Extreme Expression of DNA Repair Protein Apurinic/Apyrimidinic Endonuclease 1 (APE1) in Human Breast Cancer As Measured by Liquid Chromatography and Isotope Dilution Tandem Mass Spectrometry.

Apurinic/apyrimidinic endonuclease 1 (APE1) is a DNA repair protein and plays other important roles. Increased levels of APE1 in cancer have been reported. However, available methods for measuring APE1 levels are indirect and not quantitative. We previously developed an approach using liquid chromatography and tandem mass spectrometry with isotope dilution to accurately measure APE1 levels. Here, we applied this methodology to measure APE1 levels in normal and cancerous human breast tissues. Extreme expression of APE1 in malignant tumors was observed, suggesting that breast cancer cells may require APE1 for survival. Accurate measurement of APE1 may be essential for the development of novel treatment strategies and APE1 inhibitors as anticancer drugs.

The fused anthranilate synthase from Streptomyces venezuelae functions as a monomer.

Recently, we showed that the fused chorismate-utilizing enzyme from the antibiotic-producing soil bacterium Streptomyces venezuelae is an anthranilate synthase (designated SvAS), not a 2-amino-2-deoxyisochorismate (ADIC) synthase, as was predicted based on its amino acid sequence similarity to the phenazine biosynthetic enzyme PhzE (an ADIC synthase). Here, we report the characterization of SvAS using steady-state kinetics, gel filtration chromatography, and laser light scattering. The recombinant His-tagged enzyme has Michaelis constants Km with respect to substrates chorismate and glutamine of 8.2 ± 0.2 µM and 0.84 ± 0.05 mM, respectively, and a catalytic rate constant k cat of 0.57 ± 0.02 s(-1) at 30 °C. Unlike most other anthranilate synthases, SvAS does not utilize ammonia as a substrate. The enzyme is competitively but non-cooperatively inhibited by tryptophan (K i = 11.1 ± 0.1 µM) and is active as a monomer. The finding that SvAS is a monomer jibes with the variety of association modes that have been observed for anthranilate synthases from different microorganisms, and it identifies the enzyme's minimal functional unit as a single TrpE-TrpG pair.

Identification and quantification of human DNA repair protein NEIL1 by liquid chromatography/isotope-dilution tandem mass spectrometry.

Accumulated evidence points to DNA repair capacity as an important factor in cancer and other diseases. DNA repair proteins are promising drug targets and are emerging as prognostic and therapeutic biomarkers. Thus, the knowledge of the overexpression or underexpression levels of DNA repair proteins in tissues will be of fundamental importance. In this work, mass spectrometric assays were developed for the measurement in tissues of the human DNA repair protein NEIL1 (hNEIL1), which is involved in base excision and nucleotide excision repair pathways of oxidatively induced DNA damage. Liquid chromatography/isotope-dilution tandem mass spectrometry (LC-MS/MS), in combination with a purified and fully characterized recombinant (15)N-labeled analogue of hNEIL1 ((15)N-hNEIL1) as an internal standard, was utilized to develop an accurate method for the quantification of hNEIL1. Both hNEIL1 and (15)N-hNEIL1 were hydrolyzed with trypsin, and 18 tryptic peptides from each protein were identified by LC-MS/MS on the basis of their full-scan mass spectra. These peptides matched the theoretical peptides expected from trypsin hydrolysis of hNEIL1 and provided a statistically significant protein score that would unequivocally identify hNEIL1. The product ion spectra of the tryptic peptides from both proteins were recorded, and the characteristic product ions were defined. Selected-reaction monitoring was used to analyze mixtures of hNEIL1 and (15)N-hNEIL1 on the basis of product ions. Additional confirmation of positive identification was demonstrated via separation of the proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and in-gel tryptic digestion followed by LC-MS/MS analysis. These results suggest that the developed assays would be highly suitable for the in vivo positive identification and accurate quantification of hNEIL1 in tissues.

