Chitooligosaccharide binding to CIA17 (Coccinia indica agglutinin). Thermodynamic characterization and formation of higher order complexes.

CIA17 is a PP2-like, homodimeric lectin made up of 17 kDa subunits present in the phloem exudate of ivy gourd (Coccinia indica). Isothermal titration calorimetric (ITC) studies on the interaction of chitooligosaccharides [(GlcNAc)2-6] showed that the dimeric protein has two sugar binding sites which recognize chitotriose with ~70-fold higher affinity than chitobiose, indicating that the binding site is extended in nature. ITC, atomic force microscopic and non-denaturing gel electrophoresis studies revealed that the high-affinity interaction of CIA17 with chitohexaose (Ka = 1.8 × 107 M-1) promotes the formation of protein oligomers. Computational studies involving homology modeling, molecular docking and molecular dynamics simulations on the binding of chitooligosaccharides to CIA17 showed that the protein binding pocket accommodates up to three GlcNAc residues. Interestingly, docking studies with chitohexaose indicated that its two triose units could interact with binding sites on two protein molecules to yield dimeric complexes of the type CIA17-(GlcNAc)6-CIA17, which can extend in length by the binding of additional chitohexaose and CIA17 molecules. These results suggest that PP2 proteins play a role in plant defense against insect/pathogen attack by directly binding with the higher chain length chitooligosaccharides and forming extended, filamentous structures, which facilitate wound sealing.

Biochemical and structural studies of mutants indicate concerted movement of the dimer interface and ligand-binding region of Mycobacterium tuberculosis pantothenate kinase.

Two point mutants and the corresponding double mutant of Mycobacterium tuberculosis pantothenate kinase have been prepared and biochemically and structurally characterized. The mutants were designed to weaken the affinity of the enzyme for the feedback inhibitor CoA. The mutants exhibit reduced activity, which can be explained in terms of their structures. The crystals of the mutants are not isomorphous to any of the previously analysed crystals of the wild-type enzyme or its complexes. The mycobacterial enzyme and its homologous Escherichia coli enzyme exhibit structural differences in their nucleotide complexes in the dimer interface and the ligand-binding region. In three of the four crystallographically independent mutant molecules the structure is similar to that in the E. coli enzyme. Although the mutants involve changes in the CoA-binding region, the dimer interface and the ligand-binding region move in a concerted manner, an observation which might be important in enzyme action. This work demonstrates that the structure of the mycobacterial enzyme can be transformed into a structure similar to that of the E. coli enzyme through minor perturbations without external influences such as those involving ligand binding.

Distortion of the ligand molecule as a strategy for modulating binding affinity: Further studies involving complexes of jacalin with ß-substituted disaccharides.

Crystal structures of jacalin in complex with GlcNAc ß-(1,3) Gal-ß-OMe and Gal ß-(1,3) Gal-ß-OMe have been determined. The binding of the ligands to jacalin is similar to that of analogous α-substituted disaccharides. However, the ß-substituted ß-(1,3) linked disaccharides get distorted at the anomeric center and the glycosidic linkage. The distortion results in higher internal energies of the ligands leading to lower affinity to the lectin. This confirms the possibility of using ligand distortion as a strategy for modulating binding affinity. Unlike in the case of ß-substituted monosaccharides bound to jacalin, where a larger distortion at the anomeric center was observed, smaller distortions are distributed among two centers in the structures of the two ß-substituted ß-(1,3) linked disaccharides presented here. These disaccharides, like the unsubstituted and α-substituted counterparts, bind jacalin with the reducing Gal at the primary binding site, indicating that the lower binding affinity of ß-substituted disaccharides is not enough to overcome the intrinsic propensity of Gal ß-(1,3) Gal-based disaccharides to bind jacalin with the reducing sugar at the primary site. © 2017 IUBMB Life, 69(2):72-78, 2017.

Structural plasticity in Mycobacterium tuberculosis uracil-DNA glycosylase (MtUng) and its functional implications.

