Biochimie 110 (2015) 73e80 Contents lists available at ScienceDirect Biochimie journal homepage: www.elsevier.com/locate/biochi Research paper Insight into the structural flexibility and function of Mycobacterium tuberculosis isocitrate lyase Harish Shukla a, Vikash Kumar a, Amit Kumar Singh a, Neha Singh a, Md. Kashif a, Mohammad Imran Siddiqi a, c, Manju Yasoda Krishnan b, c, Md. Sohail Akhtar a, c, * a b c Molecular and Structural Biology Division, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Lucknow, PIN 226 031, India Microbiology Division, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Lucknow, PIN 226 031, India Academy of Scientific and Innovative Research, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Lucknow, PIN 226 031, India a r t i c l e i n f o a b s t r a c t Article history: Received 11 August 2014 Accepted 26 December 2014 Available online 7 January 2015 Isocitrate lyase (ICL), is a key enzyme of the glyoxylate shunt crucial for the survival of Mycobacterium tuberculosis (Mtb) in macrophages during persistent infection. MtbICL catalyses the first step of this carbon anaplerosis cycle and is considered as a potential anti-tubercular drug target. The MtbICL is a tetramer with 222 symmetry, and each subunit of the enzymeis composed of 14 a-helices and 14 bstrands. We studied the conformational flexibility of the enzyme to get a deeper insight into its stability and function. Our studies show that the mutation of His180, close to the MtbICL signature sequence (K193KCGH197) completely abolishes the oligomeric conformation and function of the enzyme. Molecular dynamics studies suggest that the loss of interaction between His180 and Tyr89 most likely alters the orientation of Tyr89 side chain, thereby causing the movement of helices a6, a12, a13 and a14 in the vicinity and affecting the tetrameric assembly. We further show that the oligomerization of MtbICL is primarily mediated by the inter subunit interactions, and strengthened by the helix swapping of a12 ea13 between adjacent subunits. Furthermore, the enzyme activity is influenced by the interactions between the residues of lid region (P411NSSTTALTGSTEEGQFH428) and the loop region (T391KHQREV397). Mutation of glutamates of the lid region to non homologous residues (E423A or E424A) or basic residues (E423K or E424K) inactivates the enzyme, whereas the activity is not much compromised in case of homologous mutations (E423D or E424D). te française de biochimie et biologie Mole culaire (SFBBM). All rights © 2015 Elsevier B.V. and Socie reserved. Keywords: Isocitrate lyase Structure Activity Drug target Kinetics 1. Introduction Tuberculosis (TB) remains an expanding global health crisis responsible for about 2 million deaths annually [1,2]. Nearly about one-third of the world's population harbors latent Mycobacterium tuberculosis (Mtb) and would have a positive skin test for the infection [1e4]. There is no effective vaccine against the infection and the prolonged therapy frequently leads to the emergence of multi-drug resistance not only to the first line, but also to some of the second-line drugs [1e5]. The extended therapy is needed to tackle the non replicating dormant population of Mtb in patient lesions which are refractory to killing by the currently used anti-TB Abbreviation: Mtb, Mycobacterium tuberculosis; ICL, isocitrate lyase; MtbICL, Mtuberculosis isocitrate lyase; SEC, size-exclusion chromatography. * Corresponding author. Molecular and Structural Biology Division, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Lucknow, PIN 226 031, India. E-mail address: [email protected] (Md.S. Akhtar). drugs, which are capable of killing only the actively dividing bacilli [6]. Thus combating TB requires new therapeutic strategies which can effectively target the dormant bacilli and act in synergy with the available frontline drugs [1]. Hence, the hunt for novel, safer TB drugs effective in treating drug resistant as well as persistent infection is still on. Many recent works are beginning to elucidate the metabolic adaptation by Mtb during persistence, for instance, the dependence on purine, amino acid and pantothenate biosynthesis, iron acquisition, glyoxylate shunt etc [7]. Beta-oxidation, gluconeogenesis