Structures of exoglucanase from Clostridium cellulovorans: cellotetraose binding and cleavage.

Exoglucanase/cellobiohydrolase (EC 3.2.1.176) hydrolyzes a ß-1,4-glycosidic bond from the reducing end of cellulose and releases cellobiose as the major product. Three complex crystal structures of the glycosyl hydrolase 48 (GH48) cellobiohydrolase S (ExgS) from Clostridium cellulovorans with cellobiose, cellotetraose and triethylene glycol molecules were solved. The product cellobiose occupies subsites +1 and +2 in the open active-site cleft of the enzyme-cellotetraose complex structure, indicating an enzymatic hydrolysis function. Moreover, three triethylene glycol molecules and one pentaethylene glycol molecule are located at active-site subsites -2 to -6 in the structure of the ExgS-triethylene glycol complex shown here. Modelling of glucose into subsite -1 in the active site of the ExgS-cellobiose structure revealed that Glu50 acts as a proton donor and Asp222 plays a nucleophilic role.

A Lon-like protease with no ATP-powered unfolding activity.

Lon proteases are a family of ATP-dependent proteases involved in protein quality control, with a unique proteolytic domain and an AAA(+) (ATPases associated with various cellular activities) module accommodated within a single polypeptide chain. They were classified into two types as either the ubiquitous soluble LonA or membrane-inserted archaeal LonB. In addition to the energy-dependent forms, a number of medically and ecologically important groups of bacteria encode a third type of Lon-like proteins in which the conserved proteolytic domain is fused to a large N-terminal fragment lacking canonical AAA(+) motifs. Here we showed that these Lon-like proteases formed a clade distinct from LonA and LonB. Characterization of one such Lon-like protease from Meiothermus taiwanensis indicated that it formed a hexameric assembly with a hollow chamber similar to LonA/B. The enzyme was devoid of ATPase activity but retained an ability to bind symmetrically six nucleotides per hexamer; accordingly, structure-based alignment suggested possible existence of a non-functional AAA-like domain. The enzyme degraded unstructured or unfolded protein and peptide substrates, but not well-folded proteins, in ATP-independent manner. These results highlight a new type of Lon proteases that may be involved in breakdown of excessive damage or unfolded proteins during stress conditions without consumption of energy.

Molecular basis of the recognition of the ap65-1 gene transcription promoter elements by a Myb protein from the protozoan parasite Trichomonas vaginalis.

Iron-inducible transcription of the ap65-1 gene in Trichomonas vaginalis involves at least three Myb-like transcriptional factors (tvMyb1, tvMyb2 and tvMyb3) that differentially bind to two closely spaced promoter sites, MRE-1/MRE-2r and MRE-2f. Here, we defined a fragment of tvMyb2 comprising residues 40-156 (tvMyb240₋156) as the minimum structural unit that retains near full binding affinity with the promoter DNAs. Like c-Myb in vertebrates, the DNA-free tvMyb240₋156 has a flexible and open conformation. Upon binding to the promoter DNA elements, tvMyb240₋156 undergoes significant conformational re-arrangement and structure stabilization. Crystal structures of tvMyb240₋156 in complex with promoter element-containing DNA oligomers showed that 5'-a/gACGAT-3' is the specific base sequence recognized by tvMyb240₋156, which does not fully conform to that of the Myb binding site sequence. Furthermore, Lys49, which is upstream of the R2 motif (amino acids 52-102) also participates in specific DNA sequence recognition. Intriguingly, tvMyb240₋156 binds to the promoter elements in an orientation opposite to that proposed in the HADDOCK model of the tvMyb135₋141/MRE-1-MRE-2r complex. These results shed new light on understanding the molecular mechanism of Myb-DNA recognition and provide a framework to study the molecular basis of transcriptional regulation of myriad Mybs in T. vaginalis.

Structural and functional analysis of three ß-glucosidases from bacterium Clostridium cellulovorans, fungus Trichoderma reesei and termite Neotermes koshunensis.