17 independent crystal structures of family I uracil-DNA glycosylase from Mycobacterium tuberculosis (MtUng) and its complexes with uracil and its derivatives, distributed among five distinct crystal forms, have been determined. Thermodynamic parameters of binding in the complexes have been measured using isothermal titration calorimetry. The two-domain protein exhibits open and closed conformations, suggesting that the closure of the domain on DNA binding involves conformational selection. Segmental mobility in the enzyme molecule is confined to a 32-residue stretch which plays a major role in DNA binding. Uracil and its derivatives can bind to the protein in two possible orientations. Only one of them is possible when there is a bulky substituent at the 5' position. The crystal structures of the complexes provide a reasonable rationale for the observed thermodynamic parameters. In addition to providing fresh insights into the structure, plasticity and interactions of the protein molecule, the results of the present investigation provide a platform for structure-based inhibitor design.

Jacalin-carbohydrate interactions: distortion of the ligand molecule as a determinant of affinity.

Jacalin is among the most thoroughly studied lectins. Its carbohydrate-binding site has also been well characterized. It has been postulated that the lower affinity of ß-galactosides for jacalin compared with α-galactosides is caused by steric interactions of the substituents in the former with the protein. This issue has been explored energetically and structurally using different appropriate carbohydrate complexes of jacalin. It turns out that the earlier postulation is not correct. The interactions of the substituent with the binding site remain essentially the same irrespective of the anomeric nature of the substitution. This is achieved through a distortion of the sugar ring in ß-galactosides. The difference in energy, and therefore in affinity, is caused by a distortion of the sugar ring in ß-galactosides. The elucidation of this unprecedented distortion of the ligand as a strategy for modulating affinity is of general interest. The crystal structures also provide a rationale for the relative affinities of the different carbohydrate ligands for jacalin.

Methionine down-regulates TLR4/MyD88/NF-κB signalling in osteoclast precursors to reduce bone loss during osteoporosis.

BACKGROUND AND PURPOSE: Studies have demonstrated that a moderate intake of amino acids is associated with development of bone health. Methionine, a sulphur-containing essential amino acid, has been largely implicated for improving cartilage formation, however its physiological significance on bone integrity and functionality have not been elucidated. We investigated whether methionine can prevent osteoporotic bone loss. EXPERIMENTAL APPROACH: The anti-resorptive effect of methionine, (250 mg kg(-1) body wt administered in drinking water for 10 weeks), was evaluated in ovariectomized (OVX) rats by monitoring changes in bone turnover, formation of osteoclasts from blood-derived mononuclear cells and changes in the synthesis of pro-osteoclastogenic cytokines. KEY RESULTS: Methionine improved bone density and significantly decreased the degree of osteoclast development from blood mononuclear cells in OVX rats, as indicated by decreased production of osteoclast markers tartarate resistant acid phosphatase b (TRAP5b) and MIP-1α. siRNA-mediated knockdown of myeloid differentiation primary response 88 [MyD88], a signalling molecule in the toll-like receptor (TLR) signalling cascade, abolished the synthesis of both TRAP5b and MIP-1α in developing osteoclasts. Methionine supplementation disrupted osteoclast development by inhibiting TLR-4/MyD88/NF-κB pathway. CONCLUSIONS AND IMPLICATIONS: TLR-4/MyD88/NF-κB signalling pathway is integral for osteoclast development and this is down-regulated in osteoporotic system on methionine treatment. Methionine treatment could be beneficial for the treatment of postmenopausal osteoporosis.

Cloning, expression, purification, crystallization and preliminary X-ray studies of argininosuccinate lyase (Rv1659) from Mycobacterium tuberculosis.

The last enzyme in the arginine-biosynthesis pathway, argininosuccinate lyase, from Mycobacterium tuberculosis has been cloned, expressed, purified and crystallized, and preliminary X-ray studies have been carried out on the crystals. The His-tagged tetrameric enzyme with a subunit molecular weight of 50.9 kDa crystallized with two tetramers in the asymmetric unit of the orthorhombic unit cell, space group P2(1)2(1)2(1). Molecular-replacement calculations and self-rotation calculations confirmed the space group and the tetrameric nature of the molecule.