and glyoxylate shunt are important for the survival of Mtb inside the phagosomes of macrophages, which are glucose deficient, but fatty acid replete [8]. Isocitrate lyase (ICL), one of the key enzymes of the glyoxylate shunt catalyses the conversion of isocitrate to succinate and glyoxylate. The enzyme is essential for carbon anaplerosis in the TCA cycle during growth on C2 substrates such as fatty acids [9,10]. The glyoxylate shunt is absent invertebrates, but are widespread among prokaryotes, lower eukaryotes and plants http://dx.doi.org/10.1016/j.biochi.2014.12.016 te française de biochimie et biologie Mole culaire (SFBBM). All rights reserved. 0300-9084/© 2015 Elsevier B.V. and Socie 74 H. Shukla et al. / Biochimie 110 (2015) 73e80 [11]. The expression of ICL is upregulated during infection of macrophages by Mycobacterium sp. and the disruption of ICL gene inhibits the persistence of Mtb in macrophage and in mice [12e14]. Hence ICL of Mtb (MtbICL) is considered as one of the potential and attractive drug targets against persistent infection. The crystal structures of MtbICL in apo form as well as in complex with inhibitors are solved [15]. MtbICL (428 amino acids) is a tetramer with 222 symmetry (Fig. 1). Each subunit of MtbICL is composed of 14 a-helices and 14 b-strands. The core of the structure is an unusual a/b-barrel composed of eight a-helices (a4ea11) and eight b-strands (b2eb5, b8, b12eb14) forming the larger domain of the enzyme. Helix a12 (residues 349e367) projects away from the barrel and together with the two consequent helices a13 (residues 370e384) and a14 (residues 399e409), forms interactions exclusively with the neighboring subunits. A small bdomain consisting of a short five stranded b-sheet (b6, b7, b9, b10, b11) lies atop of the a/b barrel and contains several of the active site residues. The striking feature of this structure is the inter-subunit helix-swapping of a12 and a13 between two non crystallographically related subunits responsible for the tetrameric structure. A similar helix swapping has been proposed to enable the formation of stable dimers in other proteins [16]. The mechanism of MtbICL function was elucidated based on the crystal structure of apo MtbICL and MtbICLC191Sin complex with glyoxylate and the non-reactive succinate analog 3nitropropionate [15]. The conformation of apo MtbICL differs from its substrate analog bound forms, especially in regions that control access to the active site [1,15]. The first region is an active site loop (L185ASEKKCGHLGG196) that contains the ICL signature sequence (K189KCGH193) and the second region consists of the last 18 residues (P411NSSTTALTGSTEEGQFH428) at the C-terminal end of the adjacent subunit (lid). The active site loop is flexible in the ‘open’ conformation where as in the substrate bound form it moves by 10e15 Ǻ and attains a closed conformation. The closed conformation brings catalytic Cys191 of ICL signature sequence next to the substrate and completely closes off the active site from bulk solvent. The electrostatic interaction of Lys189 within the negative patches triggers movements in the active site loop to adopt the ‘closed’ conformation. Closure of the active site loop blocks the accessibility to the catalytic residues and invokes a movement of C-terminus lid (residues 411e428) of the adjacent subunit completing the catalytic conformation. It is not clear what invokes the movement of lid on top of the active site loop, locking it into a catalytic conformation? It appears that the two conserved glutamates in the lid region might play an important role during enzyme catalytsis [17]. Furthermore, apart from the residues of signature sequence (KKCGH), the function of ICL is also influenced by the histidine residue (His184) adjacent to the ICL signature sequence in Escherichia coli. The mutation of His184 to Lys, Arg, Leu or Gln resulted in either inactive ICL or a minimally active ICL [17]. Conformational analysis of MtbICL with respect to its stability and function is important and might help in the identification of suitable inhibitors. The biological phenomena depend on the molecular recognition and play a significant role in protein folding, stability and ligand binding. Understanding of the relationship between the structure of proteins, their stability, binding with micro or macromolecules etc. are useful to understand how biological systems work. This would facilitate drug discovery, such as in molecular mechanism and energetics of drug-macromolecules interactions or in order to estimate binding constants of two molecules [18,19]. In the present study, we systematically looked into the structural elements involved in the oligomerization and deactivation of MtbICL. 