Identification and quantification of DNA repair proteins by liquid chromatography/isotope-dilution tandem mass spectrometry using their fully 15N-labeled analogues as internal standards.

Oxidatively induced DNA damage is implicated in disease, unless it is repaired by DNA repair. Defects in DNA repair capacity may be a risk factor for various disease processes. Thus, DNA repair proteins may be used as early detection and therapeutic biomarkers in cancer and other diseases. For this purpose, the measurement of the expression level of these proteins in vivo will be necessary. We applied liquid chromatography/isotope-dilution tandem mass spectrometry (LC-MS/MS) for the identification and quantification of DNA repair proteins human 8-hydroxyguanine-DNA glycosylase (hOGG1) and Escherichia coli formamidopyrimidine DNA glycosylase (Fpg), which are involved in base-excision repair of oxidatively induced DNA damage. We overproduced and purified (15)N-labeled analogues of these proteins to be used as suitable internal standards to ensure the accuracy of quantification. Unlabeled and (15)N-labeled proteins were digested with trypsin and analyzed by LC-MS/MS. Numerous tryptic peptides of both proteins were identified on the basis of their full-scan mass spectra. These peptides matched the theoretical peptide fragments expected from trypsin digestion and provided statistically significant protein scores that would unequivocally identify these proteins. We also recorded the product ion spectra of the tryptic peptides and defined the characteristic product ions. Mixtures of the analyte proteins and their (15)N-labeled analogues were analyzed by selected-reaction monitoring on the basis of product ions. The results obtained suggest that the methodology developed would be highly suitable for the positive identification and accurate quantification of DNA repair proteins in vivo as potential biomarkers for cancer and other diseases.

Stable isotope-labeling of DNA repair proteins, and their purification and characterization.

Reduced DNA repair capacity is associated with increased risk for a variety of disease processes including carcinogenesis. Thus, DNA repair proteins have the potential to be used as important predictive, prognostic and therapeutic biomarkers in cancer and other diseases. The measurement of the expression level of these enzymes may be an excellent tool for this purpose. Mass spectrometry is becoming the technique of choice for the identification and quantification of proteins. However, suitable internal standards must be used to ensure the precision and accuracy of measurements. An ideal internal standard in this case would be a stable isotope-labeled analog of the analyte protein. In the present work, we over-expressed, purified and characterized two stable isotope-labeled DNA glycosylases, i.e., (15)N-labeled Escherichia coli formamidopyrimidine DNA glycosylase (Fpg) and (15)N-labeled human 8-oxoguanine-DNA glycosylase (hOGG1). DNA glycosylases are involved in the first step of the base excision repair of oxidatively induced DNA damage by removing modified DNA bases. The measurement by MALDI-ToF mass spectrometry of the molecular mass and isotopic purity proved the identity of the (15)N-labeled proteins and showed that the (15)N-labeling of both proteins was more than 99.7%. We also measured the DNA glycosylase activities using gas chromatography/mass spectrometry with isotope-dilution. The enzymic activities of both (15)N-labeled Fpg and (15)N-labeled hOGG1 were essentially identical to those of their respective unlabeled counterparts, ascertaining that the labeling did not perturb their catalytic sites. The procedures described in this work may be used for obtaining stable isotope-labeled analogs of other DNA repair proteins for mass spectrometric measurements of these proteins as disease biomarkers.

Fusing an insoluble protein to GroEL apical domain enhances soluble expression in Escherichia coli.

A protocol for increasing soluble protein expression by fusing the chaperone GroEL apical domain with a gene of interest is described herein. GroEL apical domain, the minichaperone that functions independently of GroES and ATP in protein folding, is cloned downstream of the lambda CII ribosome binding site in the parent pRE vector. The pRE vector has tightly controlled transcription suitable for expressing toxic proteins. The GroEL minichaperone is fused to a glycine-serine rich linker followed by the enterokinase protease recognition sequence. A number of genes that are recalcitrant to protein production in the parent pRE vector 5were cloned into the pRE:GroEL fusion vector and successfully expressed as fusion proteins in Escherichia coli.

