Ligand-specific scoring functions: improved ranking of docking solutions.

Molecular docking is a powerful computational method that has been widely used in many biomolecular studies to predict geometry of a protein-ligand complex. However, while its conformational search algorithms are usually able to generate correct conformation of a ligand in the binding site, the scoring methods often fail to discriminate it among many false variants. We propose to treat this problem by applying more precise ligand-specific scoring filters to re-rank docking solutions. In this way specific features of interactions between protein and different types of compounds can be implicitly taken into account. New scoring functions were constructed including hydrogen bonds, hydrophobic and hydrophilic complementarity terms. These scoring functions also discriminate ligands by the size of the molecule, the total hydrophobicity, and the number of peptide bonds for peptide ligands. Weighting coefficients of the scoring functions were adjusted using a training set of 60 protein-ligand complexes. The proposed method was then tested on the results of docking obtained for an additional 70 complexes. In both cases the success rate was 5-8% better compared to the standard functions implemented in popular docking software.

A model for the binding of low molecular weight inhibitors to the active site of thrombin.

This paper describes the construction, validation and application of an active site model of the serine protease thrombin. Initial use was made of medium resolution X-ray crystallographic structures of thrombin complexed with low molecular weight, non-specific inhibitors to create a computationally useable active site shell of the enzyme. Molecular mechanics methods were then applied to dock known ligands into the active site region in order to derive a model that would accurately predict binding conformations. Validation of the modelling process was achieved by comparison of the predicted enzyme-bound conformations with their known, crystallographic binding conformations. The resultant model was used extensively for predictive purposes prior to obtaining confirmatory crystal data relating to a ligand possessing a novel and unexpected binding component complexed to thrombin. The data served both to confirm the accuracy of the binding site model and to provide information for the further refinement of the model.

The 1.2 A crystal structure of hirustasin reveals the intrinsic flexibility of a family of highly disulphide-bridged inhibitors.

BACKGROUND: Leech-derived inhibitors have a prominent role in the development of new antithrombotic drugs, because some of them are able to block the blood coagulation cascade. Hirustasin, a serine protease inhibitor from the leech Hirudo medicinalis, binds specifically to tissue kallikrein and possesses structural similarity with antistasin, a potent factor Xa inhibitor from Haementeria officinalis. Although the 2.4 A structure of the hirustasin-kallikrein complex is known, classical methods such as molecular replacement were not successful in solving the structure of free hirustasin. RESULTS: Ab initio real/reciprocal space iteration has been used to solve the structure of free hirustasin using either 1.4 A room temperature data or 1.2 A low temperature diffraction data. The structure was also solved independently from a single pseudo-symmetric gold derivative using maximum likelihood methods. A comparison of the free and complexed structures reveals that binding to kallikrein causes a hinge-bending motion between the two hirustasin subdomains. This movement is accompanied by the isomerisation of a cis proline to the trans conformation and a movement of the P3, P4 and P5 residues so that they can interact with the cognate protease. CONCLUSIONS: The inhibitors from this protein family are fairly flexible despite being highly cross-linked by disulphide bridges. This intrinsic flexibility is necessary to adopt a conformation that is recognised by the protease and to achieve an optimal fit, such observations illustrate the pitfalls of designing inhibitors based on static lock-and-key models. This work illustrates the potential of new methods of structure solution that require less or even no prior phase information.

Structure of recombinant human CPP32 in complex with the tetrapeptide acetyl-Asp-Val-Ala-Asp fluoromethyl ketone.

The cysteine protease CPP32 has been expressed in a soluble form in Escherichia coli and purified to >95% purity. The three-dimensional structure of human CPP32 in complex with the irreversible tetrapeptide inhibitor acetyl-Asp-Val-Ala-Asp fluoromethyl ketone was determined by x-ray crystallography at a resolution of 2.3 A. The asymmetric unit contains a (p17/p12)2 tetramer, in agreement with the tetrameric structure of the protein in solution as determined by dynamic light scattering and size exclusion chromatography. The overall topology of CPP32 is very similar to that of interleukin-1beta-converting enzyme (ICE); however, differences exist at the N terminus of the p17 subunit, where the first helix found in ICE is missing in CPP32. A deletion/insertion pattern is responsible for the striking differences observed in the loops around the active site. In addition, the P1 carbonyl of the ketone inhibitor is pointing into the oxyanion hole and forms a hydrogen bond with the peptidic nitrogen of Gly-122, resulting in a different state compared with the tetrahedral intermediate observed in the structure of ICE and CPP32 in complex with an aldehyde inhibitor. The topology of the interface formed by the two p17/p12 heterodimers of CPP32 is different from that of ICE. This results in different orientations of CPP32 heterodimers compared with ICE heterodimers, which could affect substrate recognition. This structural information will be invaluable for the design of small synthetic inhibitors of CPP32 as well as for the design of CPP32 mutants.