2. Experimental 2.1. Cloning and site directed mutagenesis The cloning of full length MtbICL (MtbICL/MtbICLa12,a13,a14) has been described previously and was a kind gift from Dr. Ranjeet Kumar, Lucknow [20]. Truncated ICL without a14 (tMtbICLa12,a13), tMtbICLa12 (without a13 and a14) and tMtbICL (without a12, a13 and a14) were amplified using suitable primers (Table 1). For all the samples, PCR reactions were carried out in a total volume of 50 ml with Platinum Pfx DNA polymerase (Invitrogen). The amplification condition was 94 C-3 min; 94 C-30 s, 53 C-1 min, 68 C-1 min (30 cycles); 68 C-10 min. These amplified gene fragments were digested with NheI and HindIII and then ligated into the pET-23a (þ) vector cut with the same enzymes. Competent E. coli DH5-a cells were transformed with the plasmid constructs and screened for positive clones. The mutant ICLs, MtbICLH180A, MtbICLY89A, MtbICLE423D, MtbICLE424D, MtbICLE423D E424D, MtbICLE423A, MtbICLE424A, MtbICLE423AE424A, MtbICLE423K, MtbICLE424k, MtbICLE423KE424K, MtbICLE423L, MtbICLE424L and MtbICLE423LE424L were generated using mutagenic primer pairs (Table 1). The DNA sequencing of all the amplified genes confirmed the homogeneity of the sequences. Fig. 1. Diagrammatic representation of the structure of MtbICL e Crystal structure of monomeric, dimeric and tetrameric MtbICL (left to right). The helices a12 and a13 involved in the oligomeization are marked as red. The structure was generated with the help of PISA server [28]. H. Shukla et al. / Biochimie 110 (2015) 73e80 75 2.2. Preparation of recombinant proteins The expression of MtbICL was carried out as described previously [20]. For the expression of remaining proteins, the corresponding plasmids were introduced into C41(DE3) cells and initially grown in LuriaeBertani media containing 100 mg/ml ampicillin at 30 C until A600~ 0.6 and later induced at 20 C with 0.1 mM isopropyl-1-thio-b-D-galactopyranoside (IPTG). Induced cultures were grown further for 8 h with rigorous shaking at the same temperature. The cleared cell lysates were loaded onto a NiNTA column pre-equilibrated with 20 mM TriseCl, pH 8 and 0.3 M NaCl and washed with the same buffer followed by washing with 20 mM and 40 mM imidazole buffer. The proteins were eluted with a linear gradient of 40e400 mM imidazole buffer and the desired peaks were dialyzed against 50 mM phosphate buffer, pH 8 containing 0.1 M NaCl. The purity and molecular weight determinations of the recombinant proteins were carried out by SDS as well as with size exclusion chromatography (SEC) experiments on a SuperdexTM 200,10/300 GL column (manufacture's exclusion limit 600 kDa) on AKTA FPLC (GE Healthcare). The column was preequilibrated and run with 50 mM phosphate buffer pH 8.0 at 25 C with a flow rate of 0.5 ml/min, with detection at 280 nm. The column was calibrated with standard molecular weight markers. Table 1 Different primers used in PCR for MtbICL, its truncations and mutants. Serial no. Gene/truncations/ mutants name Primers 1 tMtbICLa12,a13 2 tMtbICLa12 3 tMtbICL 4 MtbICLH180A 5 MtbICLY89A 6 MtbICLE423D 7 MtbICLE424D 8 MtbICLE423D,E424D 9 MtbICLE423A 10 MtbICLE424A 11 MtbICLE423A,E424A 12 MtbICLE423K 13 MtbICLEE424K 14 MtbICLE423K,E424K 15 MtbICLE423L 16 MtbICLE424L 17 MtbICLE423L,424L F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CCC AAG CTT GCC TAC TTC ACG CTG GTG CTT GGT 30 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CAA AAG CTT CTG GTT CTG GGC GTA GCC GTA GGC 30 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CCC AAG CTT CAG CGT GAT GAA CTG GAA CTT GAA30 F 50 GGC GTT GCG GGT TCG GCT TGG GAG GAT CAG TTG30 R 50 CAA CTG ATC CTC CCA AGC CGA ACC CGC AAC GCC 30 F 50 GGC CTG AAG GCC ATC GCC CTG TCG GGC TGG CA30 R 50 TGC CAG CCC GAC