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Stabilization of Torpedo californica acetylcholinesterase by the divalent cations Ca+2, Mg+2, and Mn+2 was investigated. All three substantially protect the enzyme from thermal inactivation. Electron paramagnetic resonance revealed one high‐affinity binding site for Mn+2 and several much weaker sites. Differential scanning calorimetry showed a single irreversible thermal transition. All three cations raise both the temperature of the transition and the activation energy, with the transition becoming more cooperative. The crystal structures of the Ca+2 and Mg+2 complexes with Torpedo acetylcholinesterase were solved. A principal binding site was identified. In both cases, it consists of four aspartates (a 4D motif), within which the divalent ion is embedded, together with several water molecules. It makes direct contact with two of the aspartates, and indirect contact, via waters, with the other two. The 4D motif has been identified in 31 acetylcholinesterase sequences and 28 butyrylcholinesterase sequences. Zebrafish acetylcholinesterase also contains the 4D motif; it, too, is stabilized by divalent metal ions. The ASSAM server retrieved 200 other proteins that display the 4D motif, in many of which it is occupied by a divalent cation. It is a very versatile motif, since, even though tightly conserved in terms of RMSD values, it can contain from one to as many as three divalent metal ions, together with a variable number of waters. This novel motif, which binds primarily divalent metal ions, is shared by a broad repertoire of proteins. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:Protein_Science:3. PDB‐ID(s): 7B38, 7B8E and 7B2W; [ABSTRACT FROM AUTHOR]

The photosensitizer, methylene blue (MB), generates singlet oxygen that irreversibly inhibits Torpedo californica acetylcholinesterase ( TcAChE). In the dark, it inhibits reversibly. Binding is accompanied by a bathochromic absorption shift, used to demonstrate displacement by other acetylcholinesterase inhibitors interacting with the catalytic 'anionic' subsite (CAS), the peripheral 'anionic' subsite (PAS), or bridging them. MB is a noncompetitive inhibitor of TcAChE, competing with reversible inhibitors directed at both 'anionic' subsites, but a single site is involved in inhibition. MB also quenches TcAChE's intrinsic fluorescence. It binds to TcAChE covalently inhibited by a small organophosphate (OP), but not an OP containing a bulky pyrene. Differential scanning calorimetry shows an ∼8° increase in the denaturation temperature of the MB/ TcAChE complex relative to native TcAChE, and a less than twofold increase in cooperativity of the transition. The crystal structure reveals a single MB stacked against Trp279 in the PAS, oriented down the gorge toward the CAS; it is plausible that irreversible inhibition is associated with photooxidation of this residue and others within the active-site gorge. The kinetic and spectroscopic data showing that inhibitors binding at the CAS can impede binding of MB are reconciled by docking studies showing that the conformation adopted by Phe330, midway down the gorge, in the MB/ TcAChE crystal structure, precludes simultaneous binding of a second MB at the CAS. Conversely, binding of ligands at the CAS dislodges MB from its preferred locus at the PAS. The data presented demonstrate that TcAChE is a valuable model for understanding the molecular basis of local photooxidative damage. [ABSTRACT FROM AUTHOR]

Plant peroxidases are presently used extensively in a wide range of biotechnological applications owing to their high environmental and thermal stability. As part of efforts towards the discovery of appealing new biotechnological enzymes, the peroxidase from leaves of the palm tree Chamaerops excelsa (CEP) was extracted, purified and crystallized in its native form. An X-ray diffraction data set was collected at a synchrotron source and data analysis showed that the CEP crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 70.2, b = 100.7, c = 132.3 Å. [ABSTRACT FROM AUTHOR]

Abstract: The homopentameric B-subunit of Shiga toxin (STxB) is used as a tool to deliver antigenic peptides and proteins to the cytosolic compartment of dendritic cells (DCs). In this study, a series of interface mutants of STxB has been constructed. All mutants retained their overall conformation, while a loss in thermal stability was observed. This effect was even more pronounced in trifluoroethanol solutions that mimic the membrane environment. Despite this, all mutants were equally efficient at delivering a model antigenic protein into the MHC class I-restricted antigen presentation pathway of mouse DCs, suggesting that the structural stability of STxB is not a key factor in the membrane translocation process. [Copyright &y& Elsevier]

Royal palm tree peroxidase (RPTP), which was isolated from Roystonea regia leaves, has an unusually high stability that makes it a promising candidate for diverse applications in industry and analytical chemistry [Caramyshev et al. (2005), Biomacromolecules, 6, 1360–1366]. Here, the purification and crystallization of this plant peroxidase and its X-ray diffraction data collection are described. RPTP crystals were obtained by the hanging-drop vapour-diffusion method and diffraction data were collected to a resolution of 2.8 Å. The crystals belong to the trigonal space group P3121, with unit-cell parameters a =  b = 116.83, c = 92.24 Å, and contain one protein molecule per asymmetric unit. The VM value and solvent content are 4.07 Å3 Da−1 and 69.8%, respectively. [ABSTRACT FROM AUTHOR]

