Jiban K. Dattagupta

REFINED CRYSTAL STRUCTURE (2.3 A) OF A DOUBLE-HEADED WINGED BEAN ALPHA -CHYMOTRYPSIN INHIBITOR AND LOCATION OF ITS SECOND REACTIVE SITE

The crystal structure of a double‐headed α‐chymotrypsin inhibitor, WCI, from winged bean seeds has now been refined at 2.3 Å resolution to an R‐factor of 18.7% for 9,897 reflections. The crystals belong to the hexagonal space group P6122 with cell parameters a = b = 61.8 Å and c = 212.8 Å. The final model has a good stereochemistry and a root mean square deviation of 0.011 Å and 1.14° from ideality for bond length and bond angles, respectively. A total of 109 ordered solvent molecules were localized in the structure. This improved structure at 2.3 Å led to an understanding of the mechanism of inhibition of the protein against α‐chymotrypsin. An analysis of this higher resolution structure also helped us to predict the location of the second reactive site of the protein, about which no previous biochemical information was available. The inhibitor structure is spherical and has twelve anti‐parallel β‐strands with connecting loops arranged in a characteristic β‐trefoil fold common to other homologous serine protease inhibitors in the Kunitz (STI) family as well as to some non homologous functionally unrelated proteins. A wide variation in the surface loop regions is seen in the latter ones. Proteins 1999;35:321–331.

Improving thermostability of papain through structure-based protein engineering

Papain is a plant cysteine protease of industrial importance having a two-domain structure with its catalytic cleft located at the domain interface. A structure-based rational design approach has been used to improve the thermostability of papain, without perturbing its enzymatic activity, by introducing three mutations at its interdomain region. A thermostable homologue in papain family, Ervatamin C, has been used as a template for this purpose. A single (K174R), a double (K174RV32S) and a triple (K174RV32SG36S) mutant of papain have been generated, of which the triple mutant shows maximum thermostability with the half-life (t(1/2)) extended by 94 min at 60 degrees C and 45 min at 65 degrees C compared to the wild type (WT). The temperature of maximum enzymatic activity (T(max)) and 50% maximal activity (T(50)) for the triple mutant increased by 15 and 4 degrees C, respectively. Moreover, the triple mutant exhibits a faster inactivation rate beyond T(max) which may be a desirable feature for an industrial enzyme. The values of t(1/2) and T(max) for the double mutant lie between those of the WT and the triple mutant. The single mutant however turns out to be unstable for biochemical characterization. These results have been substantiated by molecular modeling studies which also indicate highest stability for the triple mutant based on higher number of interdomain H-bonds/salt-bridges, less interdomain flexibility and lower stability free-energy compared to the WT. In silico studies also explain the unstable behavior of the single mutant.

Production and recovery of recombinant propapain with high yield.

Papain (EC 3.4.22.2), the archetypal cysteine protease of C1 family, is of considerable commercial significance. In order to obtain substantial quantities of active papain, the DNA coding for propapain, the papain precursor, has been cloned and expressed at a high level in Escherichia coli BL21(DE3) transformed with two T7 promoter based pET expression vectors - pET30 Ek/LIC and pET28a(+) each containing the propapain gene. In both cases, recombinant propapain was expressed as an insoluble His-tagged fusion protein, which was solubilized, and purified by nickel chelation affinity chromatography under denaturing conditions. By systematic variation of parameters influencing the folding, disulfide bond formation and prevention of aggregate formation, a straightforward refolding procedure, based on dilution method, has been designed. This refolded protein was subjected to size exclusion chromatography to remove impurities and around 400mg of properly refolded propapain was obtained from 1L of bacterial culture. The expressed protein was further verified by Western blot analysis by cross-reacting it with a polyclonal anti-papain antibody and the proteolytic activity was confirmed by gelatin SDS-PAGE. This refolded propapain could be converted to mature active papain by autocatalytic processing at low pH and the recombinant papain so obtained has a specific activity closely similar to the native papain. This is a simple and efficient expression and purification procedure to obtain a yield of active papain, which is the highest reported so far for any recombinant plant cysteine protease.

Cryocrystallography of a Kunitz-type serine protease inhibitor: the 90 K structure of winged bean chymotrypsin inhibitor (WCI) at 2.13 A resolution.