A comparative biochemical and structural analysis of the intracellular chorismate mutase (Rv0948c) from Mycobacterium tuberculosis H(37)R(v) and the secreted chorismate mutase (y2828) from Yersinia pestis.

The Rv0948c gene from Mycobacterium tuberculosis H(37)R(v) encodes a 90 amino acid protein as the natural gene product with chorismate mutase (CM) activity. The protein, 90-MtCM, exhibits Michaelis-Menten kinetics with a k(cat) of 5.5+/-0.2s(-1) and a K(m) of 1500+/-100microm at 37 degrees C and pH7.5. The 2.0A X-ray structure shows that 90-MtCM is an all alpha-helical homodimer (Protein Data Bank ID: 2QBV) with the topology of Escherichia coli CM (EcCM), and that both protomers contribute to each catalytic site. Superimposition onto the structure of EcCM and the sequence alignment shows that the C-terminus helix3 is shortened. The absence of two residues in the active site of 90-MtCM corresponding to Ser84 and Gln88 of EcCM appears to be one reason for the low k(cat). Hence, 90-MtCM belongs to a subfamily of alpha-helical AroQ CMs termed AroQ(delta.) The CM gene (y2828) from Yersinia pestis encodes a 186 amino acid protein with an N-terminal signal peptide that directs the protein to the periplasm. The mature protein, *YpCM, exhibits Michaelis-Menten kinetics with a k(cat) of 70+/-5s(-1) and K(m) of 500+/-50microm at 37 degrees C and pH7.5. The 2.1A X-ray structure shows that *YpCM is an all alpha-helical protein, and functions as a homodimer, and that each protomer has an independent catalytic unit (Protein Data Bank ID: 2GBB). *YpCM belongs to the AroQ(gamma) class of CMs, and is similar to the secreted CM (Rv1885c, *MtCM) from M.tuberculosis.

Platform development for expression and purification of stable isotope labeled monoclonal antibodies in Escherichia coli.

The widespread use of monoclonal antibodies (mAbs) as a platform for therapeutic drug development in the pharmaceutical industry has led to an increased interest in robust experimental approaches for assessment of mAb structure, stability and dynamics. The ability to enrich proteins with stable isotopes is a prerequisite for the in-depth application of many structural and biophysical methods, including nuclear magnetic resonance (NMR), small angle neutron scattering, neutron reflectometry, and quantitative mass spectrometry. While mAbs can typically be produced with very high yields using mammalian cell expression, stable isotope labeling using cell culture is expensive and often impractical. The most common and cost-efficient approach to label proteins is to express proteins in Escherichia coli grown in minimal media; however, such methods for mAbs have not been reported to date. Here we present, for the first time, the expression and purification of a stable isotope labeled mAb from a genetically engineered E. coli strain capable of forming disulfide bonds in its cytoplasm. It is shown using two-dimensional NMR spectral fingerprinting that the unlabeled mAb and the mAb singly or triply labeled with 13C, 15N, 2H are well folded, with only minor structural differences relative to the mammalian cell-produced mAb that are attributed to the lack of glycosylation in the Fc domain. This advancement of an E. coli-based mAb expression platform will facilitate the production of mAbs for in-depth structural characterization, including the high resolution investigation of mechanisms of action.

Structure of the class IV adenylyl cyclase reveals a novel fold.