Chemical modification of Torpedo californica acetylcholinesterase by the natural thiosulfinate allicin produces an inactive enzyme through reaction with the buried cysteine Cys 231. Optical spectroscopy shows that the modified enzyme is 'native-like,' and inactivation can be reversed by exposure to reduced glutathione. The allicin-modified enzyme is, however, metastable, and is converted spontaneously and irreversibly, at room temperature, with t1/2 ≃ 100 min, to a stable, partially unfolded state with the physicochemical characteristics of a molten globule. Osmolytes, including trimethylamine- N-oxide, glycerol, and sucrose, and the divalent cations, Ca2+, Mg2+, and Mn2+ can prevent this transition of the native-like state for >24 h at room temperature. Trimethylamine- N-oxide and Mg2+ can also stabilize the native enzyme, with only slight inactivation being observed over several hours at 39°C, whereas in their absence it is totally inactivated within 5 min. The stabilizing effects of the osmolytes can be explained by their differential interaction with the native and native-like states, resulting in a shift of equilibrium toward the native state. The stabilizing effects of the divalent cations can be ascribed to direct stabilization of the native state, as supported by differential scanning calorimetry. [ABSTRACT FROM AUTHOR]

The thermal stability of peroxidase from leaves of the African oil palm tree Elaeis guineensis (AOPTP) at pH 3.0 was studied by differential scanning calorimetry (DSC), intrinsic fluorescence, CD and enzymatic assays. The spectral parameters as monitored by ellipticity changes in the far-UV CD spectrum of the enzyme as well as the increase in tryptophan intensity emission upon heating, together with changes in enzymatic activity with temperature were seen to be good complements to the highly sensitive but integral method of DSC. The data obtained in this investigation show that thermal denaturation of palm peroxidase is an irreversible process, under kinetic control, that can be satisfactorily described by the two-state kinetic scheme, N &formmu0; D, where k ␣is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated. [ABSTRACT FROM AUTHOR]

Examines the thermally induced conformational changes in horseradish peroxidase. Conduction of detailed differential scanning calorimetry (DSC); Denaturation of horseradish peroxidase isoenzyme c; Complementary of the spectral parameters to the highly sensitive of DSC.

The bacterial histidine permease is a model system for ABC transporters (traffic ATPases). The water-soluble receptor of this permease, HisJ, binds l-histidine and l-arginine (tightly) and l-lysine and l-ornithine (less tightly) in the periplasm, interacts with the membrane-bound complex (HisQMP2) and induces its ATPase activity, which results in ligand translocation. HisJ is a two-domain protein; in the absence of ligand, the cleft between two domains is open and binding of substrate stabilizes the closed conformation. Surprisingly, various liganded HisJ forms display substantial differences in their physicochemical characteristics and capacity to induce the ATPase. This is due to either different effects of the individual ligands on the respective closed structures, or to different equilibria being reached for each ligand between the open liganded form and the closed liganded form [Wolf, A., Lee, K.C., Kirsch, J.F. & Ames, G.F.-L. (1996) J. Biol. Chem. 271, 21243–21250]. In this work, time-resolved measurements of the decay of intrinsic HisJ fluorescence and of the decay of the anisotropy of the fluorescence, as well as the analysis of the steady-state near UV CD and fluorescence spectra, rule out the model in which the differences between liganded complexes reflect different equilibria. The decay of the anisotropy of the fluorescence shows that liganded complexes differ dramatically in their large-scale conformational dynamics. Differential scanning calorimetry (DSC) curves for the HisJ thermal unfolding are well described by a scheme of equilibrium two-state unfolding of two independent domains, which can be ascribed to the two-domain structure of HisJ. This is true both for apo-HisJ at various pH values, and for HisJ in the presence of its ligands at varying concentrations, at pH 8.3. The DSC and structural data suggest that all ligands interact more extensively with the larger domain. A qualitative model for the HisJ conformational... [ABSTRACT FROM AUTHOR]

Examines the compact residual structure in lentil lectil at hydrogen-ion concentration (pH) from Lens culinaris collected in the La Armuña area in Salamanca, Spain. Measurement of circular dichroism at pH 2.0; Similarity of lentil lectin with the molten globule state; Interpretation of the process of lentil lectin denaturation.