Location and conformation of pantothenate and its derivatives in Mycobacterium tuberculosis pantothenate kinase: insights into enzyme action.

Previous studies of complexes of Mycobacterium tuberculosis PanK (MtPanK) with nucleotide diphosphates and nonhydrolysable analogues of nucleoside triphosphates in the presence or the absence of pantothenate established that the enzyme has dual specificity for ATP and GTP, revealed the unusual movement of ligands during enzyme action and provided information on the effect of pantothenate on the location and conformation of the nucleotides at the beginning and the end of enzyme action. The X-ray analyses of the binary complexes of MtPanK with pantothenate, pantothenol and N-nonylpantothenamide reported here demonstrate that in the absence of nucleotide these ligands occupy, with a somewhat open conformation, a location similar to that occupied by phosphopantothenate in the `end' complexes, which differs distinctly from the location of pantothenate in the closed conformation in the ternary `initiation' complexes. The conformation and the location of the nucleotide were also different in the initiation and end complexes. An invariant arginine appears to play a critical role in the movement of ligands that takes place during enzyme action. The work presented here completes the description of the locations and conformations of nucleoside diphosphates and triphosphates and pantothenate in different binary and ternary complexes, and suggests a structural rationale for the movement of ligands during enzyme action. The present investigation also suggests that N-alkylpantothenamides could be phosphorylated by the enzyme in the same manner as pantothenate.

M. tuberculosis pantothenate kinase: dual substrate specificity and unusual changes in ligand locations.

Kinetic measurements of enzyme activity indicate that type I pantothenate kinase from Mycobacterium tuberculosis has dual substrate specificity for ATP and GTP, unlike the enzyme from Escherichia coli, which shows a higher specificity for ATP. A molecular explanation for the difference in the specificities of the two homologous enzymes is provided by the crystal structures of the complexes of the M. tuberculosis enzyme with (1) GMPPCP and pantothenate, (2) GDP and phosphopantothenate, (3) GDP, (4) GDP and pantothenate, (5) AMPPCP, and (6) GMPPCP, reported here, and the structures of the complexes of the two enzymes involving coenzyme A and different adenyl nucleotides reported earlier. The explanation is substantially based on two critical substitutions in the amino acid sequence and the local conformational change resulting from them. The structures also provide a rationale for the movement of ligands during the action of the mycobacterial enzyme. Dual specificity of the type exhibited by this enzyme is rare. The change in locations of ligands during action, observed in the case of the M. tuberculosis enzyme, is unusual, so is the striking difference between two homologous enzymes in the geometry of the binding site, locations of ligands, and specificity. Furthermore, the dual specificity of the mycobacterial enzyme appears to have been caused by a biological necessity.

Mycobacterium tuberculosis pantothenate kinase: possible changes in location of ligands during enzyme action.

The crystal structures of complexes of Mycobacterium tuberculosis pantothenate kinase with the following ligands have been determined: (i) citrate; (ii) the nonhydrolysable ATP analogue AMPPCP and pantothenate (the initiation complex); (iii) ADP and phosphopantothenate resulting from phosphorylation of pantothenate by ATP in the crystal (the end complex); (iv) ATP and ADP, each with half occupancy, resulting from a quick soak of crystals in ATP (the intermediate complex); (v) CoA; (vi) ADP prepared by soaking and cocrystallization, which turned out to have identical structures, and (vii) ADP and pantothenate. Solution studies on CoA binding and catalytic activity have also been carried out. Unlike in the case of the homologous Escherichia coli enzyme, AMPPCP and ADP occupy different, though overlapping, locations in the respective complexes; the same is true of pantothenate in the initiation complex and phosphopantothenate in the end complex. The binding site of MtPanK is substantially preformed, while that of EcPanK exhibits considerable plasticity. The difference in the behaviour of the E. coli and M. tuberculosis enzymes could be explained in terms of changes in local structure resulting from substitutions. It is unusual for two homologous enzymes to exhibit such striking differences in action. Therefore, the results have to be treated with caution. However, the changes in the locations of ligands exhibited by M. tuberculosis pantothenate kinase are remarkable and novel.