AGG GCG ATG GCC TTC AGG CC30 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC GTC CTC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC CTC GTC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC GTC GTC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC CGC CTC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC CTC CGC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC CGC CGC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAGT 30 R 50 CTC AAG CTT GTG GAA CTG ACC TTT CTC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC CTC TTT GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC TTT TTT GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC TTG CTC GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC CTC TTG GGT GGA3 F 50 CCC GCT AGC ATG TCT GTC GTC GGC ACC CCG AAG T 30 R 50 CTC AAG CTT GTG GAA CTG ACC TTG TTG GGT GGA3 2.3. Activity assay The activity of recombinant proteins was measured at 30 C in 20 mM HEPES buffer, pH 7.5, by a modification of the continuous method as described previously [21]. The reaction mixture comprised of 20 mM Hepes, pH 7.5, containing 5 mM MgCl2, 4 mM phenyl hydrazine hydrochloride, 30 mM threo-DL-isocitrate and enzyme in a final volume of 1 ml. The reaction was initiated by the addition of the enzyme (25 nM), and the isocitrate cleavage was measured at 30 C by the change in absorbance at 324 nm associated with the formation of glyoxylate phenylhydrazone (extinction coefficient at 324 nm, 17,000/M/cm). 2.4. Glutaraldehyde crosslinking 50 mg of desired protein was incubated with 5 ml of 1:25 dilution (w/v) of 25% glutaraldehyde in a 100 ml reaction volume in 20 mM HEPES buffer pH 7.5 for 30 min. The reaction was stopped by the addition of 10 ml of 1 M TriseHCl, pH 8 in the reaction mixture and the samples analyzed on 8% SDS-PAGE. 2.5. Spectroscopy measurements Tryptophan fluorescence spectra were recorded with a Perkin Elmer Life Sciences LS 50-B spectrofluorimeter in a 5 mm path length quartz cell at 25 C. Excitation wavelength of 280 nm was used and the spectra were recorded between 300 and 400 nm. The protein concentration of 3 mM was used for the studies. Circular dichroism measurements were made on JASCO J810 spectropolarimeter calibrated with ammonium (þ)-10-camphorsulfonate with a 2 mm path length cell at 25 C. The values obtained were normalized by subtracting the baseline recorded for the buffer under similar conditions. 2.6. Molecular dynamics simulation Crystal structure of MtbICL (PDB-id: 1F61) [16] was retrieved from protein data bank (www.rcsb.org). Mutant model of MtbICL was obtained after replacing His180 with alanine in USCF Chimera1.6 [22]. Molecular dynamics simulation of wild and mutant MtbICL was carried out for 10 ns using Gromacs4.5 [23]. 76 H. Shukla et al. / Biochimie 110 (2015) 73e80 Protein molecule was solvated in cubic box with TIP3P water model under periodic boundary condition. Whole system was neutralized by adding Naþ ion. After energy minimization solvated system was subjected to NVT and NPT equilibration. Final production simulation was performed under NPT condition for 10 ns. For trajectory analysis VMD software [24] and xmgrace were used. 3. Results and discussion 3.1. Purification of full length, truncated and mutant MtbICL The recombinant enzyme and mutants such as MtbICL/MtbICLa12,a13,a14, tMtbICLa12,a13, tMtbICLa12, tMtbICL, MtbICLH180A, MtbICLY89A MtbICLE423D, MtbICLE424D, MtbICLE423D E424D, MtbICLE423A, MtbICLE424A, MtbICLE423AE424A, MtbICLE423K, MtbICLE424k, MtbICLE423KE424K, MtbICLE423L, MtbICLE424L and MtbICLE423LE424L were purified by the methods described under “Experimental”. The molecular mass of the purified proteins was determined by SDS-PAGE and the oligomeric status by size exclusion chromatography. The secondary and tertiary structure of mutants was determined by spectroscopic studies and were observed to be similar to those of MtbICL (Supplemental Fig. 1). 