ß-glucosidases (EC 3.2.1.21) cleave ß-glucosidic linkages in disaccharide or glucose-substituted molecules and play important roles in fundamental biological processes. ß-Glucosidases have been widely used in agricultural, biotechnological, industrial and medical applications. In this study, a high yield expression (70-250 mg/l) in Escherichia coli of the three functional ß-glucosidase genes was obtained from the bacterium Clostridium cellulovorans (CcBglA), the fungus Trichoderma reesei (TrBgl2), and the termite Neotermes koshunensis (NkBgl) with the crystal structures of CcBglA, TrBgl2 and NkBgl, determined at 1.9Å, 1.63Å and 1.34Å resolution, respectively. The overall structures of these enzymes are similar to those belonging to the ß-retaining glycosyl hydrolase family 1, which have a classical (α/ß)(8)-TIM barrel fold. Each contains a slot-like active site cleft and a more variable outer opening, related to its function in processing different lengths of ß-1,4-linked glucose derivatives. The two essential glutamate residues for hydrolysis are spatially conserved in the active site. In both TrBgl2 and NkBgl structures, a Tris molecule was found to bind at the active site, explaining the slight inhibition of hydrolase activity observed in Tris buffer. Manganese ions at 10mM exerted an approximate 2-fold enzyme activity enhancement of all three ß-glucosidases, with CcBglA catalyzing the most efficiently in hydrolysis reaction and tolerating Tris as well as some metal inhibition. In summary, our results for the structural and functional properties of these three ß-glucosidases from various biological sources open important avenues of exploration for further practical applications.

Directed evolution of a lysosomal enzyme with enhanced activity at neutral pH by mammalian cell-surface display.

Human beta-glucuronidase, due to low intrinsic immunogenicity in humans, is an attractive enzyme for tumor-specific prodrug activation, but its utility is hindered by low activity at physiological pH. Here we describe the development of a high-throughput screening procedure for enzymatic activity based on the stable retention of fluorescent reaction product in mammalian cells expressing properly folded glycoproteins on their surface. We utilized this procedure on error-prone PCR and saturation mutagenesis libraries to isolate beta-glucuronidase tetramers that were up to 60-fold more active (k(cat)/K(m)) at pH 7.0 and were up to an order of magnitude more effective at catalyzing the conversion of two structurally disparate glucuronide prodrugs to anticancer agents. The screening procedure described here can facilitate investigation of eukaryotic enzymes requiring posttranslational modifications for biological activity.

Identification of CD46 binding sites within the adenovirus serotype 35 fiber knob.

Species B human adenoviruses (Ads) are often associated with fatal illnesses in immunocompromised individuals. Recently, species B Ads, most of which use the ubiquitously expressed complement regulatory protein CD46 as a primary attachment receptor, have gained interest for use as gene therapy vectors. In this study, we focused on species B Ad serotype 35 (Ad35), whose trimeric fiber knob domain binds to three CD46 molecules with a KD (equilibrium dissociation constant) of 15.5 nM. To study the Ad35 knob-CD46 interaction, we generated an expression library of Ad35 knobs with random mutations and screened it for CD46 binding. We identified four critical residues (Phe242, Arg279, Ser282, and Glu302) which, when mutated, ablated Ad35 knob binding to CD46 without affecting knob trimerization. The functional importance of the identified residues was validated in surface plasmon resonance and competition binding studies. To model the Ad35 knob-CD46 interaction, we resolved the Ad35 knob structure at 2-A resolution by X-ray crystallography and overlaid it onto the existing structure for Ad11-CD46 interaction. According to our model, all identified Ad35 residues are in regions that interact with CD46, whereby one CD46 molecule binds between two knob monomers. This mode of interaction might have potential consequences for CD46 signaling and intracellular trafficking of Ad35. Our findings are also fundamental for better characterization of species B Ads and design of antiviral drugs, as well as for application of species B Ads as in vivo and in vitro gene transfer vectors.

Enhanced preference for pi-bond containing substrates is correlated to Pro110 in the substrate-binding tunnel of Escherichia coli thioesterase I/protease I/lysophospholipase L(1).

