Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology最新文献

Biosensors based on acetyl cholinesterase (AChE) inhibition have been known for monitoring of pesticides in food and water samples. However, strong inhibition of the enzyme is a major drawback in practical application of the biosensor which can be overcome by reactivation of the enzyme for repeated use. In the present study, enzyme reactivation by oximes was explored for this purpose. Two oximes viz., 1,1′-trimethylene bis 4-formylpyridinium bromide dioxime (TMB-4) and pyridine 2-aldoxime methiodide (2-PAM) were compared for the reactivation of the immobilized AChE. TMB-4 was found to be a more efficient reactivator under repeated use, retaining more than 60% of initial activity after 11 reuses, whereas in the case of 2-PAM, the activity retention dropped to less than 50% after only 6 reuses. Investigations also showed that reactivation must be effected within 10 min after each analysis to eliminate the ageing effect, which reduces the efficiency of reactivation.

Two strains (B728a and Y37) of the phytopathogenic bacterium Pseudomonas syringae pv. syringae isolated from bean (Phaseolus vulgaris) plants were shown to produce in culture both syringomycin, a lipodepsinonapeptide secreted by the majority of the strains of the bacterium, and a new form of syringopeptin, SP₂₂Phv. The structure of the latter metabolite was elucidated by the combined use of mass spectrometry (MS), nuclear magnetic resonance (NMR) spectroscopy and chemical procedures. Comparative phytotoxic and antimicrobial assays showed that SP₂₂Phv did not differ substantially from the previously characterized syringopeptin 22 (SP₂₂) as far as toxicity to plants was concerned, but was less active in inhibiting the growth of the test fungi Rhodotorula pilimanae and Geotrichum candidum and of the Gram-positive bacterium Bacillus megaterium.

The stoichiometry of oxygen consumption during tyrosinase-catalyzed oxidation of an o-diphenol (4-tert-butylcatechol, TBC) and a monophenol (4-tert-butylphenol, TBP) has been determined. At high [substrate]/[enzyme] ratios, in the case of o-diphenols, the stoichiometry of the enzyme-catalyzed reaction was always 1 O₂/2 o-diphenols, although if the o-quinone product was unstable, the apparent stoichiometry could tend to 1 O₂/1 o-diphenol due to regeneration of an o-diphenol in a side reaction. In the case of monophenols, the stoichiometry could be 1 O₂/1 monophenol or 1.5 O₂/1 monophenol depending if the o-quinone product was stable or unstable, respectively. However, at low [substrate]/[enzyme] ratios, the oxygen/substrate stoichiometry could, even in the case where stable products are formed, be lower than 1 O₂/2 substrates for o-diphenols or higher than 1 O₂/1 substrate for monophenols. These data supported the mechanism proposed by Rodrı́guez-López et al. [J. Biol. Chem. 267 (1992) 3801–3810], in which, during hydroxylation of monophenols, tyrosinase first transformed monophenol to o-diphenol and then either catalyzed a further oxidation to form o-quinone or released it into the reaction medium. In this second case, subsequent oxidation of the o-diphenol resulted in additional oxygen consumption.

The misfolded isoform of the prion protein (PrP^Sc) possesses many unusual physiochemical properties. Previously, we and others reported on the differential partitioning of PrP^Sc from plasma derived therapeutic proteins during their purification processes. To understand the driving force behind these partitioning differences, we investigated the effects of various solvent conditions on the precipitation of PrP^Sc. In a physiological buffer, PrP^Sc remained in the supernatant after low speed centrifugation. At pH 5, PrP^Sc precipitation was nearly complete regardless of the salt content. PrP^Sc could also be precipitated at pH 8 by adding ethanol, but this precipitation was salt dependent. Based on these observations, an empirical mathematical model was constructed in which the PrP^Sc precipitation trends were fully described as a function of solvent pH, salt, and ethanol concentration. This model consistently predicted PrP^Sc partitioning during cold ethanol precipitation steps used in plasma protein purification processes, as shown by experimentally determined distributions of PrP^Sc and transmissible spongiform encephalopathy (TSE) infectivity. These results indicate that pH, salt, and ethanol content are the major solvent factors determining the precipitation of the infectious PrP^Sc in these processes and may provide a useful tool for assessing the differential partitioning of PrP^Sc in a given solvent environment.