The thermal unfolding and domain structure of myosin subfragment 1 (S1) from rabbit skeletal muscles and their changes induced by nucleotide binding were studied by differential scanning calorimetry. The binding of ADP to S1 practically does not influence the position of the thermal transition (maximum at 47.2°C), while the binding of the non-hydrolysable analogue of ATP, adenosine 5′-[β,γ-imido]triphosphate (AdoPP[NH]P) to S1, or trapping of ADP in S1 by ortho-vanadate (Vi), shift the maximum of the heat adsorption curve for S1 up to 53.2 and 56.1 °C, respectively. Such an increase of S1 thermostability in the complexes S1-AdoPP[NH]P and S1-ADP- Vi is confirmed by results of turbidity and tryptophan fluorescence measurements. The total heat adsorption curves for S1 and its complexes with nucleotides were decomposed into elementary peaks corresponding to the melting of structural domains in the S1 molecule. Quantitative analysis of the data shows that the domain structure of S1 in the complexes S1-AdoPP[NH]P and S1-ADP-Vi is similar and differs radically from that of nucleotide-free S1 and S1 in the S1-ADP complex. These data are the first direct evidence that the S1 molecule can be in two main conformations which may correspond to different states during the ATP hydrolysis: one of them corresponds to nucleotide-free S1 and to the complex S1-ADP, and the other corresponds to the intermediate complexes S1-ATP and S1-ADP-Pi. Surprisingly it turned out that the domain structure of S1 with ADP trapped by p-phenylene-N, N′-dimaleimide (pPDM) thiol cross-linking almost does not differ from that of the nucleotide-free S1. This means that pPDM-cross-linked S1 in contrast to S1-AdoPP[NH]P and S1- ADP-Vi can not be considered a structural analogue of the intermediate complexes S1-ATP and S1- ADP-Pi. [ABSTRACT FROM AUTHOR]

In a continuation of our earlier study [Ruiz-Arribas, A., Santamaría, R.I., Zhadan, G. G., Villar, E. & Shnyrov, V. L. (1994) Differential scanning calorimetric study of the thermal stability of xylanase from Streptomyces halstedii JM8, Biochemistry 33, 13787-13791], we used high-sensitivity differential scanning microcalorimetry, intrinsic tryptophan fluorescence and far-ultraviolet circular dichroism to study the effect of regional sequence differences on the thermodynamic stability of xylanase (Xys1) from Streptomyces halstedii JM8 (1,4-β-D-xylanohydrolase, EC3.2.1.8). Thermal transitions were measured for original xylanase (Xys1S) and two variants. Thermal denaturation of all the xylanases studied revealed two structural domains, each of which, despite its partial irreversibility, follows a two-state thermal unfolding process under our experimental conditions. Both variants were found to exibit slightly decreased stability, possessing the same activity as the original. The unfolding parameters for each domain of both variants, unlike the situation with wild-type xylanase (see our previous report), fit some correlations obtained for the most compact globular proteins. The values of enthalpy and entropy of unfolding/residue at 383 K were found to be inversely proportional to residual, well-regulated structures in unfolded states. [ABSTRACT FROM AUTHOR]

The thermal unfolding of two recombinant fragments of the head of Dictyostelium discoideum myosin II was studied by differential scanning calorimetry. These fragments M754 and M761 correspond to the globular motor portion of the myosin head that contains ATP- and actin-binding sites but lacks the light chain binding domain. Our results show that M754 is less thermostable than M761 : the maximum of the thermal transition occurred at 41.7 +C for M754 and at 45.6 +C for M761, and the calorimetric enthalpy value determined for M754 (677 kJ/mol) was about half of that for M761 (1417 kJ/mol). This indicates that the region containing residues 755-761 plays a very important role in the structural stabilization of the entire globular motor part of the myosin head. ADP binding induces structural changes in both myosin fragments which are reflected in a 2-3.5 +C shift of the thermal transitions to higher temperature. The formation of stable ternary complexes of these myosin fragments with ADP and phosphate analogues such as orthovanadate, beryllium fluoride or aluminium fluoride causes additional structural changes which are reflected in a pronounced increase of thermal stability. The effect of beryllium fluoride was less distinct than that of aluminium fluoride or orthovanadate. In general, the changes caused by various phosphate analogues were similar to those observed with skeletal myosin subfragment 1. Thus, structural changes revealed by differential scanning calorimetry in the myosin head, that are due to the formation of stable ternary complexes with ADP and Pi analogues, occur mainly in the globular motor portion of the head. [ABSTRACT FROM AUTHOR]

Native zinc/cobalt-containing ATP sulfurylase (ATPS; EC 2.7.7.4; MgATP:sulfate adenylyltransferase) from Desulfovibrio desulfuricans ATCC 27774 was purified to homogeneity and crystallized. The orthorhombic crystals diffracted to beyond 2.5 A resolution and the X-ray data collected should allow the determination of the structure of the zinc-bound form of this ATPS. Although previous biochemical studies of this protein indicated the presence of a homotrimer in solution, a dimer was found in the asymmetric unit. Elucidation of this structure will permit a better understanding of the role of the metal in the activity and stability of this family of enzymes.