Pravindra Kumar

Structure and function of proteins involved in milk allergies.

Allergy to milk proteins has been defined as any adverse reaction mediated by immunological mechanisms to one or several of proteins found in milk. The milk allergy has been classified according to the onset of symptoms as immediate or delayed type. The milk allergy seems to be manifested by three major proteins found in milk: alpha-lactalbumin, beta-lactoglobulin and caseins. The structural comparison of allergenic sites in alpha-lactalbumin and beta-lactoglobulin with the structure of lactoferrin has clearly shown that yet another major milk protein lactoferrin also possesses allergenic sites and thus may qualify to be an allergen. The heat treatment of milk proteins considerably reduces their allergenicity.

Purification and characterization of a trypsin inhibitor from Putranjiva roxburghii seeds.

A highly stable and potent trypsin inhibitor was purified to homogeneity from the seeds of Putranjiva roxburghii belonging to Euphorbiaceae family by acid precipitation, cation-exchange and anion-exchange chromatography. SDS-PAGE analysis, under reducing condition, showed that protein consists of a single polypeptide chain with molecular mass of approximately 34 kDa. The purified inhibitor inhibited bovine trypsin in 1:1 molar ratio. Kinetic studies showed that the protein is a competitive inhibitor with an equilibrium dissociation constant of 1.4x10(-11) M. The inhibitor retained the inhibitory activity over a broad range of pH (pH 2-12), temperature (20-80 degrees C) and in DTT (up to100 mM). The complete loss of inhibitory activity was observed above 90 degrees C. CD studies, at increasing temperatures, demonstrated the structural stability of inhibitor at high temperatures. The polypeptide backbone folding was retained up to 80 degrees C. The CD spectra of inhibitor at room temperature exhibited an alpha, beta pattern. N-terminal amino acid sequence of 10 residues did not show any similarities to known serine proteinase inhibitors, however, two peptides obtained by internal partial sequencing showed significant resemblance to Kunitz-type inhibitors.

The tautomeric half-reaction of BphD, a C-C bond hydrolase. Kinetic and structural evidence supporting a key role for histidine 265 of the catalytic triad.

BphD of Burkholderia xenovorans LB400 catalyzes an unusual C-C bond hydrolysis of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to afford benzoic acid and 2-hydroxy-2,4-pentadienoic acid (HPD). An enol-keto tautomerization has been proposed to precede hydrolysis via a gem-diol intermediate. The role of the canonical catalytic triad (Ser-112, His-265, Asp-237) in mediating these two half-reactions remains unclear. We previously reported that the BphD-catalyzed hydrolysis of HOPDA (λmax is 434 nm for the free enolate) proceeds via an unidentified intermediate with a red-shifted absorption spectrum (λmax is 492 nm) (Horsman, G. P., Ke, J., Dai, S., Seah, S. Y. K., Bolin, J. T., and Eltis, L. D. (2006) Biochemistry 45, 11071-11086). Here we demonstrate that the S112A variant generates and traps a similar intermediate (λmax is 506 nm) with a similar rate, 1/τ ∼ 500 s-1. The crystal structure of the S112A:HOPDA complex at 1.8-Å resolution identified this intermediate as the keto tautomer, (E)-2,6-dioxo-6-phenyl-hex-3-enoate. This keto tautomer did not accumulate in either the H265A or the S112A/H265A double variants, indicating that His-265 catalyzes tautomerization. Consistent with this role, the wild type and S112A enzymes catalyzed tautomerization of the product HPD, whereas H265A variants did not. This study thus identifies a keto intermediate, and demonstrates that the catalytic triad histidine catalyzes the tautomerization half-reaction, expanding the role of this residue from its purely hydrolytic function in other serine hydrolases. Finally, the S112A:HOPDA crystal structure is more consistent with hydrolysis occurring via an acyl-enzyme intermediate than a gem-diol intermediate as solvent molecules have poor access to C6, and the closest ordered water is 7Å away.

Structural Insight Into the Expanded Pcb-Degrading Abilities of a Biphenyl Dioxygenase Obtained by Directed Evolution.

The biphenyl dioxygenase of Burkholderia xenovorans LB400 is a multicomponent Rieske-type oxygenase that catalyzes the dihydroxylation of biphenyl and many polychlorinated biphenyls (PCBs). The structural bases for the substrate specificity of the enzymes oxygenase component (BphAE(LB400)) are largely unknown. BphAE(p4), a variant previously obtained through directed evolution, transforms several chlorobiphenyls, including 2,6-dichlorobiphenyl, more efficiently than BphAE(LB400), yet differs from the parent oxygenase at only two positions: T335A/F336M. Here, we compare the structures of BphAE(LB400) and BphAE(p4) and examine the biochemical properties of two BphAE(LB400) variants with single substitutions, T335A or F336M. Our data show that residue 336 contacts the biphenyl and influences the regiospecificity of the reaction, but does not enhance the enzymes reactivity toward 2,6-dichlorobiphenyl. By contrast, residue 335 does not contact biphenyl but contributes significantly to expansion of the enzymes substrate range. Crystal structures indicate that Thr335 imposes constraints through hydrogen bonds and nonbonded contacts to the segment from Val320 to Gln322. These contacts are lost when Thr is replaced by Ala, relieving intramolecular constraints and allowing for significant movement of this segment during binding of 2,6-dichlorobiphenyl, which increases the space available to accommodate the doubly ortho-chlorinated congener 2,6-dichlorobiphenyl. This study provides important insight about how Rieske-type oxygenases can expand substrate range through mutations that increase the plasticity and/or mobility of protein segments lining the catalytic cavity.