Napin from Brassica juncea: thermodynamic and structural analysis of stability.

The napin from Brassica juncea, oriental mustard, is highly thermostable, proteolysis resistant and allergenic in nature. It consists of two subunits - one small (29 amino acid residues) and one large (86 amino acids residues) - held together by disulfide bonds. The thermal unfolding of napin has been followed by differential scanning calorimetry (DSC) and circular dichroism (CD) measurements. The thermal unfolding is characterized by a three state transition with T(M1) and T(M2) at 323.5 K and 335.8 K, respectively; DeltaC(P1) and DeltaC(P2) are 2.05 kcal mol(-1) K(-1) and 1.40 kcal mol(-1) K(-1), respectively. In the temperature range 310-318 K, the molecule undergoes dimerisation. Isothermal equilibrium unfolding by guanidinium hydrochloride also follows a three state transition, N <_-_-> I <_-_-> U with DeltaG(1H2O) and DeltaG(2H2O) values of 5.2 kcal mol(-1) and 5.1 kcal mol(-1) at 300 K, respectively. Excess heat capacity values obtained, are similar to those obtained from DSC measurements. There is an increase in hydrodynamic radius from 20 A to 35.0 A due to unfolding by guanidinium hydrochloride. In silico alignment of sequences of napin has revealed that the internal repeats (40%) spanning residues 31 to 60 and 73 to 109 are conserved in all Brassica species. The internal repeats may contribute to the greater stability of napin. A thorough understanding of the structure and stability of these proteins is essential before they can be exploited for genetic improvements for nutrition.

Structural studies on peanut lectin complexed with disaccharides involving different linkages: further insights into the structure and interactions of the lectin.

Crystal structures of peanut lectin complexed with Galbeta1-3Gal, methyl-T-antigen, Galbeta1-6GalNAc, Galalpha1-3Gal and Galalpha1-6Glc and that of a crystal grown in the presence of Galalpha1-3Galbeta1-4Gal have been determined using data collected at 100 K. The use of water bridges as a strategy for generating carbohydrate specificity was previously deduced from the complexes of the lectin with lactose (Galbeta1-4Glc) and T-antigen (Galbeta1-3GalNAc). This has been confirmed by the analysis of the complexes with Galbeta1-3Gal and methyl-T-antigen (Galbeta1-3GalNAc-alpha-OMe). A detailed analysis of lectin-sugar interactions in the complexes shows that they are more extensive when the beta-anomer is involved in the linkage. As expected, the second sugar residue is ill-defined when the linkage is 1-->6. There are more than two dozen water molecules which occur in the hydration shells of all structures determined at resolutions better than 2.5 A. Most of them are involved in stabilizing the structure, particularly loops. Water molecules involved in lectin-sugar interactions are also substantially conserved. The lectin molecule is fairly rigid and does not appear to be affected by changes in temperature.

Invariance and variability in bacterial PanK: a study based on the crystal structure of Mycobacterium tuberculosis PanK.

Pantothenate kinase (PanK) is a ubiquitous and essential enzyme that catalyzes the first step of the universal coenzyme A biosynthetic pathway. In this step, pantothenate (vitamin B(5)) is converted to 4'-phosphopantothenate, which subsequently forms coenzyme A in four enzymatic steps. The complex of this enzyme from Mycobacterium tuberculosis (MtPanK) with a derivative of the feedback inhibitor coenzyme A has been crystallized in two forms and its structure solved. The structure was refined in both forms using room-temperature and low-temperature X-ray data. In both forms, the MtPanK subunit has a mononucleotide-binding fold with a seven-stranded central beta-sheet and helices on either side. However, there is a small though significant difference in subunit association between the two forms. The structure is also grossly similar to the enzyme from Escherichia coli. The active-site pocket and the dimeric interface are on two opposite sides of the PanK subunit. The enzymes from M. tuberculosis and E. coli exhibit several differences, particularly at the dimeric interface. On the other hand, the coenzyme A-binding region is almost entirely conserved. A delineation of the invariant and variable features of the PanK structure further indicates that the dimeric interface is very variable, while the coenzyme A-binding site is substantially invariant. A sequence alignment involving various bacterial PanKs is in agreement with this conclusion. The strong correlation between structural plasticity, evolutionary conservation and variability and function exhibited by the molecule could be important in the design of species-specific inhibitors of the enzyme.

