1P and K(m) = 6.5 +/- 0.7 microM for the activator beta-d-glucose 1,6-bisphosphate (betaG1,6bisP). The observed transient accumulation of [(14)C]betaG1,6bisP (12% at approximately 0.1 s) in the single turnover reaction carried out with excess betaPGM (40 microM) and limiting [(14)C]betaG1P (5 microM) and betaG1,6bisP (5 microM) supported the role of betaG1,6bisP as a reaction intermediate in the conversion of the betaG1P to G6P. Single turnover reactions of [(14)C]betaG1,6bisP with excess betaPGM were carried out to demonstrate that phosphoryl transfer rather than ligand binding is rate-limiting and to show that the betaG1,6bisP binds to the active site in two different orientations (one positioning the C(1)phosphoryl group for reaction with Asp8, and the other orientation positioning the C(6)phosphoryl group for reaction with Asp8) with roughly the same efficiency. Single turnover reactions carried out with betaPGM, [(14)C]betaG1P, and unlabeled betaG1,6bisP demonstrated complete exchange of label to the betaG1,6bisP during the catalytic cycle. Thus, the reorientation of the betaG1,6bisP intermediate that is required to complete the catalytic cycle occurs by diffusion into solvent followed by binding in the opposite orientation. Published X-ray structures of betaG1P suggest that the reorientation and phosphoryl transfer from betaG1,6bisP occur by conformational cycling of the enzyme between the active site open and closed forms via cap domain movement. Last, the equilibrium ratio of betaG1,6bisP to betaG1P plus G6P was examined to evidence a significant stabilization of betaPGM aspartyl phosphate.

The Reaction of Phosphohexomutase from Pseudomonas aeruginosa

The enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa catalyzes the reversible conversion of 1-phospho to 6-phospho-sugars. The reaction entails two phosphoryl transfers, with an intervening 180 degrees reorientation of the reaction intermediate (e.g. glucose 1,6-bisphosphate) during catalysis. Reorientation of the intermediate occurs without dissociation from the active site of the enzyme and is, thus, a simple example of processivity, as defined by multiple rounds of catalysis without release of substrate. Structural characterization of two PMM/PGM-intermediate complexes with glucose 1,6-bisphosphate provides new insights into the reaction catalyzed by the enzyme, including the reorientation of the intermediate. Kinetic analyses of site-directed mutants prompted by the structural studies reveal active site residues critical for maintaining association with glucose 1,6-bisphosphate during its unique dynamic reorientation in the active site of PMM/PGM.

Crystal structures of N-acetylglucosamine-phosphate mutase, a member of the alpha-D-phosphohexomutase superfamily, and its substrate and product complexes

N-acetylglucosamine-phosphate mutase (AGM1) is an essential enzyme in the synthetic process of UDP-N-acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc is a UDP sugar that serves as a biosynthetic precursor of glycoproteins, mucopolysaccharides, and the cell wall of bacteria. Thus, a specific inhibitor of AGM1 from pathogenetic fungi could be a new candidate for an antifungal reagent that inhibits cell wall synthesis. AGM1 catalyzes the conversion of N-acetylglucosamine 6-phosphate (GlcNAc-6-P) into N-acetylglucosamine 1-phosphate (GlcNAc-1-P). This enzyme is a member of the alpha-D-phosphohexomutase superfamily, which catalyzes the intramolecular phosphoryl transfer of sugar substrates. Here we report the crystal structures of AGM1 from Candida albicans for the first time, both in the apoform and in the complex forms with the substrate and the product, and discuss its catalytic mechanism. The structure of AGM1 consists of four domains, of which three domains have essentially the same fold. The overall structure is similar to those of phosphohexomutases; however, there are two additional beta-strands in domain 4, and a circular permutation occurs in domain 1. The catalytic cleft is formed by four loops from each domain. The N-acetyl group of the substrate is recognized by Val-370 and Asn-389 in domain 3, from which the substrate specificity arises. By comparing the substrate and product complexes, it is suggested that the substrate rotates about 180 degrees on the axis linking C-4 and the midpoint of the C-5-O-5 bond in the reaction.