The crystal structure of the class IV adenylyl cyclase (AC) from Yersinia pestis (Yp) is reported at 1.9 A resolution. The class IV AC fold is distinct from the previously described folds for class II and class III ACs. The dimeric AC-IV folds into an antiparallel eight-stranded barrel whose connectivity has been seen in only three previous structures: yeast RNA triphosphatase and two proteins of unknown function from Pyrococcus furiosus and Vibrio parahaemolyticus. Eight highly conserved ionic residues E10, E12, K14, R63, K76, K111, D126, and E136 lie in the barrel core and form the likely binding sites for substrate and divalent cations. A phosphate ion is observed bound to R63, K76, K111, and R113 near the center of the conserved cluster. Unlike the AC-II and AC-III active sites that utilize two-Asp motifs for cation binding, the AC-IV active site is relatively enriched in glutamate and features an ExE motif as its most conserved element. Homologs of Y. pestis AC-IV, including human thiamine triphosphatase, span the three kingdoms of life and delineate an ancient family of phosphonucleotide processing enzymes.

Crystallization of the class IV adenylyl cyclase from Yersinia pestis.

The class IV adenylyl cyclase from Yersinia pestis has been cloned and crystallized in both a triclinic and an orthorhombic form. An amino-terminal His-tagged construct, from which the tag was removed by thrombin, crystallized in a triclinic form diffracting to 1.9 A, with one dimer per asymmetric unit and unit-cell parameters a = 33.5, b = 35.5, c = 71.8 A, alpha = 88.7, beta = 82.5, gamma = 65.5 degrees. Several mutants of this construct crystallized but diffracted poorly. A non-His-tagged native construct (179 amino acids, MW = 20.5 kDa) was purified by conventional chromatography and crystallized in space group P2(1)2(1)2(1). These crystals have unit-cell parameters a = 56.8, b = 118.6, c = 144.5 A, diffract to 3 A and probably have two dimers per asymmetric unit and VM = 3.0 A3 Da(-1). Both crystal forms appear to require pH below 5, complicating attempts to incorporate nucleotide ligands into the structure. The native construct has been produced as a selenomethionine derivative and crystallized for phasing and structure determination.

Methods to Characterize Ricin for the Development of Reference Materials.

Ricin is an abundant protein from the castor bean plant Ricinus communis. Because of its high toxicity and the simplicity of producing mass quantities, ricin is considered a biological terrorism agent. We have characterized ricin extensively with a view to develop Reference Materials that could be used to test and calibrate detection devices. The characterization of ricin includes: 1) purity test of a commercial batch of ricin using electrophoresis in polyacrylamide gels, 2) biological activity assay by measuring its ability to inhibit protein synthesis, 3) quantitation of protein concentration by amino acid analysis, 4) detection of ricin by an immunoassay using a flow cytometer, and 5) detection of ricin genomic DNA by polymerase chain reaction using nine different primer sets. By implementing these five methods of characterization, we are in a position to develop a reference material for ricin.

Production, Purification, and Characterization of ¹5N-Labeled DNA Repair Proteins as Internal Standards for Mass Spectrometric Measurements.

Oxidatively induced DNA damage is caused in living organisms by a variety of damaging agents, resulting in the formation of a multiplicity of lesions, which are mutagenic and cytotoxic. Unless repaired by DNA repair mechanisms before DNA replication, DNA lesions can lead to genomic instability, which is one of the hallmarks of cancer. Oxidatively induced DNA damage is mainly repaired by base excision repair pathway with the involvement of a plethora of proteins. Cancer tissues develop greater DNA repair capacity than normal tissues by overexpressing DNA repair proteins. Increased DNA repair in tumors that removes DNA lesions generated by therapeutic agents before they became toxic is a major mechanism in the development of therapy resistance. Evidence suggests that DNA repair capacity may be a predictive biomarker of patient response. Thus, knowledge of DNA-protein expressions in disease-free and cancerous tissues may help predict and guide development of treatments and yield the best therapeutic response. Our laboratory has developed methodologies that use mass spectrometry with isotope dilution for the measurement of expression of DNA repair proteins in human tissues and cultured cells. For this purpose, full-length (15)N-labeled analogs of a number of human DNA repair proteins have been produced and purified to be used as internal standards for positive identification and accurate quantification. This chapter describes in detail the protocols of this work. The use of (15)N-labeled proteins as internal standards for the measurement of several DNA repair proteins in vivo is also presented.