Characterization, crystallization and preliminary crystallographic analysis of human recombinant cyclooxygenase-2.

Purified recombinant membrane apoprotein cyclooxygenase-2 (COX-2) has been reconstituted with heme and characterized. The holoprotein has been crystallized in complex with the selective inhibitor CGP 28238 [6-(2,4-difluorophenoxy)-5- methylsulfonylamino-l-indanone] by the sitting-drop method of vapor diffusion using polyethylene glycol 2000 monomethyl ether as precipitant, in the presence of the nonionic detergent beta-octylglucoside. The crystals are orthorhombic, belonging to the space group P2(1)2(1)2 with cell dimensions a = 209.56, b = 71.28 and c = 93.82 A, and diffract to 2.5 A resolution. The asymmetric unit contains two COX-2 monomers, as confirmed by the molecular replacement solution and in agreement with the dimeric structure of the detergent-solubilized protein found with dynamic light scattering and size-exclusion chromatography. Structural work is in progress.

A new structural class of serine protease inhibitors revealed by the structure of the hirustasin-kallikrein complex.

BACKGROUND: Hirustasin belongs to a class of serine protease inhibitors characterized by a well conserved pattern of cysteine residues. Unlike the closely related inhibitors, antistasin/ghilanten and guamerin, which are selective for coagulation factor Xa or neutrophil elastase, hirustasin binds specifically to tissue kallikrein. The conservation of the pattern of cysteine residues and the significant sequence homology suggest that these related inhibitors possess a similar three-dimensional structure to hirustasin. RESULTS: The crystal structure of the complex between tissue kallikrein and hirustasin was analyzed at 2.4 resolution. Hirustasin folds into a brick-like structure that is dominated by five disulfide bridges and is sparse in secondary structural elements. The cysteine residues are connected in an abab cdecde pattern that causes the polypeptide chain to fold into two similar motifs. As a hydrophobic core is absent from hirustasin the disulfide bridges maintain the tertiary structure and present the primary binding loop to the active site of the protease. The general structural topography and disulfide connectivity of hirustasin has not previously been described. CONCLUSIONS: The crystal structure of the kallikrein-hirustasin complex reveals that hirustasin differs from other serine protease inhibitors in its conformation and its disulfide bond connectivity, making it the prototype for a new class of inhibitor. The disulfide pattern shows that the structure consists of two domains, but only the C-terminal domain interacts with the protease. The disulfide pattern of the N-terminal domain is related to the pattern found in other proteins. Kallikrein recognizes hirustasin by the formation of an antiparallel beta sheet between the protease and the inhibitor. The P1 arginine binds in a deep negatively charged pocket of the enzyme. An additional pocket at the periphery of the active site accommodates the sidechain of the P4 valine.

Recombinant hirustasin: production in yeast, crystallization, and interaction with serine proteases.

A synthetic gene coding for the 55-amino acid protein hirustasin, a novel tissue kallikrein inhibitor from the leech Hirudo medicinalis, was generated by polymerase chain reaction using overlapping oligonucleotides, fused to the yeast alpha-factor leader sequence and expressed in Saccharomyces cerevisiae. Recombinant hirustasin was secreted mainly as incompletely processed fusion protein, but could be processed in vitro using a soluble variant of the yeast yscF protease. The processed hirustasin was purified to better than 97% purity. N-terminal sequence analysis and electrospray ionization mass spectrometry confirmed a correctly processed N-terminus and the expected amino acid sequence and molecular mass. The biological activity of recombinant hirustasin was identical to that of the authentic leech protein. Crystallized hirustasin alone and in complex with tissue kallikrein diffracted beyond 1.4 A and 2.4 A, respectively. In order to define the reactive site of the inhibitor, the interaction of hirustasin with kallikrein, chymotrypsin, and trypsin was investigated by monitoring complex formation in solution as well as proteolytic cleavage of the inhibitor. During incubation with high, nearly equimolar concentration of tissue kallikrein, hirustasin was cleaved mainly at the peptide bond between Arg 30 and Ile 31, the putative reactive site, to yield a modified inhibitor. In the corresponding complex with chymotrypsin, mainly uncleaved hirustasin was found and cleaved hirustasin species accumulated only slowly. Incubation with trypsin led to several proteolytic cleavages in hirustasin with the primary scissile peptide bond located between Arg 30 and Ile 31. Hirustasin appears to fall into the class of protease inhibitors displaying temporary inhibition.