Aedes aegypti phosphohexomutases and uridine diphosphate-hexose pyrophosphorylases: comparison of primary sequences, substrate specificities and temporal transcription

Phosphohexomutases reversibly catalyse the transfer of the phosphate group of a glycosyl phosphate between the C6 and C1 positions, and uridine diphosphate (UDP)-hexose pyrophosphorylases catalyse the synthesis of UDP-hexose from uridine triphosphate (UTP) and hexose-1-phosphate. Both enzyme families are essential for nucleoside diphosphate hexose biosynthesis and are therefore critical for various physiological functions in the midgut of mosquitoes after a blood meal. We cloned and sequenced three phosphohexomutase and two UDP-hexose pyrophosphorylase cDNAs from Aedes aegypti. The products of the cDNAs were expressed and substrate specificities were examined. Herein we describe Ae. aegypti phosphoglucomutase 1, phosphoglucomutase 2, phosphoacetylglucosamine mutase, UDP-glucose pyrophosphorylase, and UDP-N-acetylglucosamine pyrophosphorylase. Transcripts of the genes expressing the enzymes are constitutively present in all life stages and blood-feeding does not seem to influence transcript abundance.

Active site sharing and subterminal hairpin recognition in a new class of DNA transposases

Many bacteria harbor simple transposable elements termed insertion sequences (IS). In Helicobacter pylori, the chimeric IS605 family elements are particularly interesting due to their proximity to genes encoding gastric epithelial invasion factors. Protein sequences of IS605 transposases do not bear the hallmarks of other well-characterized transposases. We have solved the crystal structure of full-length transposase (TnpA) of a representative member, ISHp608. Structurally, TnpA does not resemble any characterized transposase; rather, it is related to rolling circle replication (RCR) proteins. Consistent with RCR, Mg2+ and a conserved tyrosine, Tyr127, are essential for DNA nicking and the formation of a covalent intermediate between TnpA and DNA. TnpA is dimeric, contains two shared active sites, and binds two DNA stem loops representing the conserved inverted repeats near each end of ISHp608. The cocrystal structure with stem-loop DNA illustrates how this family of transposases specifically recognizes and pairs ends, necessary steps during transposition.

Dual substrate recognition of aminotransferases

Pyridoxal 5'-phosphate-dependent aminotransferases reversibly catalyzes the transamination reaction in which the alpha-amino group of amino acid 1 is transferred to the 2-oxo acid of amino acid 2 (usually 2-oxoglutarate) to produce the 2-oxo acid of amino acid 1 and amino acid 2 (glutamate). An aminotransferase must thus be able to recognize and bind two kinds of amino acids (amino acids 1 and 2), the side chains of which are different in shape and properties, from among many other small molecules. The dual substrate recognition mechanism has been discovered based on three-dimensional structures of aromatic amino acids, histidinol phosphate, glutamine:phenylpyruvate, acetylornithine, and branched-chain amino acid aminotransferases. There are two representative strategies for dual substrate recognition. An aromatic amino acid aminotransferase prepares charged and neutral pockets for acidic and aromatic side chains, respectively, at the same place by a large-scale rearrangement of the hydrogen-bond network caused by the induced fit. In a branched-chain aminotransferase, the same hydrophobic cavity implanted with hydrophilic sites accommodates both hydrophobic and acidic side chains without side-chain rearrangements of the active-site residues, which is reminiscent of the lock and key mechanism. Dual substrate recognition in other aminotransferases is attained by combining the two representative methods.

Characterization of a Thermostable Enzyme with Phosphomannomutase/Phosphoglucomutase Activities from the Hyperthermophilic Archaeon Pyrococcus horikoshii OT3

The phosphomannomutase/phosphoglucomutase (PMM/PGM) enzyme catalyzes reversibly the intra-molecular phosphoryl interconverting reaction of mannose-6-phosphate and mannose-1-phosphate or glucose-6-phosphate and glucose-1-phosphate. Glucose-6-phosphate and glucose-1-phosphate are known to be utilized for energy metabolism and cell surface construction, respectively. PMM/PGM has been isolated from many microorganisms. By performing similarity searches using existing PMM/PGM sequences, the homologous ORFs PH0923 and PH1210 were identified from the genomic data of Pyrococcus horikoshii OT3. Since PH0923 appears to be part of an operon consisting of four carbohydrate metabolic enzymes, PH0923 was selected as the first target for the investigation of PMM/PGM activity in P. horikoshii OT3. The coding region of PH0923 was cloned and the purified recombinant protein was utilized for an examination of its biochemical properties. The enzyme retained half its initial activity after treatment at 95 degrees C for 90 min. Detailed analyses of activities showed that this protein is capable of utilizing a variety of metal ions that are not utilized by previously characterized PMM/PGM proteins. A mutated protein with an alanine residue replacing the active site serine residue indicated that this residue plays an important but non-essential role in PMM/PGM activity.