Protein labeling in Escherichia coli with (2)H, (13)C, and (15)N.

A number of structural biology techniques such as nuclear magnetic resonance spectroscopy and small-angle neutron scattering can be performed with proteins with nuclei at natural isotope abundance. However, the use of proteins labeled with stable isotopes ((2)H, (13)C, and (15)N) enables greater experimental flexibility. In this chapter, several methods for uniform and fractional protein labeling with stable isotopes using Escherichia coli in a defined media are described. The methods described can be used for labeling with single or multiple isotopes.

The HI0073/HI0074 protein pair from Haemophilus influenzae is a member of a new nucleotidyltransferase family: structure, sequence analyses, and solution studies.

The crystal structure of HI0074 from Haemophilus influenzae, a protein of unknown function, has been determined at a resolution of 2.4 A. The molecules form an up-down, four-helix bundle, and associate into homodimers. The fold is most closely related to the substrate-binding domain of KNTase, yet the amino acid sequences of the two proteins exhibit no significant homology. Sequence analyses of completely and incompletely sequenced genomes reveal that the two adjacent genes, HI0074 and HI0073, and their close relatives comprise a new family of nucleotidyltransferases, with 15 members at the time of writing. The analyses also indicate that this is one of eight families of a large nucleotidyltransferase superfamily, whose members were identified based on the proximity of the nucleotide- and substrate-binding domains on the respective genomes. Both HI0073 and HI0074 were annotated "hypothetical" in the original genome sequencing publication. HI0073 was cloned, expressed, and purified, and was shown to form a complex with HI0074 by polyacrylamide gel electrophoresis under nondenaturing conditions, analytic size exclusion chromatography, and dynamic light scattering. Double- and single-stranded DNA binding assays showed no evidence of DNA binding to HI0074 or to HI0073/HI0074 complex despite the suggestive shape of the putative binding cleft formed by the HI0074 dimer.

Identification and quantification of DNA repair protein apurinic/apyrimidinic endonuclease 1 (APE1) in human cells by liquid chromatography/isotope-dilution tandem mass spectrometry.

Unless repaired, DNA damage can drive mutagenesis or cell death. DNA repair proteins may therefore be used as biomarkers in disease etiology or therapeutic response prediction. Thus, the accurate determination of DNA repair protein expression and genotype is of fundamental importance. Among DNA repair proteins involved in base excision repair, apurinic/apyrimidinic endonuclease 1 (APE1) is the major endonuclease in mammals and plays important roles in transcriptional regulation and modulating stress responses. Here, we present a novel approach involving LC-MS/MS with isotope-dilution to positively identify and accurately quantify APE1 in human cells and mouse tissue. A completely (15)N-labeled full-length human APE1 was produced and used as an internal standard. Fourteen tryptic peptides of both human APE1 (hAPE1) and (15)N-labeled hAPE1 were identified following trypsin digestion. These peptides matched the theoretical peptides expected from trypsin digestion and provided a statistically significant protein score that would unequivocally identify hAPE1. Using the developed methodology, APE1 was positively identified and quantified in nuclear and cytoplasmic extracts of multiple human cell lines and mouse liver using selected-reaction monitoring of typical mass transitions of the tryptic peptides. We also show that the methodology can be applied to the identification of hAPE1 variants found in the human population. The results describe a novel approach for the accurate measurement of wild-type and variant forms of hAPE1 in vivo, and ultimately for defining the role of this protein in disease development and treatment responses.

Active-site structure of class IV adenylyl cyclase and transphyletic mechanism.