The crystal structure of a Kunitz-type double-headed alpha--chymotrypsin inhibitor from winged bean seeds has been refined at 2.13 A resolution using data collected from cryo-cooled (90 K) crystals which belong to the hexagonal space group P6(1)22 with unit-cell parameters a = b = 60.84, c = 207.91 A. The volume of the unit cell is reduced by 5.3% on cooling. The refinement converged to an R value of 20.0% (R(free) = 25.8%) for 11100 unique reflections and the model shows good stereochemistry, with r.m.s. deviations from ideal values for bond lengths and bond angles of 0.011 A and 1.4 degrees, respectively. The structural architecture of the protein consists of 12 antiparallel beta-strands joined in the form of a characteristic beta-trefoil fold, with the two reactive-site regions, Asn38-Leu43 and Gln63-Phe68, situated on two external loops. Although the overall protein fold is the same as that of the room-temperature model, some conformational changes are observed in the loop regions and in the side chains of a few surface residues. A total of 176 ordered water molecules and five sulfate ions are included in the model.

Proposed amino acid sequence and the 1.63 A X-ray crystal structure of a plant cysteine protease, ervatamin B: some insights into the structural basis of its stability and substrate specificity.

The crystal structure of a cysteine protease ervatamin B, isolated from the medicinal plant Ervatamia coronaria, has been determined at 1.63 Å. The unknown primary structure of the enzyme could also be traced from the high‐quality electron density map. The final refined model, consisting of 215 amino acid residues, 208 water molecules, and a thiosulfate ligand molecule, has a crystallographic R‐factor of 15.9% and a free R‐factor of 18.2% for F > 2σ(F). The protein belongs to the papain superfamily of cysteine proteases and has some unique properties compared to other members of the family. Though the overall fold of the structure, comprising two domains, is similar to the others, a few natural substitutions of conserved amino acid residues at the interdomain cleft of ervatamin B are expected to increase the stability of the protein. The substitution of a lysine residue by an arginine (residue 177) in this region of the protein may be important, because Lys → Arg substitution is reported to increase the stability of proteins. Another substitution in this cleft region that helps to hold the domains together through hydrogen bonds is Ser36, replacing a conserved glycine residue in the others. There are also some substitutions in and around the active site cleft. Residues Tyr67, Pro68, Val157, and Ser205 in papain are replaced by Trp67, Met68, Gln156, and Leu208, respectively, in ervatamin B, which reduces the volume of the S2 subsite to almost one‐fourth that of papain, and this in turn alters the substrate specificity of the enzyme. Proteins 2003;51:489–497.

Structure of a Kunitz-type chymotrypsin from winged bean seeds at 2.95 A resolution.

Thc crystal structure of an alpha-chymotrypsin inhibitor (P6(1)22; a = 61.4, c = 210.9 A) isolated from winged bean (Psophocarpus. tetragonolobus) seeds has been determined at 2.95 A resolution by the molecular-replacement method using the 2.6 A coordinates of Erythrina trypsin inhibitor (ETI) as the starting model (57% sequence homology). This protease inhibitor, WCI, belongs to the Kunitz (STI) family and is a single polypeptide chain with 183 amino-acid residues having a molecular weight of 20 244 Da. Structure refinement with RESTRAIN and X-PLOR has led to a crystallographic R factor of 19.1% for 3469 observed reflections (I > 2sigma) in the resolution range 8-2.95 A. A total of 56 water molecules have been incorporated in the refined model containing 181 amino-acid residues. In the refined structure the deviations of bond lengths and bond angles from ideal values are 0.015 A and 2.2 degrees, respectively. The inhibitor molecule is spherical and consists of 12 antiparallel beta-strands with connecting loops arranged in a characteristic folding (a six-stranded beta-barrel and a six-stranded lid on one hollow end of the barrel) common to other homologous serine protease inhibitors in the Kunitz (STI) family as well as to some non-homologous proteins like interleukin-lalpha and interleukin-lbeta. In the structure the conformation of the protruding reactive-site loop is stabilized through hydrogen bonds mainly formed by the side chain of Asnl4, which intrudes inside the cavity of the reactive-site loop, with the side-chain and main-chain atoms of some residues in the loop region. A pseudo threefold axis exists parallel to the barrel axis of the structure. Each of the three subdomains comprises of four beta-strands with connecting loops.

Structural insights into the substrate specificity and activity of ervatamins, the papain‐like cysteine proteases from a tropical plant, Ervatamia coronaria