A phosphohexomutase from the archaeon Sulfolobus solfataricus is covalently modified by phosphorylation on serine

A phosphoserine-containing peptide was identified from tryptic digests from Sulfolobus solfataricus P1 by liquid chromatography-tandem mass spectrometry. Its amino acid sequence closely matched that bracketing Ser-309 in the predicted protein product of open reading frame sso0207, a putative phosphohexomutase, in the genome of S. solfataricus P2. Open reading frame sso0207 was cloned, and its protein product expressed in Escherichia coli. The recombinant protein proved capable of interconverting mannose 1-phosphate and mannose 6-phosphate, as well as glucose 1-phosphate and glucose 6-phosphate, in vitro. It displayed no catalytic activity toward glucosamine 6-phosphate or N-acetylglucosamine 6-phosphate. Models constructed using the X-ray crystal structure of a homologous phosphohexomutase from Pseudomonas aeruginosa predicted that Ser-309 of the archaeal protein lies within the substrate binding site. The presence of a phosphoryl group at this location would be expected to electrostatically interfere with the binding of negatively charged phosphohexose substrates, thus attenuating the catalytic efficiency of the enzyme. Using site-directed mutagenesis, Ser-309 was substituted by aspartic acid to mimic the presence of a phosphoryl group. The V(max) of the mutationally altered protein was only 4% that of the unmodified form. Substitution of Ser-309 with larger, but uncharged, amino acids, including threonine, also decreased catalytic efficiency, but to a lesser extent--three- to fivefold. We therefore predict that phosphorylation of the enzyme in vivo serves to regulate its catalytic activity.

Understanding the control of Pseudomonas aeruginosa alginate synthesis and the prospects for management of chronic infections in cystic fibrosis

Decades of research have been dedicated to the study of the opportunistic pathogen Pseudomonas aeruginosa, a Gram-negative, environmental bacterium that secretes the exopolysaccharide alginate during chronic lung infection of cystic fibrosis (CF) patients. Although P. aeruginosa utilizes a variety of factors to establish a successful infection in the lungs of CF patients, alginate has stood out as one of the best-studied prognostic indicators of chronic lung infection. While the genetics, biosynthesis and regulation of alginate are well understood, questions still remain concerning its role in biofilm development and its potential as a therapeutic target. The purpose of this review is to provide a brief summary of alginate biosynthesis and regulation, and to highlight recent discoveries in the areas of alginate production, biofilm formation and vaccine design. This information is placed in context with a proposed P. aeruginosa infectious pathway, highlighting avenues for the use of existing therapies as well as the potential for novel agents to reduce or eliminate chronic infections in CF patients.

Catalytic Cycling in β-Phosphoglucomutase: A Kinetic and Structural Analysis,

Lactococcus lactis beta-phosphoglucomutase (beta-PGM) catalyzes the interconversion of beta-d-glucose 1-phosphate (beta-G1P) and beta-d-glucose 6-phosphate (G6P), forming beta-d-glucose 1,6-(bis)phosphate (beta-G16P) as an intermediate. Beta-PGM conserves the core domain catalytic scaffold of the phosphatase branch of the HAD (haloalkanoic acid dehalogenase) enzyme superfamily, yet it has evolved to function as a mutase rather than as a phosphatase. This work was carried out to identify the structural basis underlying this diversification of function. In this paper, we examine beta-PGM activation by the Mg(2+) cofactor, beta-PGM activation by Asp8 phosphorylation, and the role of cap domain closure in substrate discrimination. First, the 1.90 A resolution X-ray crystal structure of the Mg(2+)-beta-PGM complex is examined in the context of previously reported structures of the Mg(2+)-alpha-d-galactose-1-phosphate-beta-PGM, Mg(2+)-phospho-beta-PGM, and Mg(2+)-beta-glucose-6-phosphate-1-phosphorane-beta-PGM complexes to identify conformational changes that occur during catalytic turnover. The essential role of Asp8 in nucleophilic catalysis was confirmed by demonstrating that the D8A and D8E mutants are devoid of catalytic activity. Comparison of the ligands to Mg(2+) in the different complexes shows that a single Mg(2+) coordination site must alternatively accommodate water, phosphate, and the phosphorane intermediate during catalytic turnover. Limited involvement of the HAD family metal-binding loop in Mg(2+) anchoring in beta-PGM is consistent with the relatively loose binding indicated by the large K(m) for Mg(2+) activation (270 +/- 20 microM) and with the retention of activity found in the E169A/D170A double loop mutant. Comparison of the relative positions of cap and core domains in the different complexes indicated that interaction of cap domain Arg49 with the "nontransferring" phosphoryl group of the substrate ligand might stabilize the cap-closed conformation, as required for active site desolvation and alignment of Asp10 for acid-base catalysis. Kinetic analyses of the specificity of beta-PGM toward phosphoryl group donors and the specificity of phospho-beta-PGM toward phosphoryl group acceptors were carried out. The results support a substrate induced-fit mechanism of beta-PGM catalysis, which allows phosphomutase activity to dominate over the intrinsic phosphatase activity. Last, we present evidence that the autophosphorylation of beta-PGM by the substrate beta-G1P accounts for the origin of phospho-beta-PGM in the cell.