3.2. H180A mutation compromises the subunit assembly of MtbICL Among the four highly conserved regions of ICL derived from different organisms, the first conserved region (residues 173e197) contains an ICL signature sequence (K193KCGH197) (Fig. 2A) [25,17] and a conserved histidine at position 180 (His180). The ICL signature sequence is important for substrate binding and catalysis because any non-homologous mutation in the region severely affects the function of ICL in E. coli [25,26]. We mutated the corresponding histidine (His180) of MtbICL (MtbICLH180A) and checked its effect on the oligomerization of the enzyme by SEC (Fig. 2B). The elution volumes of MtbICL (12.4 ml) and MtbICLH180A (15.3 ml) correspond to the molecular weights of ~192 kDa (tetramer) and ~48 kDa (monomer) and are suggestive of the dissociation of tetrameric MtbICL. The subsequent glutaraldehyde crosslinking experiment further suggested the complete dissociation of the mutant MtbICLH180A to monomer (Fig. 2B, inset A). The dissociation of MtbICLH180A from tetramer to monomer also leads to the complete loss in the enzymatic activity (Fig. 2C). We subsequently looked for the salt strength at which the MtbICL could be dissociated without His180 mutation and observed the formation of dimeric as well as monomeric MtbICL at about 0.75 M Na2SO4(Supplemental Fig. 2). The dimeric and monomeric MtbICL were enzymatically inactive (data not shown). We subsequently measured the enzymatic activity of MtbICL between 0.5 M2.0 M NaCl to help differentiate, whether the observed inactivity of MtbICL was due to high Na2SO4 concentration or because of the change in the oligomeric conformation. The MtbICL had no apparent change in the enzymatic activity up to 1.5 M NaCl (Supplemental Fig. 3) however, tended to precipitate after 1.75 M NaCl with the loss in the enzymatic activity. The oligomeric status of MtbICL remained unchanged up to 2 M NaCl (data not shown). 3.3. H180A mutation affects subunit assembly through destabilization of the a6 and C-terminal helices (a12, a13 and a14) With molecular dynamics simulation (10 ns), we analyzed the trajectories of MtbICL and its His180 mutant (MtbICLH180A). Both the proteins showed large deviation (up to 4 Ǻ) from the starting structure (Fig. 3A and Supplemental Fig. 4). The main reason behind the high value of root mean square deviation (RMSD) can be attributed to the highly flexible C-terminal region. It has been reported that the C-terminal region of MtbICL is more dynamic and helps in oligomerization through helix swapping of a12 and a13 with the neighboring subunits [15]. We carried out MD study on the single subunit of MtbICL, in which the C-terminal region is no longer stabilized due to the loss of swapping. Despite this, any effect of H180A mutation on monomeric structure could be analyzed. As evident from RMSD trajectory, mutant MtbICL showed irregular deviation with respect to wild MtbICL, indicating its structural instability. The observed effects of H180A mutation on the structure Fig. 2. H180A mutation breaks oligomerization in MtbICL e (A) The conserved region of Isocitrate Lyases containing the signature sequence (KKCGH) and histidine. (B) The SEC profiles of MtbICL and MtbICLH180A at pH 8.0 and 25 C. The inset A shows the glutaraldehyde cross-linked samples where lanes 1e4 represent, crosslinked MtbICL, molecular weight marker, uncrosslinked MtbICL and cross-linked MtbICLH180A respectively. The inset B shows the standard molecular weight marker used for the calibration of SEC column. (C) Relative enzymatic activity of MtbICL and MtbICLH180A. H. Shukla et al. / Biochimie 110 (2015) 73e80 77 Fig. 3. MtbICLH180A destabilizes the a6 and C-terminus helices e (A) Trajectory showing Ca-RMSD of wild and mutant MtbICL during 10 ns of dynamics simulation. (B) Superimposed wild (Orange) and mutant (Sky blue) MtbICL after 10 ns of MD simulation. Here last frame coordinates were used. C-terminal domains of both proteins are highlighted in the square, where major differences are visible. (C) Superimposed wild (Orange) and mutant (Sky blue) MtbICL after 10 ns of MD simulation (left). Close view of differences in and near active site pocket where H180A mutation exists (right). (D) Trajectory showing per residue RMSF of wild and mutant ICL during 10 ns of dynamics simulation. of MtbICL are largely on the a6 and C-terminal helices a12, a13 and a14 (Fig. 3B). In case of Aspergillus nidulans, His185 makes hydrogen bond with hydroxyl group of Tyr95 side [27]. The corresponding residues (His180 and Tyr89) are