Escherichia coli thioesterase I/protease I/lysophospholipase L(1) (TAP) possesses multifunctional enzyme with thioesterase, esterase, arylesterase, protease, and lysophospholipase activities. Leu109, located at the substrate-binding tunnel, when substituted with proline (Pro) in TAP, shifted the substrate-preference from medium-to-long acyl chains to shorter acyl chains of triglyceride and p-nitrophenyl ester, and increased the preference for aromatic-amino acid-derived esters. In the three-dimensional TAP structures, the only noticeable alteration of backbone and side chain conformation was located at the downstream Pro110-Ala123 region rather than at Pro109 itself. The residue Pro110, adjacent to Leu109 or Pro109, was found to contribute to the substrate preference of TAP enzymes for esters containing acyl groups with pi bond(s) or aromatic group(s). Some of the interactions between the enzyme protein and the substrate may be contributed by an attractive force between the Pro110 C-H donor and the substrate pi-acceptor.

Inhibitory effects of nontoxic protein volvatoxin A1 on pore-forming cardiotoxic protein volvatoxin A2 by interaction with amphipathic alpha-helix.

Volvatoxin A2, a pore-forming cardiotoxic protein, was isolated from the edible mushroom Volvariella volvacea. Previous studies have demonstrated that volvatoxin A consists of volvatoxin A2 and volvatoxin A1, and the hemolytic activity of volvatoxin A2 is completely abolished by volvatoxin A1 at a volvatoxin A2/volvatoxin A1 molar ratio of 2. In this study, we investigated the molecular mechanism by which volvatoxin A1 inhibits the cytotoxicity of volvatoxin A2. Volvatoxin A1 by itself was found to be nontoxic, and furthermore, it inhibited the hemolytic and cytotoxic activities of volvatoxin A2 at molar ratios of 2 or lower. Interestingly, volvatoxin A1 contains 393 amino acid residues that closely resemble a tandem repeat of volvatoxin A2. Volvatoxin A1 contains two pairs of amphipathic alpha-helices but it lacks a heparin-binding site. This suggests that volvatoxin A1 may interact with volvatoxin A2 but not with the cell membrane. By using confocal microscopy, it was demonstrated that volvatoxin A1 could not bind to the cell membrane; however, volvatoxin A1 could inhibit binding of volvatoxin A2 to the cell membrane at a molar ratio of 2. Via peptide competition assay and in conjunction with pull-down and co-pull-down experiments, we demonstrated that volvatoxin A1 and volvatoxin A2 may form a complex. Our results suggest that this occurs via the interaction of one molecule of volvatoxin A1, which contains two amphipathic alpha-helices, with two molecules of volvatoxin A2, each of which contains one amphipathic alpha-helix. Taken together, the results of this study reveal a novel mechanism by which volvatoxin A1 regulates the cytotoxicity of volvatoxin A2 via direct interaction, and potentially provide an exciting new strategy for chemotherapy.

Substrate specificities of Escherichia coli thioesterase I/protease I/lysophospholipase L1 are governed by its switch loop movement.

Escherichia coli thioesterase I/protease I/lysophospholipase L(1) (TAP) is a multifunctional lysophospholipase and acyl-CoA thioesterase with a SGNH-hydrolase fold. The relationship between TAP's structure and its versatile substrate specificity, however, is unclear. Here, we present the crystal structure of TAP in complex with octanoic acid (TAP-OCA; OCA, a free fatty acid with eight carbon atoms, C(8)). A structural comparison of native TAP with TAP-OCA reveals a remarkable conformational change in loop(75)(-)(80), called "switch loop movement", upon OCA binding to the substrate-binding crevice of TAP. OCA binding to the substrate-binding crevice results in a continuous hydrophobic surface, which triggers switch loop movement. The switch loop movement is acyl chain length dependent, with an effect of stabilizing the Michaelis complex (MC) of TAP during catalysis, and is essential for TAP's substrate preference. The finding of a sulfate ion binding site in the TAP structures, together with previous enzyme kinetic analyses, leads us to postulate that a putative CoA binding site is essential for efficient catalysis of thioesters in TAP. We also present the crystal structure of L109P-OCA (TAP's L109P mutant in complex with OCA), in which Leu109 mutated to Pro109 abolishes switch loop movement. This result strengthens our hypothesis that the switch loop movement is induced by hydrophobic interactions.