Structure of the complex of leech-derived tryptase inhibitor (LDTI) with trypsin and modeling of the LDTI-tryptase system.

BACKGROUND: Tryptase is a trypsin-like serine proteinase stored in the cytoplasmic granules of mast cells, which has been implicated in a number of mast cell related disorders such as asthma and rheumatoid arthritis. Unlike almost all other serine proteinases, tryptase is fully active in plasma and in the extracellular space, as there are no known natural inhibitors of tryptase in humans. Leech-derived tryptase inhibitor (LDTI), a protein of 46 amino acids, is the first molecule found to bind tightly to and specifically inhibit human tryptase in the nanomolar range. LDTI also inhibits trypsin and chymotrypsin with similar affinities. The structure of LDTI in complex with an inhibited proteinase could be used as a template for the development of low molecular weight tryptase inhibitors. RESULTS: The crystal structure of the complex between trypsin and LDTI was solved at 2.0 A resolution and a model of the LDTI-tryptase complex was created, based on this X-ray structure. LDTI has a very similar fold to the third domain of the turkey ovomucoid inhibitor. LDTI interacts with trypsin almost exclusively through its binding loop (residues 3-10) and especially through the sidechain of the specificity residue Lys8. Our modeling studies indicate that these interactions are maintained in the LDTI-tryptase complex. CONCLUSIONS: The insertion of nine residues after residue 174 in tryptase, relative to trypsin and chymotrypsin, prevents inhibition by other trypsin inhibitors and is certainly responsible for the higher specificity of tryptase relative to trypsin. In LDTI, the disulfide bond between residues 4 and 25 causes a sharp turn from the binding loop towards the N terminus, holding the N terminus away from the 174 loop of tryptase.

The crystal structure of TGF-beta 3 and comparison to TGF-beta 2: implications for receptor binding.

Transforming growth factors beta belong to a group of cytokines that control cellular proliferation and differentiation. Five isoforms are known that share approximately 75% sequence identity, but exert different biological activities. The structure of TGF-beta 3 was solved by X-ray crystallography and refined to a final R-factor of 17.5% at 2.0 A resolution. Comparison with the structure of TGF-beta 2 (Schlunegger MP, Grütter MG, 1992, Nature 358:430-434; Daopin S, Piez KA, Ogawa Y, Davies DR, 1992, Science 257:369-373) reveals a virtually identical central core. Differences exist in the conformations of the N-terminal alpha-helix and in the beta-sheet loops. In TGF-beta 3, the N-terminal alpha-helix has moved approximately 1 A away from the central core. This movement can be correlated with the mutation of Leu 17 to Val and Ala 47 to Pro in TGF-beta 3. The beta-sheet loops rotate as a rigid body 9 degrees around an axis that runs approximately parallel to the dimer axis. If these differences are recognized by the TGF-beta receptors, they might account for the individual cellular responses. A molecule of the precipitating agent dioxane is bound in a crystal contact, forming a hydrogen bond with Trp 32. This dioxane may occupy a carbohydrate-binding site, because dioxane possesses some structural similarity with a carbohydrate. The dioxane is in contact with two tryptophans, which are often involved in carbohydrate recognition.

Active site binding loop stabilization in the subtilisin inhibitor eglin c: structural and functional studies on specifically designed mutants in complex with subtilisin and the uncomplexed inhibitor.