ATP:citrate lyase (ACL), an important enzyme in lipid synthesis, has been purified from Aspergillus nidulans to a specific activity of 19.6 μmol min⁻¹ mg⁻¹, almost twice that of any other purified ACL and shown to be distinct from any previously purified ACL. The enzyme is a 371±31 kDa hexamer of 3α, 3β proteins, unlike the 4α tetramer found in rats or yeasts. The molecular weights of the α and β protein subunits were determined by SDS-PAGE to be 70 and 55 kDa.

ACL in A. nidulans (unlike Aspergillus niger) appears to be regulated by the carbon source present in the media. In crude extracts, it was found at high activity (88 μmol min⁻¹ mg protein⁻¹) in glucose-grown cells but only at low activity (10 μmol min⁻¹ mg protein⁻¹) in acetate-grown cells.

Human glutathione transferase A1-1 (GST A1-1) has a flexible C-terminal segment that forms a helix (α9) closing the active site upon binding of glutathione and a small electrophilic substrate such as 1-chloro-2,4-dinitrobenzene (CDNB). In the absence of active-site ligands, the C-terminal segment is not fixed in one position and is not detectable in the crystal structure. A key residue in the α9-helix is Phe 220, which can interact with both the enzyme-bound glutathione and the second substrate, and possibly guide the reactants into the transition state. Mutation of Phe 220 into Ala and Thr was shown to reduce the catalytic efficiency of GST A1-1. The mutation of an additional residue, Phe 222, caused further decrease in activity. The presence of a viscosogen in the reaction medium decreased the kinetic parameters K_cat and K_cat/K_m for the conjugation of CDNB catalyzed by wild-type GST A1-1, in agreement with the view that product release is rate limiting for the substrate-saturated enzyme. The mutations cause a decrease of the viscosity dependence of both kinetic parameters, indicating that the motion of the α9-helix is linked to catalysis in wild-type GST A1-1. The isomerization reaction with the alternative substrate Δ⁵-androstene-3,17-dione (AD) is affected in a similar manner by the viscosogens. The transition state energy of the isomerization reaction, like that of the CDNB conjugation, is lowered by Phe 220 as indicated by the effects of the mutations on K_cat/K_m. The results demonstrate that Phe 220 and Phe 222, in the dynamic C-terminal segment, influence rate-determining steps in the catalytic mechanism of both the substitution and the isomerization reactions.

The thermal transition of elongation factor EF-Tu from Thermus thermophilus in the presence of low-molecular weight effectors was studied by differential scanning calorimetry. The effectors of GTPase activity used were the antibiotic kirromycin and the cations Li⁺, Na⁺, K⁺ and NH₄⁺ in the chloride form. The temperature of thermal denaturation and the cooperativity of the transition of nucleotide-free EF-Tu (EF-Tu_f) in the presence of kirromycin are comparable with those of the EF-Tu·guanosine-5′-[β,γ-imido]triphosphate (GppNHp) form, indicating similar conformational states. Increased concentrations of Na⁺ and K⁺ stabilized EF-Tu_f in a manner similar to GppNHp. NH₄⁺ decreased the transition temperature of EF-Tu_f and Li⁺ decreased both the temperature and the calorimetric enthalpy of the thermal transition of EF-Tu_f. In the presence of salts, binding of kirromycin had a stabilizing effect on EF-Tu_f. Correlation between the GTPase activity and thermodynamic characteristics of EF-Tu_f induced by kirromycin in the absence or presence of the cations is discussed.