GDSL family of serine esterases/lipases.

GDSL esterases and lipases are hydrolytic enzymes with multifunctional properties such as broad substrate specificity and regiospecificity. They have potential for use in the hydrolysis and synthesis of important ester compounds of pharmaceutical, food, biochemical, and biological interests. This new subclass of lipolytic enzymes possesses a distinct GDSL sequence motif different from the GxSxG motif found in many lipases. Unlike the common lipases, GDSL enzymes do not have the so called nucleophile elbow. Studies show that GDSL hydrolases have a flexible active site that appears to change conformation with the presence and binding of the different substrates, much like the induced fit mechanism proposed by Koshland. Some of the GDSL enzymes have thioesterase, protease, arylesterase, and lysophospholipase activity, yet they appear to be the same protein with similar molecular weight ( approximately 22-60 kDa for most esterases), although some have multiple glycosylation sites with higher apparent molecular weight. GDSL enzymes have five consensus sequence (I-V) and four invariant important catalytic residues Ser, Gly, Asn, and His in blocks I, II, III, and V, respectively. The oxyanion structure led to a new designation of these enzymes as SGNH-hydrolase superfamily or subfamily. Phylogenetic analysis revealed that block IIA which belonged to the SGNH-hydrolases was found only in clade I. Therefore, this family of hydrolases represents a new example of convergent evolution of lipolytic enzymes. These enzymes have little sequence homology to true lipases. Another important differentiating feature of GDSL subfamily of lipolytic enzymes is that the serine-containing motif is closer to the N-terminus unlike other lipases where the GxSxG motif is near the center. Since the first classification of these subclass or subfamily of lipases as GDSL(S) hydrolase, progress has been made in determining the consensus sequence, crystal structure, active site and oxyanion residues, secondary structure, mechanism of catalysis, and understanding the conformational changes. Nevertheless, much still needs to be done to gain better understanding of in vivo biological function, 3-D structure, how this group of enzymes evolved to utilize many different substrates, and the mechanism of reactions. Protein engineering is needed to improve the substrate specificity, enantioselectivity, specific activity, thermostability, and heterologous expression in other hosts (especially food grade microorganisms) leading to eventual large scale production and applications. We hope that this review will rekindle interest among researchers and the industry to study and find uses for these unique enzymes.

Crystal structures and electron micrographs of fungal volvatoxin A2.

Membrane adhesion and insertion of protein are essential to all organisms, but the underlying mechanisms remain largely unknown. Membrane pore-forming toxins (PFTs) are potential model systems for studying these mechanisms. We have determined the crystal structures of volvatoxin A2 (VVA2), a fungal PFT from Volvariella volvacea, using Br-multiple-wavelength anomalous diffraction (MAD). The VVA2 structures obtained at pH 4.6, pH 5.5 and pH 6.5 were refined to resolutions of 1.42 A, 2.6 A and 3.2 A, respectively. The structures reveal that the VVA2 monomer contains a single alpha/beta domain. Most of the VVA2 surface is occupied by its oligomerization motif and two putative heparin-binding motifs. Residues Ala91 to Ala101 display several conformations at different pH values, which might be under the control of His87. We also found that the shape of one putative heparin-binding motif in VVA2 appears similar to those found in fibroblast growth factors, and the other one displays a linear polypeptide. Our results suggest several possible intermediates of protein assembly in solution and protein adhering to cell membranes before conformational changes. The electron micrographs of VVA2 molecules in solution, at a protein concentration of 1 microg ml(-1), show that they can assemble into filament-like or braid-like oligomers in a pH-dependent way. In addition, the arc-shaped VVA2 structure obtained at pH 6.5 suggests that VVA2 could form a two-layered helical oligomer with 18 subunits per turn. The structures presented here could be used to elucidate the pore-formation mechanisms of VVA2 and its structural neighbors, Cyt toxins from Bacillus thuringiensis.