Formation and Reorientation of Glucose 1,6-Bisphosphate in the PMM/PGM Reaction: Transient-State Kinetic Studies

The interconversion of glucose 1-phosphate and glucose 6-phosphate, catalyzed by Pseudomonas aeruginosa phosphomannomutase/phosphoglucomutase, has been studied by transient-state kinetic techniques. Glucose 1,6-bisphosphate is formed as an intermediate in the reaction, but an obligatory step in the catalytic cycle appears to be the formation of an enzyme-glucose 1,6-bisphosphate complex that is not competent to form either glucose 1-phosphate or glucose 6-phosphate directly. We suggest that during the lifetime of this complex the glucose 1,6-bisphosphate intermediate undergoes the 180 degrees reorientation that is required for completion of the catalytic cycle. The formation of glucose 1,6-bisphosphate from glucose 1-phosphate is in rapid equilibrium relative to the rest of the reaction, where K(eq) = 0.14. In the opposite direction, glucose 1,6-bisphosphate is formed from glucose 6-phosphate with a rate constant of 12 s(-)(1), and the reverse step occurs with a rate constant of 255 s(-)(1). The interconversion of the productive and nonproductive glucose 1,6-bisphosphate complexes occurs with a rate constant of 64 s(-)(1) in one direction and 48 s(-)(1) in the other direction. Glucose 1,6-bisphosphate remains associated with the enzyme during reorientation. Isotope trapping studies indicate that it partitions to form glucose 1-phosphate or glucose 6-phosphate 14.3 times more frequently than it dissociates from the enzyme.

Evolutionary trace analysis of the alpha-D-phosphohexomutase superfamily

The alpha-D-phosphohexomutase superfamily is composed of four related enzymes that catalyze a reversible, intramolecular phosphoryl transfer on their sugar substrates. The enzymes in this superfamily play important and diverse roles in carbohydrate metabolism in organisms from bacteria to humans. Recent structural and mechanistic studies of one member of this superfamily, phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa, have provided new insights into enzyme mechanism and substrate recognition. Here we use sequence-sequence and sequence-structure comparisons via evolutionary trace analysis to examine 71 members of the alpha-D-phosphohexomutase superfamily. These analyses show that key residues in the active site, including many of those involved in substrate contacts in the P. aeruginosa PMM/PGM complexes, are conserved throughout the enzyme family. Several important regions show class-specific differences in sequence that appear to be correlated with differences in substrate specificity exhibited by subgroups of the family. In addition, we describe the translocation of a 20-residue segment containing the catalytic phosphoserine of phosphoacetylglucosamine mutase, which uniquely identifies members of this subgroup.

Detailed Kinetic Studies of an Aggregating Inhibitor; Inhibition of Phosphomannomutase/Phosphoglucomutase by Disperse Blue 56†

Phosphomannomutase/phosphoglucomutase occupies a central position in the pathways by which several virulence factors are synthesized in Pseudomonas aeruginosa. Virtual screening was used to identify potential inhibitors of phosphomannomutase/ phosphoglucomutase, and one compound, the anthraquinone-based dye Disperse Blue 56, showed potent inhibition in vitro. The kinetics of inhibition was complex; the time courses for reactions in the presence of the inhibitor were biphasic, suggestive of slow-binding inhibition. Quantitative analysis of the progress curves and preincubation experiments demonstrated that slow-binding inhibition was not occurring, however. Initial velocity kinetic studies indicated that Disperse Blue 56 was a parabolic, noncompetitve inhibitor. Progress curves for reactions in the presence of Disperse Blue 56 could be fitted very well by a model in which 2 equiv of the inhibitor bound to free enzyme or the enzyme-substrate complex. The inhibition was largely relieved by the inclusion of 0.01% Triton X-100 in the assay solutions, which has been suggested to be the hallmark for inhibition by compounds that exert their effect through aggregates [McGovern, S. L., Caselli, E., Grigorieff, N., and Shiochet, B. K. (2002) J. Med. Chem. 45, 1712-1722]. Our kinetic data appear to be consistent with either inhibition by a dimer of Disperse Blue 56 or inhibition by a Disperse Blue 56 aggregate, but the latter appears much more likely. We present a detailed analysis of the system to provide further information that may help in the recognition of inhibition through aggregation.