Crystallographic identification of an ordered C-terminal domain and a second nucleotide-binding site in RecA: new insights into allostery.

RecA protein is a crucial and central component of the homologous recombination and DNA repair machinery. Despite numerous studies on the protein, several issues concerning its action, including the allosteric regulation mechanism have remained unclear. Here we report, for the first time, a crystal structure of a complex of Mycobacterium smegmatis RecA (MsRecA) with dATP, which exhibits a fully ordered C-terminal domain, with a second dATP molecule bound to it. ATP binding is an essential step for all activities of RecA, since it triggers the formation of active nucleoprotein filaments. In the crystal filament, dATP at the first site communicates with a dATP of the second site of an adjacent subunit, through conserved residues, suggesting a new route for allosteric regulation. In addition, subtle but definite changes observed in the orientation of the nucleotide at the first site and in the positions of the segment preceding loop L2 as well as in the segment 102-105 situated between the 2 nt, all appear to be concerted and suggestive of a biological role for the second bound nucleotide.

Unusual sugar specificity of banana lectin from Musa paradisiaca and its probable evolutionary origin. Crystallographic and modelling studies.

The crystal structure of a complex of methyl-alpha-D-mannoside with banana lectin from Musa paradisiaca reveals two primary binding sites in the lectin, unlike in other lectins with beta-prism I fold which essentially consists of three Greek key motifs. It has been suggested that the fold evolved through successive gene duplication and fusion of an ancestral Greek key motif. In other lectins, all from dicots, the primary binding site exists on one of the three motifs in the three-fold symmetric molecule. Banana is a monocot, and the three motifs have not diverged enough to obliterate sequence similarity among them. Two Greek key motifs in it carry one primary binding site each. A common secondary binding site exists on the third Greek key. Modelling shows that both the primary sites can support 1-2, 1-3, and 1-6 linked mannosides with the second residue interacting in each case primarily with the secondary binding site. Modelling also readily leads to a bound branched mannopentose with the nonreducing ends of the two branches anchored at the two primary binding sites, providing a structural explanation for the lectin's specificity for branched alpha-mannans. A comparison of the dimeric banana lectin with other beta-prism I fold lectins, provides interesting insights into the variability in their quaternary structure.

Structural basis for the energetics of jacalin-sugar interactions: promiscuity versus specificity.

Jacalin, a tetrameric lectin, is one of the two lectins present in jackfruit (Artocarpus integrifolia) seeds. Its crystal structure revealed, for the first time, the occurrence of the beta-prism I fold in lectins. The structure led to the elucidation of the crucial role of a new N terminus generated by post-translational proteolysis for the lectin's specificity for galactose. Subsequent X-ray studies on other carbohydrate complexes showed that the extended binding site of jacalin consisted of, in addition to the primary binding site, a hydrophobic secondary site A composed of aromatic residues and a secondary site B involved mainly in water-bridges. A recent investigation involving surface plasmon resonance and the X-ray analysis of a methyl-alpha-mannose complex, had led to a suggestion of promiscuity in the lectin's sugar specificity. To explore this suggestion further, detailed isothermal titration calorimetric studies on the interaction of galactose (Gal), mannose (Man), glucose (Glc), Me-alpha-Gal, Me-alpha-Man, Me-alpha-Glc and other mono- and oligosaccharides of biological relevance and crystallographic studies on the jacalin-Me-alpha-Glc complex and a new form of the jacalin-Me-alpha-Man complex, have been carried out. The binding affinity of Me-alpha-Man is 20 times weaker than that of Me-alpha-Gal. The corresponding number is 27, when the binding affinities of Gal and Me-alpha-Gal, and those of Man and Me-alpha-Man are compared. Glucose (Glc) shows no measurable binding, while the binding affinity of Me-alpha-Glc is slightly less than that of Me-alpha-Man. The available crystal structures of jacalin-sugar complexes provide a convincing explanation for the energetics of binding in terms of interactions at the primary binding site and secondary site A. The other sugars used in calorimetric studies show no detectable binding to jacalin. These results and other available evidence suggest that jacalin is specific to O-glycans and its affinity to N-glycans is extremely weak or non-existent and therefore of limited value in processes involving biological recognition.