Structural basis of binding and inhibition of novel tarantula toxins in mammalian voltage-dependent potassium channels.

Voltage-dependent potassium channel Kv2.1 is widely expressed in mammalian neurons and was suggested responsible for mediating the delayed rectifier (I(K)) currents. Further investigation of the central role of this channel requires the development of specific pharmacology, for instance, the utilization of spider venom toxins. Most of these toxins belong to the same structural family with a short peptide reticulated by disulfide bridges and share a similar mode of action. Hanatoxin 1 (HaTx1) from a Chilean tarantula was one of the earliest discussed tools regarding this and has been intensively applied to characterize the channel blocking not through the pore domain. Recently, more related novel toxins from African tarantulas such as heteroscordratoxins (HmTx) and stromatoxin 1 (ScTx1) were isolated and shown to act as gating modifiers such as HaTx on Kv2.1 channels with electrophysiological recordings. However, further interaction details are unavailable due to the lack of high-resolution structures of voltage-sensing domains in such mammalian Kv channels. Therefore, in the present study, we explored structural observation via molecular docking simulation between toxins and Kv2.1 channels based upon the solution structures of HaTx1 and a theoretical basis of an individual S3(C) helical channel fragment in combination with homology modeling for other novel toxins. Our results provide precise chemical details for the interactions between these tarantula toxins and channel, reasonably correlating the previously reported pharmacological properties to the three-dimensional structural interpretation. In addition, it is suggested that certain subtle structural variations on the interaction surface of toxins may discriminate between the related toxins with different affinities for Kv channels. Evolutionary links between spider peptide toxins and a "voltage sensor paddles" mechanism most recently found in the crystal structure of an archaebacterial K(+) channel, KvAP, are also delineated in this paper.

Sequential structural changes of Escherichia coli thioesterase/protease I in the serial formation of Michaelis and tetrahedral complexes with diethyl p-nitrophenyl phosphate.

Escherichia coli thioesterase/protease I (TEP-I) belongs to a new subclass of lipolytic enzymes of the serine hydrolase superfamily. Here we report the first direct NMR observation of the formation of the Michaelis complex (MC) between TEP-I and diethyl p-nitrophenyl phosphate (DENP), an active site directed inhibitor of serine protease, and its subsequent conversion to the tetrahedral complex (TC). NMR, ESI-MS, and kinetic data showed that DENP binds to TEP-I in a two-step process, a fast formation of MC followed by a slow conversion to TC. NMR chemical shift perturbation further revealed that perturbations were confined mainly to four conserved segments comprising the active site. Comparable magnitudes of chemical shift perturbations were detected in both steps. The largest chemical shift perturbation occurred around the catalytic Ser(10). In MC, the conformation of the mobile Ser(10) was stabilized, and its amide resonance became observable. From the large chemical shift perturbation upon conversion from MC to TC, we propose that the amide protons of Ser(10) and Gly(44) serve as the oxyanion hole proton donors that stabilize the tetrahedral adduct. The pattern of residues perturbed in both steps suggests a sequential, stepwise structural change upon binding of DENP. The present study also demonstrates the important catalytic roles of conserved residues in the SGNH family of proteins.

Crystal structure of Escherichia coli thioesterase I/protease I/lysophospholipase L1: consensus sequence blocks constitute the catalytic center of SGNH-hydrolases through a conserved hydrogen bond network.