also conserved in Mtb and our simulation studies support the above finding. We observed that His180 and Tyr89 side chains are involved in pep stacking interaction and H-bonding (approximately 50% occupancy) in MtbICL (Fig. 3C). In addition, HE2 hydrogen of His180 appeared to make Hbond with the backbone of Leu90 residue. We propose that the loss of the above interactions between His180 and the hydroxyl group of Tyr89, alters the orientation of Tyr89 side chain leading to the movement of the helices in the vicinity. Like MtbICLH180A, the Tyr89 mutant (MtbICLY89A) also leads to the formation of inactive monomer (Supplemental Fig. 5). Supplemental Fig. 6 shows the key residues of the active site in the simulated structures of MtbICL. Root mean square fluctuation (RMSF) plot (Fig. 3D) clearly shows that the a6 and C-terminal helices of mutant MtbICL undergo more fluctuation than those of wild MtbICL. Overall, our simulation results show that the H180A mutation alters the orientation of a6 and C-terminal helices, which in turn may impede the oligomerization process. 3.4. Inter helix domain swapping is not exclusive for the tetrameric conformation of MtbICL We have observed that the mutant MtbICLH180A show structural instability and affects largely the helices a6, a12, a13 and a14. The helix a12 (residues 349e367) together with the two helices a13 (residues 370e384) and a14 (residues 399e409) interacts with the neighboring subunits. Helix a12 and a13 are mainly involved in helix swapping, essential for tetrameric oligomerization [15]. For studying the effect of helix-swapping on the molecular assembly of MtbICL, we systematically truncated the a-helices and looked for the effect on oligomerization and enzymatic activity. Fig. 4A summarizes the result of the size exclusion profile of MtbICL/MtbICLa12,a13,a14, tMtbICLa12,a13, tMtbICLa12 and tMtbICL showing oligomeric stability. For full length as well as truncated MtbICLs a similar peak at about 12.25 ± 0.4 ml was observed corresponding to the tetramer (~192 ± 24 kDa). The oligomeric stability of full length as well as the truncated MtbICLs was also confirmed by glutaraldehyde crosslinking (Fig. 4A inset). The crosslinked band of proteins observed at about 192 kDa corresponds to a tertameric configuration. The truncation of MtbICL leads to a slight change in the secondary structure (Supplemental Fig. 7). The stability of the truncated enzyme was also studied by optical spectroscopy in the presence of increasing concentrations of chaotropic agents. Fig. 4B and C summarize the GdnHCl and urea induced changes in the structural properties of recombinant MtbICLs. The observed Cm (concentration of denaturant where 50% denaturation is observed) of MtbICL/MtbICLa12,a13,a14, tMtbICLa12,a13, tMtbICLa12 and tMtbICL are presented in Table 2 which primarily suggest that the removal of a-helices affects the overall stability of MtbICL. In case of the truncated MtbICLs, the presence of small peaks in the void volume suggests their overall instability as 78 H. Shukla et al. / Biochimie 110 (2015) 73e80 Fig. 4. The oligomeric conformation of MtbICL is not dependent on helix swapping e (A) The SEC profiles represent MtbICL/MtbICLa12,a13,a14, tMtbICLa12,a13, tMtbICLa12 and tMtbICL at pH 8.0 and 25 C. The inset shows the glutaraldehyde cross-linked MtbICL and its truncations. Lanes 1e6 represents, molecular weight markers, uncrosslinked MtbICL, cross-linked MtbICL, cross-linked tMtbICLa12,a13, cross-linked tMtbICLa12 and cross-linked tMtbICL respectively. (B) Plot of the fractional change in the wavelength maxima of fluorescence of MtbICL ( ), tMtbICLa12,a13 (◊), tMtbICLa12 (D) and tMtbICL (7) with increasing concentration of urea. (C) The plot of the fractional change in the wavelength maxima of fluorescence of MtbICL ( ), tMtbICLa12,a13 (◊), tMtbICLa12 (D) and tMtbICL (7) with increasing concentration of GdnHCl. (D) The dimer of MtbICL showing complementarity of surface involved in inter-subunit interaction and the interacting residues of a6 helix from both the monomeric subunits. Chain A and B are colored in orange and blue respectively. Hydrogen bonds are displayed in black dotted lines. The non-polar hydrogens are not shown to avoid crowding. (E) The