Expression, purification, crystallization and preliminary X-ray crystallographic analysis of pantothenate kinase from Mycobacterium tuberculosis.

Pantothenate kinase is an essential enzyme in the bacterial life cycle. It catalyzes the phosphorylation of pantothenate (vitamin B5) to 4'-phosphopantothenate, the first step in the coenzyme A biosynthetic pathway. The enzyme from Mycobacterium tuberculosis, MW 35.7 kDa, has been cloned, expressed, purified and crystallized in two different trigonal crystal forms, both belonging to space group P3(1)21. Two complete data sets of resolution 2.5 A (form I) and 2.9 A (form II) from crystals with unit-cell parameters a = b = 78.3, c = 115.45 A and a = b = 107.63, c = 89.85 A, respectively, were collected at room temperature on a home X-ray source. Structures of both crystal forms were solved for one subunit in the asymmetric unit by molecular replacement.

Purification, crystallization and preliminary X-ray structure analysis of the banana lectin from Musa paradisiaca.

The banana lectin from Musa paradisiaca, MW 29.4 kDa, has been isolated, purified and crystallized. The trigonal crystals contain one dimeric molecule in the asymmetric unit. The structure has been solved using molecular replacement to a resolution of 3 A. The structure of the subunit is similar to that of jacalin-like lectins.

Effect of glycosylation on the structure of Erythrina corallodendron lectin.

The three-dimensional structure of the recombinant form of Erythrina corallodendron lectin, complexed with lactose, has been elucidated by X-ray crystallography at 2.55 A resolution. Comparison of this non-glycosylated structure with that of the native glycosylated lectin reveals that the tertiary and quaternary structures are identical in the two forms, with local changes observed at one of the glycosylation sites (Asn17). These changes take place in such a way that hydrogen bonds with the neighboring protein molecules in rECorL compensate those made by the glycan with the protein in ECorL. Contrary to an earlier report, this study demonstrates that the glycan attached to the lectin does not influence the oligomeric state of the lectin. Identical interactions between the lectin and the non-covalently bound lactose in the two forms indicate, in line with earlier reports, that glycosylation does not affect the carbohydrate specificity of the lectin. The present study, the first of its kind involving a glycosylated protein with a well-defined glycan and the corresponding deglycosylated form, provides insights into the structural aspects of protein glycosylation.

Structural basis for the carbohydrate specificities of artocarpin: variation in the length of a loop as a strategy for generating ligand specificity.

Artocarpin, a tetrameric lectin of molecular mass 65 kDa, is one of the two lectins extracted from the seeds of jackfruit. The structures of the complexes of artocarpin with mannotriose and mannopentose reported here, together with the structures of artocarpin and its complex with Me-alpha-mannose reported earlier, show that the lectin possesses a deep-seated binding site formed by three loops. The binding site can be considered as composed of two subsites; the primary site and the secondary site. Interactions at the primary site composed of two of the loops involve mainly hydrogen bonds, while those at the secondary site comprising the third loop are primarily van der Waals in nature. Mannotriose in its complex with the lectin interacts through all the three mannopyranosyl residues; mannopentose interacts with the protein using at least three of the five mannose residues. The complexes provide a structural explanation for the carbohydrate specificities of artocarpin. A detailed comparison with the sugar complexes of heltuba, the only other mannose-specific jacalin-like lectin with known three-dimensional structure in sugar-bound form, establishes the role of the sugar-binding loop constituting the secondary site, in conferring different specificities at the oligosaccharide level. This loop is four residues longer in artocarpin than in heltuba, providing an instance where variation in loop length is used as a strategy for generating carbohydrate specificity.