Adenylyl cyclases (ACs) belonging to three nonhomologous classes (II, III, and IV) have been structurally characterized, enabling a comparison of the mechanisms of cyclic adenosine 3',5'-monophosphate biosynthesis. We report the crystal structures of three active-site complexes for Yersinia pestis class IV AC (AC-IV)-two with substrate analogs and one with product. Mn(2+) binds to all three phosphates, and to Glu12 and Glu136. Electropositive residues Lys14, Arg63, Lys76, Lys111, and Arg113 also form hydrogen bonds to phosphates. The conformation of the analogs is suitable for in-line nucleophilic attack by the ribose O3' on α-phosphate (distance ∼4 Å). In the product complex, a second Mn ion is observed to be coordinated to both ribose 2' oxygen and ribose 3' oxygen. Observation of both metal sites, together with kinetic measurements, provides strong support for a two-cation mechanism. Eleven active-site mutants were also made and kinetically characterized. These findings and comparisons with class II and class III enzymes enable a detailed transphyletic analysis of the AC mechanism. Consistent with its lack of coordination to purine, Y. pestis AC-IV cyclizes both ATP and GTP. As in other classes of AC, the ribose is loosely bound, and as in class III, no base appears to ionize the O3' nucleophile. Different syn/anti conformations suggest that the mechanism involves a conformational transition, and further evidence suggests a role for ribosyl pseudorotation. With resolutions of 1.6-1.7 Å, these are the most detailed active-site ligand complexes for any class of this ubiquitous signaling enzyme.

Evidence for upregulated repair of oxidatively induced DNA damage in human colorectal cancer.

Carcinogenesis may involve overproduction of oxygen-derived species including free radicals, which are capable of damaging DNA and other biomolecules in vivo. Increased DNA damage contributes to genetic instability and promote the development of malignancy. We hypothesized that the repair of oxidatively induced DNA base damage may be modulated in colorectal malignant tumors, resulting in lower levels of DNA base lesions than in surrounding pathologically normal tissues. To test this hypothesis, we investigated oxidatively induced DNA damage in cancerous tissues and their surrounding normal tissues of patients with colorectal cancer. The levels of oxidatively induced DNA lesions such as 4,6-diamino-5-formamidopyrimidine, 2,6-diamino-4-hydroxy-5-formamidopyrimidine, 8-hydroxyguanine and (5'S)-8,5'-cyclo-2'-deoxyadenosine were measured by gas chromatography/isotope-dilution mass spectrometry and liquid chromatography/isotope-dilution tandem mass spectrometry. We found that the levels of these DNA lesions were significantly lower in cancerous colorectal tissues than those in surrounding non-cancerous tissues. In addition, the level of DNA lesions varied between colon and rectum tissues, being lower in the former than in the latter. The results strongly suggest upregulation of DNA repair in malignant colorectal tumors that may contribute to the resistance to therapeutic agents affecting the disease outcome and patient survival. The type of DNA base lesions identified in this work suggests the upregulation of both base excision and nucleotide excision pathways. Development of DNA repair inhibitors targeting both repair pathways may be considered for selective killing of malignant tumors in colorectal cancer.

Profound asymmetry in the structure of the cAMP-free cAMP Receptor Protein (CRP) from Mycobacterium tuberculosis.

The cyclic AMP receptor protein (CRP, also called catabolite gene activator protein or CAP) plays a key role in metabolic regulation in bacteria and has become a widely studied model allosteric transcription factor. On binding its effector cAMP in the N-terminal domain, CRP undergoes a structural transition to a conformation capable of specific DNA binding in the C-terminal domain and transcription initiation. The crystal structures of Escherichia coli CRP (EcCRP) in the cAMP-bound state, both with and without DNA, are known, although its structure in the off state (cAMP-free, apoCRP) remains unknown. We describe the crystal structure at 2.0A resolution of the cAMP-free CRP homodimer from Mycobacterium tuberculosis H(37)R(v) (MtbCRP), whose sequence is 30% identical with EcCRP, as the first reported structure of an off-state CRP. The overall structure is similar to that seen for the cAMP-bound EcCRP, but the apo MtbCRP homodimer displays a unique level of asymmetry, with a root mean square deviation of 3.5A between all Calpha positions in the two subunits. Unlike structures of on-state EcCRP and other homologs in which the C-domains are asymmetrically positioned but possess the same internal conformation, the two C-domains of apo MtbCRP differ both in hinge structure and in internal arrangement, with numerous residues that have completely different local environments and hydrogen bond interactions, especially in the hinge and DNA-binding regions. Comparison of the structures of apo MtbCRP and DNA-bound EcCRP shows how DNA binding would be inhibited in the absence of cAMP and supports a mechanism involving functional asymmetry in apoCRP.