Escherichia coli thioesterase I (TAP) is a multifunctional enzyme possessing activities of thioesterase, esterase, arylesterase, protease, and lysophospholipase. In particular, TAP has stereoselectivity for amino acid derivative substrates, hence it is useful for the kinetic resolution of racemic mixtures of industrial chemicals. In the present work, the crystal structure of native TAP was determined at 1.9A, revealing a minimal SGNH-hydrolase fold. The structure of TAP in complex with a diethyl phosphono moiety (DEP) identified its catalytic triad, Ser10-Asp154-His157, and oxyanion hole, Ser10-Gly44-Asn73. The oxyanion hole of TAP consists of three residues each separated from the other by more than 3.5A, implying that all of them are highly polarized when substrate bound. The catalytic (His)C(epsilon1)-H...O=C hydrogen bond usually plays a role in the catalytic mechanisms of most serine hydrolases, however, there were none present in SGNH-hydrolases. We propose that the existence of the highly polarized tri-residue-constituted oxyanion hole compensates for the lack of a (His)C(epsilon1)-H...O=C hydrogen bond. This suggests that members of the SGNH-hydrolase family may employ a unique catalytic mechanism. In addition, most SGNH-hydrolases have low sequence identities and presently there is no clear criterion to define consensus sequence blocks. Through comparison of TAP and the three SGNH-hydrolase structures currently known, we have identified a unique hydrogen bond network which stabilizes the catalytic center: a newly discovered structural feature of SGNH-hydrolases. We have defined these consensus sequence blocks providing a basis for the sub-classification of SGNH-hydrolases.

Residues involved in the catalysis, base specificity, and cytotoxicity of ribonuclease from Rana catesbeiana based upon mutagenesis and X-ray crystallography.

The Rana catesbeiana (bullfrog) ribonucleases, which belong to the RNase A superfamily, exert cytotoxicity toward tumor cells. RC-RNase, the most active among frog ribonucleases, has a unique base preference for pyrimidine-guanine rather than pyrimidine-adenine in RNase A. Residues of RC-RNase involved in base specificity and catalytic activity were determined by site-directed mutagenesis, k(cat)/K(m) analysis toward dinucleotides, and cleavage site analysis of RNA substrate. The results show that Pyr-1 (N-terminal pyroglutamate), Lys-9, and Asn-38 along with His-10, Lys-35, and His-103 are involved in catalytic activity, whereas Pyr-1, Thr-39, Thr-70, Lys-95, and Glu-97 are involved in base specificity. The cytotoxicity of RC-RNase is correlated, but not proportional to, its catalytic activity. The crystal structure of the RC-RNase.d(ACGA) complex was determined at 1.80 A resolution. Residues Lys-9, His-10, Lys-35, and His-103 interacted directly with catalytic phosphate at the P(1) site, and Lys-9 was stabilized by hydrogen bonds contributed by Pyr-1, Tyr-28, and Asn-38. Thr-70 acts as a hydrogen bond donor for cytosine through Thr-39 and determines B(1) base specificity. Interestingly, Pyr-1 along with Lys-95 and Glu-97 form four hydrogen bonds with guanine at B(2) site and determine B(2) base specificity.

A possible molecular mechanism of hanatoxin binding-modified gating in voltage-gated K+-channels.

While S4 is known as the voltage sensor in voltage-gated potassium channels, the carboxyl terminus of S3 (S3C) is of particular interest concerning the site for gating modifier toxins like hanatoxin. The thus derived helical secondary structural arrangement for S3C, as well as its surrounding environment, has since been intensively and vigorously debated. Our previous structural analysis based on molecular simulation has provided sufficient information to describe reasonable docking conformation and further experimental designs (Lou et al., 2002. J. Mol. Recognit. 15: 175-179). However, if one only relies on such information, more advanced structure-functional interpretations for the roles S3C may play in the modification of gating behavior upon toxin binding will remain unknown. In order to have better understanding of the molecular details regarding this issue, we have performed the docking simulation with the S3C sequence from the hanatoxin-insensitive K+-channel, shaker, and analyzed the conformational changes resulting from such docking. Compared with other functional data from previous studies with respect to the proximity of the S3-S4 linker region, we suggested a significant movement of drk1 S3C, but not shaker S3C, in the direction presumably towards S4, which was comprehended as a possible factor interfering with S4 translocation during drk1 gating in the presence of toxin. In combination with the discussions for structural roles of the length of the S3-S4 linker, a possible molecular mechanism to illustrate the hanatoxin binding-modified gating is proposed.