Retuning Rieske-type Oxygenases to Expand Substrate Range

Rieske-type oxygenases are promising biocatalysts for the destruction of persistent pollutants or for the synthesis of fine chemicals. In this work, we explored pathways through which Rieske-type oxygenases evolve to expand their substrate range. BphAEp4, a variant biphenyl dioxygenase generated from Burkholderia xenovorans LB400 BphAELB400 by the double substitution T335A/F336M, and BphAERR41, obtained by changing Asn338, Ile341, and Leu409 of BphAEp4 to Gln338, Val341, and Phe409, metabolize dibenzofuran two and three times faster than BphAELB400, respectively. Steady-state kinetic measurements of single- and multiple-substitution mutants of BphAELB400 showed that the single T335A and the double N338Q/L409F substitutions contribute significantly to enhanced catalytic activity toward dibenzofuran. Analysis of crystal structures showed that the T335A substitution relieves constraints on a segment lining the catalytic cavity, allowing a significant displacement in response to dibenzofuran binding. The combined N338Q/L409F substitutions alter substrate-induced conformational changes of protein groups involved in subunit assembly and in the chemical steps of the reaction. This suggests a responsive induced fit mechanism that retunes the alignment of protein atoms involved in the chemical steps of the reaction. These enzymes can thus expand their substrate range through mutations that alter the constraints or plasticity of the catalytic cavity to accommodate new substrates or that alter the induced fit mechanism required to achieve proper alignment of reaction-critical atoms or groups.

Cloning, sequence analysis and crystal structure determination of a miraculin-like protein from Murraya koenigii.

Earlier, the purification of a 21.4kDa protein with trypsin inhibitory activity from seeds of Murraya koenigii has been reported. The present study, based on the amino acid sequence deduced from both cDNA and genomic DNA, establishes it to be a miraculin-like protein and provides crystal structure at 2.9A resolution. The mature protein consists of 190 amino acid residues with seven cysteines arranged in three disulfide bridges. The amino acid sequence showed maximum homology and formed a distinct cluster with miraculin-like proteins, a soybean Kunitz super family member, in phylogenetic analyses. The major differences in sequence were observed at primary and secondary specificity sites in the reactive loop when compared to classical Kunitz family members. The crystal structure analysis showed that the protein is made of twelve antiparallel beta-strands, loops connecting beta-strands and two short helices. Despite similar overall fold, it showed significant differences from classical Kunitz trypsin inhibitors.

Structural characterization of Pandoraea pnomenusa B-356 biphenyl dioxygenase reveals features of potent polychlorinated biphenyl-degrading enzymes.

The oxidative degradation of biphenyl and polychlorinated biphenyls (PCBs) is initiated in Pandoraea pnomenusa B-356 by biphenyl dioxygenase (BPDOB356). BPDOB356, a heterohexameric (αβ)3 Rieske oxygenase (RO), catalyzes the insertion of dioxygen with stereo- and regioselectivity at the 2,3-carbons of biphenyl, and can transform a broad spectrum of PCB congeners. Here we present the X-ray crystal structures of BPDOB356 with and without its substrate biphenyl 1.6-Å resolution for both structures. In both cases, the Fe(II) has five ligands in a square pyramidal configuration: H233 Nε2, H239 Nε2, D386 Oδ1 and Oδ2, and a single water molecule. Analysis of the active sites of BPDOB356 and related ROs revealed structural features that likely contribute to the superior PCB-degrading ability of certain BPDOs. First, the active site cavity readily accommodates biphenyl with minimal conformational rearrangement. Second, M231 was predicted to sterically interfere with binding of some PCBs, and substitution of this residue yielded variants that transform 2,2′-dichlorobiphenyl more effectively. Third, in addition to the volume and shape of the active site, residues at the active site entrance also apparently influence substrate preference. Finally, comparison of the conformation of the active site entrance loop among ROs provides a basis for a structure-based classification consistent with a phylogeny derived from amino acid sequence alignments.

Structure-function studies of DNA binding domain of response regulator KdpE reveals equal affinity interactions at DNA half-sites.