Protein Crystal Engineering of YpAC-IV using the Strategy of Excess Charge Reduction.

The class IV adenylyl cyclase from Yersinia pestis has been engineered by site-specific mutagenesis to facilitate crystallization at neutral pH. The wild-type enzyme crystallized only below pH 5, consistent with the observation of a carboxyl-carboxylate H bond in a crystal contact in the refined structure 2FJT. Based on that unliganded structure at 1.9 A resolution, two different approaches were tested with the goal of producing a higher-pH crystal needed for inhibitor complexation and mechanistic studies. In one approach, Asp 19, which forms the growth-limiting dicarboxyl contact in wild-type triclinic crystals, was modified to Ala and Asn in hopes of relieving the acid-dependence of that crystal form. In the other approach, wild-type residues Met 18, Glu 25, and Asp 55 were (individually) changed to lysine to reduce the protein's excess negative charge in hopes of enabling growth of new, higher-pH forms. These 3 sites were selected based on their high solvent exposure and lack of intraprotein interactions. The D19A and D19N mutants had reduced solubility and did not crystallize. The other 3 mutants all crystallized, producing several new forms at neutral pH. One of these forms, with the D55K mutant, enabled a product complex at 1.6 A resolution, structure 3GHX. This structure shows why the new crystal form required the mutation in order to grow at neutral pH. This approach could be useful in other cases where excess negative charge inhibits the crystallization of low-pI proteins.

Biochemical and structural characterization of the secreted chorismate mutase (Rv1885c) from Mycobacterium tuberculosis H37Rv: an *AroQ enzyme not regulated by the aromatic amino acids.

The gene Rv1885c from the genome of Mycobacterium tuberculosis H37Rv encodes a monofunctional and secreted chorismate mutase (*MtCM) with a 33-amino-acid cleavable signal sequence; hence, it belongs to the *AroQ class of chorismate mutases. Consistent with the heterologously expressed *MtCM having periplasmic destination in Escherichia coli and the absence of a discrete periplasmic compartment in M. tuberculosis, we show here that *MtCM secretes into the culture filtrate of M. tuberculosis. *MtCM functions as a homodimer and exhibits a dimeric state of the protein at a concentration as low as 5 nM. *MtCM exhibits simple Michaelis-Menten kinetics with a Km of 0.5 +/- 0.05 mM and a k(cat) of 60 s(-1) per active site (at 37 degrees C and pH 7.5). The crystal structure of *MtCM has been determined at 1.7 A resolution (Protein Data Bank identifier 2F6L). The protein has an all alpha-helical structure, and the active site is formed within a single chain without any contribution from the second chain in the dimer. Analysis of the structure shows a novel fold topology for the protein with a topologically rearranged helix containing Arg134. We provide evidence by site-directed mutagenesis that the residues Arg49, Lys60, Arg72, Thr105, Glu109, and Arg134 constitute the catalytic site; the numbering of the residues includes the signal sequence. Our investigation on the effect of phenylalanine, tyrosine, and tryptophan on *MtCM shows that *MtCM is not regulated by the aromatic amino acids. Consistent with this observation, the X-ray structure of *MtCM does not have an allosteric regulatory site.