relative enzymatic activity of MtbICL, tMtbICLa12,a13, tMtbICLa12 and tMtbICL. ▫ ▫ compared to the full length MtbICL (Fig. 4A). A similar peak in the void volume was also observed in the case of MtbICLH180A (Fig. 2B). However, as mentioned above, these instabilities do not affect the oligomeric configuration and hence the other interactions in addition to helix swapping are also essential for stable MtbICL oligomerization. As our experimental study suggested that the truncation of three C-terminal helices i.e., a12, a13 and a14 respectively did not affect the oligomerization state, we looked for inter-subunit interactions essential for holding the subunits. We selected the dimer structure of MtbICL and subsequently truncated it. The resulting dimer of MtbICL had all structural elements except the a12, a13 and a14 helices. Fig. 4D shows that the interface region of the dimer, containing all the residues may be involved in the inter-subunit interactions. Detailed analysis of the interaction pattern showed a substantial number of H-bonds between two monomer units (for H-bond details, please see the Supplemental Table). Important residues, which were involved in H-bonds, are Asn75, Asp98, Gly103, Thr105, Arg123, Asn126 and Arg130 from both subunits. Also, we observed electrostatic interaction between Asp98 and Arg123 from both subunits. Apart from the above interactions, shape complementarity can be another factor that holds the subunits together. A recent study shows that besides electrostatic and hydrophobic complementarity, shape complementarity is the third key factor affecting proteineprotein interactions. As we see, the contact surfaces of the subunits appear complementary. Involvement of inter-subunit interaction in oligomerization is further substantiated by the results of MD simulation studies on the monomeric mutant MtbICL (Supplemental Fig. 4). The substrate binding to MtbICL leads to a large conformational changes in the active site loop as well as the carboxy terminal end residues (lid region) of the adjacent subunit. We studied the effect Table 2 Comparison of Urea and Guanidine hydrochloride Cm values for full length MtbICL and truncations. Protein Oligomeric state Cm urea (M) Cm GdnHCl (M) MtbICL/MtbICLa12,a13,a14 tMtbICLa12,a13 tMtbICLa12 tMtbICL Tetramer Tetramer Tetramer Tetramer 3.8 2.8 2.4 1.5 1.9 1.4 1.24 0.75 H. Shukla et al. / Biochimie 110 (2015) 73e80 79 Fig. 5. Closure of lid is influenced by electrostatic interaction e (A) Close view of lid region and active site interface showing important residues of MtbICL. (B) Relative enzymatic activity of MtbICL and the mutants where the glutamic acid of the lid region was replaced with homologous, non homologous, basic and hydrophobic residues. of truncations on enzymatic activity and observed that the truncated MtbICLs were enzymatically inactive (Fig. 4E). The loss in activity was mainly because of the absence of lid region and non closure of the active site loop as necessary to attain the catalytic conformation. 3.5. Lid closure to accomplish catalytic conformation is influenced by electrostatic interactionThe amino acid sequences of the active site loop (L185ASEKKCGHLGG196) and the C-terminal lid (P411NSSTTALTGSTEEGQFH428) suggest the possibility of electrostatic interaction between the two. From the crystal structure of closed form of MtbICL (PDB:1F8M, 1F8I) it is evident that the C-terminal lid residues participate in hydrogen bonding and electrostatic interactions with the neighboring residues. The above interactions appear crucial in terms of maintaining the appropriate conformation of lid region for the closure of active site. In the closed form of MtbICL, side chain of E423 forms H-bonds with main chain NH of Gln394 and Arg395 residues (Fig. 5A). E424 does not appear to form any Hbond with neighboring residues and the orientation of its side chain slightly differs in the crystal structures of the closed form. To check the role of E423 and E424, we mutated these residues to homologous (aspartic acids), non homologous (alanine), basic (lysine) and hydrophobic (leucine) amino acids and measured the enzymatic activity of MtbICL and its single and double mutants Table 3 Enzyme kinetic parameters of MtbICL and