As known from the x-ray crystal structure in complex with a proteinase and from NMR studies, the serine proteinase inhibitor eglin c has a wedge-like shape with a hydrophobic core and a solvent exposed active site binding loop which is stabilized by a network of non-covalent core-binding loop interactions. Previous studies implied a crucial role of the P1'-residue Asp-46 for binding loop stabilization and high inhibitory potency of eglin c towards serine proteinases such as subtilisin. In the present study, the formation of specific eglin core-binding loop interactions was modulated by replacing the wildtype Asp-46 by asparagine, glutarnate and glutamine. The x-ray crystal structures of these mutants were solved in complex with subtilisin, and the inhibitory potency towards this enzyme was determined. Our results imply a reduction of inhibitory potency with declining core-binding loop interactions. We succeeded in crystallizing free wildtype eglin c. The 1.95 angstroms x-ray crystal structure indicates that the transition from the free to the bound form of eglin is accompanied by a concerted conformational change in the binding loop, implying an induced fit to the accessible enzyme surface. Except for the binding loop domain and a few residues on the surface of eglin, the differences observed between the uncomplexed and bound form of the inhibitor are only small.

Modelling study of protein kinase inhibitors: binding mode of staurosporine and origin of the selectivity of CGP 52411.

A model for the binding mode of the potent protein kinase inhibitor staurosporine is proposed. Using the information provided by the crystal structure of the cyclic-AMP-dependent protein kinase, it is suggested that staurosporine, despite a seemingly unrelated chemical structure, exploits the same key hydrogen-bond interactions as ATP, the cofactor of the protein kinases, in its binding mode. The structure-activity relationship of the inhibitor and a docking analysis give strong support to this hypothesis. The selectivity of the dianilinophthalimide inhibitor CGP 52411 towards the EGF-receptor protein tyrosine kinase is rationalized on the basis of the model. It is proposed that this selectivity originates in the occupancy, by one of the anilino moieties of the inhibitor, of the region of the enzyme cleft that normally binds the ribose ring of ATP, which appears to possess a marked lipophilic character in this kinase.

Comparative analysis of the X-ray structures of HIV-1 and HIV-2 proteases in complex with CGP 53820, a novel pseudosymmetric inhibitor.

BACKGROUND: The human immunodeficiency virus (HIV) is the causative agent of acquired immunodeficiency syndrome (AIDS). Two subtypes of the virus, HIV-1 and HIV-2, have been characterized. The protease enzymes from these two subtypes, which are aspartic acid proteases and have been found to be essential for maturation of the infectious particle, share about 50% sequence identity. Differences in substrate and inhibitor binding between these enzymes have been previously reported. RESULTS: We report the X-ray crystal structures of both HIV-1 and HIV-2 proteases each in complex with the pseudosymmetric inhibitor, CGP 53820, to 2.2 A and 2.3 A, respectively. In both structures, the entire enzyme and inhibitor could be located. The structures confirmed earlier modeling studies. Differences between the CGP 53820 inhibitory binding constants for the two enzymes could be correlated with structural differences. CONCLUSIONS: Minor sequence changes in subsites at the active site can explain some of the observed differences in substrate and inhibitor binding between the two enzymes. The information gained from this investigation may help in the design of equipotent HIV-1/HIV-2 protease inhibitors.

Mutational analysis of the structure-function relationship in interferon-alpha.

A number of mutants of the recombinant human hybrid interferon-alpha 8[60]alpha 1[92]alpha 8 have been expressed in Escherichia coli, purified to homogeneity and characterized. The introduction of one or two negative charges results in an anomalously lower electrophoretic mobility than that expected from the molecular mass. The mutations Lys84-->Glu, Cys86-->Tyr, Cys86-->Ser, Thr87-->Ile, Tyr90-->Asp and Lys84-->Glu/Tyr90-->Asp (double mutant) result in a marked decrease of the biological activity in murine cells compared to the unmutated protein. Comparisons with published structural and homology models of interferon-beta and -alpha, imply that these residues are located primarily on the external surface of the carboxylterminus of helix C. We propose a model of interferon-receptor interaction in which these residues define a potential binding site.

A mutational analysis of receptor binding sites of interleukin-1 beta: differences in binding of human interleukin-1 beta muteins to human and mouse receptors.