Multiple proteases of the same family are quite often present in the same species in biological systems. These multiple proteases, despite having high homology in their primary and tertiary structures, show deviations in properties such as stability, activity, and specificity. It is of interest, therefore, to compare the structures of these multiple proteases in a single species to identify the structural changes, if any, that may be responsible for such deviations. Ervatamin‐A, ervatamin‐B and ervatamin‐C are three such papain‐like cysteine proteases found in the latex of the tropical plant Ervatamia coronaria, and are known not only for their high stability over a wide range of temperature and pH, but also for variations in activity and specificity among themselves and among other members of the family. Here we report the crystal structures of ervatamin‐A and ervatamin‐C, complexed with an irreversible inhibitor 1‐[l‐N‐(trans‐epoxysuccinyl)leucyl]amino‐4‐guanidinobutane (E‐64), together with enzyme kinetics and molecular dynamic simulation studies. A comparison of these results with the earlier structures helps in a correlation of the structural features with the corresponding functional properties. The specificity constants (kcat/Km) for the ervatamins indicate that all of these enzymes have specificity for a branched hydrophobic residue at the P2 position of the peptide substrates, with different degrees of efficiency. A single amino acid change, as compared to ervatamin‐C, in the S2 pocket of ervatamin‐A (Ala67→Tyr) results in a 57‐fold increase in its kcat/Km value for a substrate having a Val at the P2 position. Our studies indicate a higher enzymatic activity of ervatamin‐A, which has been subsequently explained at the molecular level from the three‐dimensional structure of the enzyme and in the context of its helix polarizibility and active site plasticity.

Structural determinants of V. cholerae CheYs that discriminate them in FliM binding: comparative modeling and MD simulation studies.

Abstract Chemotaxis of Vibrio cholerae is a complex process where multiple paralogues of various chemotaxis genes participate. V. cholerae contains five copies of the response regulator protein CheY (CheV) and the role played by these CheY homologs in chemotaxis and virulence are investigated only through a few in vivo studies. As identification of the molecular features that discriminate CheYVs in terms of FliM binding is necessary for the detailed understanding of chemotaxis and pathogenesis, we built the models of CheYVs through comparative modeling and MD simulation was performed on each model in their phosphorylated and Mg+2 bound state. Our analysis identified the key structural elements, unique to CheY3V, which complement the N-terminal part of FliMV and we explained how the structure, shape, and surface properties of the FliM binding pocket of other CheYVs abrogate this function. Furthermore, we have provided the structural basis of a putative cross species interaction between CheYE and FliMV, identified in a recent in vivo study.

Structure of diferric hen serum transferrin at 2.8 A resolution.

Hen serum transferrin in its diferric form (hST) has been isolated, purified and the three-dimensional structure determined by X-ray crystallography at 2.8 A resolution. The final refined structure of hST, comprising 5232 protein atoms, two Fe(3+) cations, two CO(3)(2-) anions, 54 water molecules and one fucose moiety, has an R factor of 21.5% and an R(free) of 26.9% for all data. The structure has been compared with the three-dimensional structure of hen ovotransferrin (hOT) and also with structures of some other transferrins, viz. rabbit serum transferrin (rST) and human lactoferrin (hLF). The overall conformation of the hST molecule is essentially the same as that of other transferrins. However, the relative orientation of the two lobes, which is related to the species-specific receptor-recognition property of transferrins, has been found to be different in hST from that in hOT, rST and hLF. On the basis of superposition of the N lobes, rotations of 5.8, 16.9 and 11.3 degrees are required to bring the C lobes of hOT, rST and hLF, respectively, into coincidence with that of hST. A number of additional hydrogen bonds between the two domains in the N and C lobes have been identified in the structure of hST compared with that of hOT, which indicate a greater compactness of the lobes of hST than those of hOT. Being products of the same gene, hST and hOT have 100% sequence identity and differ only in the attached carbohydrate moiety. On the other hand, despite having similar functions, hST and rST have only 51% sequence similarity. However, the nature of the interdomain interactions of hST are closer to rST than to hOT. A putative carbohydrate-binding site has been identified in the N lobe of hST at Asn52 and a fucose molecule could be modelled at the site. The variations in interdomain and interlobe interactions in hST, together with altered lobe orientation with respect to hOT, rST and hLF, which are the representatives of the other subfamily of transferrins, are discussed.

The structure of a thermostable mutant of pro-papain reveals its activation mechanism.

Papain is the archetype of a broad class of cysteine proteases (clan C1A) that contain a pro-peptide in the zymogen form which is required for correct folding and spatio-temporal regulation of proteolytic activity in the initial stages after expression. This study reports the X-ray structure of the zymogen of a thermostable mutant of papain at 2.6 Å resolution. The overall structure, in particular that of the mature part of the protease, is similar to those of other members of the family. The structure provides an explanation for the molecular basis of the maintenance of latency of the proteolytic activity of the zymogen by its pro-segment at neutral pH. The structural analysis, together with biochemical and biophysical studies, demonstrated that the pro-segment of the zymogen undergoes a rearrangement in the form of a structural loosening at acidic pH which triggers the proteolytic activation cascade. This study further explains the bimolecular stepwise autocatalytic activation mechanism by limited proteolysis of the zymogen of papain at the molecular level. The possible factors responsible for the higher thermal stability of the papain mutant have also been analyzed.