its mutants. Protein Km (mM) MtbICL MtbICLE423D MtbICLE424D MtbICLE423D,E424D MtbICLE423A MtbICLE424A MtbICLE423A,E424A MtbICLE423K MtbICLE424K MtbICLE423K,E424K MtbICLE423L MtbICLE424L MtbICLE423L,E424L 1.142 1.423 1.861 2.031 ND ND ND ND ND ND ND ND ND ND: not determined. ± ± ± ± 0.065 0.097 0.097 0.157 (Fig. 5B). The homologous mutants (MtbICLE423D, MtbICLE424D and MtbICLE423D E424D) were enzymatically active, albeit with reduced activity and specificity as indicated by lower Kcat values (Table 3). The other mutants (MtbICLE423A, MtbICLE424A, MtbICLE423AE424A, MtbICLE423K, MtbICLE424k, MtbICLE423KE424K, MtbICLE423L, MtbICLE424L and MtbICLE423LE424L) were enzymatically inactive. The mutations in E423 and E424 despite affecting the enzyme catalytic properties, had no apparent effect on the oligomeric structure and stability of MtbICL (Supplemental Fig. 1). From mutational analysis, it is clear that E423 and E424 have critical roles in the function of MtbICL. However, in the crystal structures, side chain of E424 is sequestered and less likely to be involved in electrostatic interaction(s). Now the question is how E424 is important? The crystal structure of the closed form of MtbICL was analyzed and it was found that K392 and R395 lie in the close vicinity of E423 and E424. However the side chains of these glutamates are not in the optimal orientation to from electrostatic interactions. We tried different rotamer combinations and a few combinations suggested the possibility of H-bonding or electrostatic interaction with E424 also (Supplemental Fig. 8). B-factor analysis of the monomeric subunit using UCSF chimera shows that the extreme C-terminal residues (410e427) have high atomic B-factor and hence high flexibility of this region. It appears that the extreme C-terminal region can adopt a different orientation and with the course can interact with K392. The observations suggest that the closure of lid to accomplish the catalytic conformation is influenced not only by the electrostatic interactions involving E423 but may also of E424. 4. Conclusion Vmax (mM1$min1) Kcat (s1) 14.67 ± 0.23 12.52 ± 0.25 12.7 ± 0.21 11.3 ± 0.28 ND ND ND ND ND ND ND ND ND 9.78 8.34 8.46 7.53 ND ND ND ND ND ND ND ND ND ± ± ± ± 0.15 0.16 0.14 0.18 The mutation of His180 leads to the destabilization and formation of monomeric MtbICL. The RMSD and RMSF trajectories of mutant MtbICL show irregular deviation with respect to wild MtbICL, which renders major conformational changes in the Cterminal helices, i.e. a6, a12, a13 and a14. Truncation of a12ea14 in MtbICL did not lead to the formation of monomer as proposed before, suggesting that the oligomerization in MtbICL is further strengthened by the inter-subunit interactions influenced mostly by electrostatic interactions. The movement of lid containing the carboxy terminus end residues (P411NSSTTALTGSTEEGQFH428) of adjacent subunit over the catalytic loop (L185ASEKKCGHLGG196) containing ICL signature sequences (K189KCGH193) is essential to accomplish the enzymatic activity (15). The closure of lid seems to 80 H. Shukla et al. / Biochimie 110 (2015) 73e80 be influenced by electrostatic interactions in order to stabilize the lid in a suitable conformation during closed form. Since the mutant MtbICLs (glutamic acids to alanine)were inactive, we assume that the movement of the lid was affected due to the absence of electrostatic interaction with neighboring K392 and R395 residues of the lid. The conformational freedom and structural dynamics associated with MtbICL seems to be ideal for rational drug design, especially based on structure-activity relationship. Apart from the screening of inhibitors against the active site pocket of MtbICL, screening of compounds docked against a region affecting the enzyme function may also be useful. It is an urgent need to have a drug for persistent infection not only of M. tuberculosis but also for other microbial pathogens that shows persistent infection and contain ICL eg. Pseudomonas21, Salmonella22, Yersinia23 and Leishmania24 etc. [15]. Conflict of interest None. Acknowledgments This work was supported in part by CSIR-YSA (YSA0001) to MSA, and CSIR-GENESIS (BSC0121) to MIS. 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