The 3-D crystal structure of interleukin-1 beta (IL-1 beta) has been used to define its receptor binding surface by mutational analysis. The surface of IL-1 beta was probed by site-directed mutagenesis. A total of 27 different IL-1 beta muteins were constructed, purified and analyzed. Receptor binding measurements on mouse and human cell lines were performed to identify receptor affinities. IL-1 beta muteins with modified receptor affinity were evaluated for structural integrity by CD spectroscopy or X-ray crystallography. Changes in six surface loops, as well as in the C- and N-termini, yielded muteins with lower binding affinities. Two muteins with intact binding affinities showed 10- to 100-fold reduced biological activity. The surface region involved in receptor binding constitutes a discontinuous area of approximately 1000 A2 formed by discontinuous polypeptide chain stretches. Based on these results, a subdivision into two distinct local areas is proposed. Differences in receptor binding affinities for human and mouse receptors have been observed for some muteins, but not for wild-type IL-1 beta. This is the first time a difference in binding affinity of IL-1 beta muteins to human and mouse receptors has been demonstrated.

Changes in interactions in complexes of hirudin derivatives and human alpha-thrombin due to different crystal forms.

The three-dimensional structures of D-Phe-Pro-Arg-chloromethyl ketone-inhibited thrombin in complex with Tyr-63-sulfated hirudin (ternary complex) and of thrombin in complex with the bifunctional inhibitor D-Phe-Pro-Arg-Pro-(Gly)4-hirudin (CGP 50,856, binary complex) have been determined by X-ray crystallography in crystal forms different from those described by Skrzypczak-Jankun et al. (Skrzypczak-Jankun, E., Carperos, V.E., Ravichandran, K.G., & Tulinsky, A., 1991, J. Mol. Biol. 221, 1379-1393). In both complexes, the interactions of the C-terminal hirudin segments of the inhibitors binding to the fibrinogen-binding exosite of thrombin are clearly established, including residues 60-64, which are disordered in the earlier crystal form. The interactions of the sulfate group of Tyr-63 in the ternary complex structure explain why natural sulfated hirudin binds with a 10-fold lower K(i) than the desulfated recombinant material. In this new crystal form, the autolysis loop of thrombin (residues 146-150), which is disordered in the earlier crystal form, is ordered due to crystal contacts. Interactions between the C-terminal fragment of hirudin and thrombin are not influenced by crystal contacts in this new crystal form, in contrast to the earlier form. In the bifunctional inhibitor-thrombin complex, the peptide bond between Arg-Pro (P1-P1') seems to be cleaved.

Changing the inhibitory specificity and function of the proteinase inhibitor eglin c by site-directed mutagenesis: functional and structural investigation.

Amino acids in the serine proteinase inhibitor eglin c important for its inhibitory specificity and activity have been investigated by site-directed mutagenesis. The specificity of eglin c could be changed from elastase to trypsin inhibition by the point mutation Leu45----Arg (L45R) in position P1 [nomenclature according to Schechter and Berger (1967) Biochem. Biophys. Res. Commun. 27, 157-162]. Model building studies based on the crystal structure of mutant L45R [Heinz et al. (1991) J. Mol. Biol. 217, 353-371] were used to rationalize this specificity change. Surprisingly, the double mutant L45R/D46S was found to be a substrate of trypsin and various other serine proteinases. Multidimensional NMR studies show that wild-type eglin c and the double mutant have virtually identical conformations. In the double mutant L45R/D46S, however, the N-H bond vector of the scissile peptide bond shows a much higher mobility, indicating that the internal rigidity of the binding loop is significantly weakened due to the loss or destabilization of the internal hydrogen bond of the P1' residue. Mutant T44P was constructed to examine the role of a proline in position P2, which is frequently found in serine proteinase inhibitors [Laskowski and Kato (1980) Annu. Rev. Biochem. 49, 593-626]. The mutant remains a potent elastase inhibitor but no longer inhibits subtilisin, which could be explained by model building. Both Arg51 and Arg53, located in the core of the molecule and participating in the hydrogen bonding network with residues in the binding loop to maintain rigidity around the scissile bond, were individually replaced with the shorter but equally charged amino acid lysine. Both mutants showed a decrease in their inhibitory potential. The crystal structure of mutant R53K revealed the loss of two hydrogen bonds between the core and the binding loop of the inhibitor, which are partially restored by a solvent molecule, leading to a decrease in inhibition of elastase by 2 orders of magnitude.

X-ray crystal structure of the serine proteinase inhibitor eglin c at 1.95 A resolution.