Expression of KdpFABC, a K+ pump that restores osmotic balance, is controlled by binding of the response regulator KdpE to a specific DNA sequence (kdpFABCBS) via the winged helix-turn-helix type DNA binding domain (KdpEDBD). Exploration of E. coli KdpEDBD and kdpFABCBS interaction resulted in the identification of two conserved, AT-rich 6 bp direct repeats that form half-sites. Despite binding to these half-sites, KdpEDBD was incapable of promoting gene expression in vivo. Structure-function studies guided by our 2.5 Å X-ray structure of KdpEDBD revealed the importance of residues R193 and R200 in the α-8 DNA recognition helix and T215 in the wing region for DNA binding. Mutation of these residues renders KdpE incapable of inducing expression of the kdpFABC operon. Detailed biophysical analysis of interactions using analytical ultracentrifugation revealed a 2∶1 stoichiometry of protein to DNA with dissociation constants of 200±100 and 350±100 nM at half-sites. Inactivation of one half-site does not influence binding at the other, indicating that KdpEDBD binds independently to the half-sites with approximately equal affinity and no discernable cooperativity. To our knowledge, these data are the first to describe in quantitative terms the binding at half-sites under equilibrium conditions for a member of the ubiquitous OmpR/PhoB family of proteins.

Structural Basis for Dual Inhibitory Role of Tamarind Kunitz Inhibitor (Tki) Against Factor Xa and Trypsin.

A Kunitz type dual inhibitor (TKI) of factor Xa (FXa) and trypsin was found in tamarind. It also shows prolongation of blood coagulation time. The deduced 185 amino acid sequence of TKI by cDNA cloning and sequence analysis revealed that it belongs to the Kunitz type soybean trypsin inhibitor (STI) family; however, it has a distorted Kunitz signature sequence due to insertion of Asn15 in the motif. TKI exhibited a competitive inhibitory activity against both FXa (Ki = 220 nm) and porcine pancreatic trypsin (Ki = 3.2 nm). The crystal structure of TKI shows a β‐trefoil fold similar to Kunitz STI inhibitors; however, a distinct mobile reactive site, an inserted residue and loop β7β8 make it distinct from classical Kunitz inhibitors. The crystal structure of TKI‐trypsin and a 3D model of TKI‐FXa complex revealed that the distinct reactive site loop probably plays a role in dual inhibition. The reactive site of TKI interacts with an active site and two exosites (36 loop and autolysis loop) of FXa. Apart from Arg66 (P1), Arg64 (P3) is one of the most important residues responsible for the specificity of TKI towards FXa. Along with the reactive site loop (β4β5), loops β1 and β7β8 also interact with FXa and could further confer selectivity for FXa. We also present the role of inserted Asn15 in the stabilization of complexes. To the best of our knowledge, this is the first structure of FXa inhibitor belonging to the Kunitz type inhibitor family and its unique structural and sequence features make TKI a novel potent inhibitor.

Biochemical Studies and Ligand-bound Structures of Biphenyl Dehydrogenase from Pandoraea pnomenusa Strain B-356 Reveal a Basis for Broad Specificity of the Enzyme

Background: BphBB-356 catalyzes the second step of the PCB catabolic pathway. Result: Apo, binary, intermediate, and ternary structures were obtained. Conclusion: Conformational changes in the substrate binding loop lead to the formation of a structurally defined pocket to catalyze a wide range of substrates. Significance: Recognition of conformational changes in the substrate binding loop and insight into the substrate specificity. Biphenyl dehydrogenase, a member of short-chain dehydrogenase/reductase enzymes, catalyzes the second step of the biphenyl/polychlorinated biphenyls catabolic pathway in bacteria. To understand the molecular basis for the broad substrate specificity of Pandoraea pnomenusa strain B-356 biphenyl dehydrogenase (BphBB-356), the crystal structures of the apo-enzyme, the binary complex with NAD+, and the ternary complexes with NAD+-2,3-dihydroxybiphenyl and NAD+-4,4′-dihydroxybiphenyl were determined at 2.2-, 2.5-, 2.4-, and 2.1-Å resolutions, respectively. A crystal structure representing an intermediate state of the enzyme was also obtained in which the substrate binding loop was ordered as compared with the apo and binary forms but it was displaced significantly with respect to the ternary structures. These five structures reveal that the substrate binding loop is highly mobile and that its conformation changes during ligand binding, starting from a disorganized loop in the apo state to a well organized loop structure in the ligand-bound form. Conformational changes are induced during ligand binding; forming a well defined cavity to accommodate a wide variety of substrates. This explains the biochemical data that shows BphBB-356 converts the dihydrodiol metabolites of 3,3′-dichlorobiphenyl, 2,4,4′-trichlorobiphenyl, and 2,6-dichlorobiphenyl to their respective dihydroxy metabolites. For the first time, a combination of structural, biochemical, and molecular docking studies of BphBB-356 elucidate the unique ability of the enzyme to transform the cis-dihydrodiols of double meta-, para-, and ortho-substituted chlorobiphenyls.