The crystal structure of eglin c, naturally occurring in the leech Hirudo medicinalis, is known from its complexes with various serine proteinases, but the crystallization of free eglin c has not yet been reported. A method is described for growing well-diffracting crystals of free eglin c from highly concentrated protein solutions (approximately 200 mg/ml). The space group of the orthorhombic crystals was determined to be P2(1)2(1)2(1) with unit cell parameters a = 32.6, b = 42.0, c = 44.1 A. The structure of free eglin c was resolved at 1.95 A resolution by Patterson search methods. The final model contains all 70 amino acids of eglin c and 125 water molecules. In comparison to the eglin structure known from its complexes with proteinases, only small differences have been observed in free eglin c. However, the reactive site-binding loop and a few residues on the surface of eglin have been found in different conformations due to crystal contacts. In contrast to the complex structures, the first seven amino acids of the highly flexible amino terminus can be located. Crystallographic refinement comprised molecular dynamics refinement, classical restrained least-squares refinement and individual isotropic atomic temperature refinement. The final R-factor is 15.8%.

Identification of a reactive lysyl residue (Lys103) of recombinant human interleukin-1 beta. Mechanism of its reactivity and implication of its functional role in receptor binding.

Recombinant human interleukin-1 beta (h-IL-1 beta) was chemically modified with 4-(N,N-dimethylamino)-4'-isothiocyanatoazobenzene-2'-sulfonic acid (S-DABITC), a water-soluble color reagent specific for lysine labeling. Modified h-IL-1 beta was digested by lysyl endopeptidase. Peptides containing labeled lysines were detected at the visible wavelength (436 nm) and isolated by HPLC. The modification sites were eventually determined by sequence analysis. The results revealed that Lys103, Lys92, Lys93, and Lys94 of h-IL-1 beta reacted selectively with S-DABITC. A 1-h incubation with 1 mM S-DABITC at room temperature resulted in a quantitative modification of Lys103, 22% of Lys92, 27% of Lys93, and 18% of Lys94, respectively. This modification was accompanied by a 20-fold decrease of the protein's ability to bind to the receptor. Furthermore, a mutant of h-IL-1 beta (M9, Glu105 substituted by Lys) exhibits markedly impaired receptor binding, and the S-DABITC reactivity of its Lys103 was found to be reduced by 90%. These findings suggest that Lys103 of h-IL-1 beta might play an important role in the h-IL-1 beta/receptor interaction.

Three-dimensional structure of the bifunctional enzyme phosphoribosylanthranilate isomerase: indoleglycerolphosphate synthase from Escherichia coli refined at 2.0 A resolution.

The three-dimensional structure of the monomeric bifunctional enzyme N-(5'-phosphoribosyl)anthranilate isomerase:indole-3-glycerol-phosphate synthase from Escherichia coli has been refined at 2.0 A resolution, using oscillation film data obtained from synchrotron radiation. The model includes the complete protein (452 residues), two phosphate ions and 628 water molecules. The final R-factor is 17.3% for all observed data between 15 and 2 A resolution. The root-mean-square deviations from ideal bond lengths and bond angles are 0.010 A and 3.2 degrees, respectively. The structure of N-(5'-phosphoribosyl)anthranilate isomerase: indole-3-glycerol-phosphate synthase from E. coli comprises two beta/alpha-barrel domains that superimpose with a root-mean-square deviation of 2.03 A for 138 C alpha-pairs. The C-terminal domain (residues 256 to 452) catalyses the PRAI reaction and the N-terminal domain (residues 1 to 255) catalyses the IGPS reaction, two sequential steps in tryptophan biosynthesis. The enzyme has the overall shape of a dumb-bell, resulting in a surface area that is considerably larger than normally observed for monomeric proteins of this size. The active sites of the PRAI and the IGPS domains, both located at the C-terminal side of the central beta-barrel, contain equivalent binding sites for the phosphate moieties of the substrates N-(5'-phosphoribosyl) anthranilate and 1-(o-carboxyphenylamino)-1-deoxyribulose-5-phosphate. These two phosphate binding sites are identical with respect to their positions within the tertiary structure of the beta/alpha-barrel, the conformation of the residues involved in phosphate binding and the hydrogen-bonding network between the phosphate ions and the protein. The active site cavities of both domains contain similar hydrophobic pockets that presumably bind the anthranilic acid moieties of the substrates. These similarities of the tertiary structures and the active sites of the two domains provide evidence that N-(5'-phosphoribosyl)anthranilate isomerase:indole-3-glycerol-phosphate synthase from E. coli results from a gene duplication event of a monomeric beta/alpha-barrel ancestor.