The cleavage specificity of protease C1, isolated from soybean (Glycine max (L.) Merrill) seedling cotyledons, was examined using oligopeptide substrates in an HPLC based assay. A series of peptides based on the sequence Ac-KVEKEESEEGE-NH₂ was used, mimicking a natural cleavage site of protease C1 in the α subunit of the storage protein β-conglycinin. A study of substrate peptides truncated from either the N- or C-terminus indicates that the minimal requirements for cleavage by protease C2 are three residues N-terminal to the cleaved bond, and two residues C-terminal (i.e. P₃-P₂′). The maximal rate of cleavage is reached with substrates containing four to five residues N-terminal to the cleaved bond and four residues C-terminal (i.e. P₄ or P₅ to P₄′). The importance of Glu residues at the P₁, P₁′, and P₄ positions was examined using a series of substituted nonapeptides (P₅-P₄′) with a base sequence of Ac-KVEKEESEE-NH₂. At the P₁ position, the relative ranking, based on k_cat/K_m, was E>Q>K>A>D>F>S. Substitutions at the P₁′ position yield the ranking E≅Q>A>S>D>K>F, while those at P₄′ had less effect on k_cat/K_m, yielding the ranking F≅S≅E≅D>K>A≅Q. These data show that protease C1 prefers to cleave at Glu-Glu and Glu-Gln bonds, and that the nature of the P₄′ position is less important. The fact that there is specificity in the cleavage of the oligopeptides suggests that the more limited specific cleavage of the α and α′ subunits of β-conglycinin by protease C1 is due to a combination of the sequence cleavage specificity of the protease and the accessibility of appropriate scissile peptide bonds on the surface of the substrate protein.

Previously, it was demonstrated that pancreatic cholesterol esterase is selectively inhibited by 6-chloro-2-pyrones with cyclic aliphatic substituents in the 3-position. Inhibition is reversible and is competitive with substrate. Pancreatic cholesterol esterase is a potential target for treatment of hypercholesterolemia. In the present study, yeast cholesterol esterase from Candida cylindracea (also called C. rugosa CRL3) was compared to porcine pancreatic cholesterol esterase for inhibition by a series of 3-alkyl- or 5-alkyl-6-chloro-2-pyrones. In addition, CRL3 was compared with the related yeast lipase CRL1. Inhibition of CRL3 by substituted 6-chloro-2-pyrones was competitive with binding of the substrate p-nitrophenyl butyrate. Inhibition constants ranged from 0.2 μM to >90 μM. Small changes in the alkyl group had profound effects on binding. The pattern of inhibition of CRL3 is quite distinct from that observed with porcine cholesterol esterase. Molecular modeling studies suggest that the orientation of binding of these inhibitors at the active site of CRL3 can vary but that the pyrone ring consistently occupies a position close to the active site serine. CRL1 is highly homologous to CRL3. Nevertheless, patterns of inhibition of CRL1 by substituted 6-chloro-2-pyrones differ markedly from patterns observed with CRL3. The substituted 6-chloro-2-pyrones are slowly hydrolyzed in the presence of CRL1 and are pseudosubstrates of CRL3, but are simple reversible inhibitors of pancreatic cholesterol esterase

Peptide display on solvent-exposed surfaces of engineered enzymes allows them to respond to anti-peptide antibodies by detectable changes in their enzymatic activity, offering a new principle for biosensor development. In this work, we show that multiple peptide insertion in the vicinity of the Escherichia coli β-galactosidase active site dramatically increases the enzyme responsiveness to specific anti-peptide antibodies. The modified enzymes HD7872A and HT7278CA, carrying eight and 12 copies respectively of a foot-and-mouth disease peptide per enzyme molecule, show antibody-mediated activation factors higher than those previously observed in the first generation enzymatic sensors, for HT7278CA being close to 400%. The analysis of the signal transduction process with multiple inserted proteins strongly suggests a new, non-exclusive mechanism of enzymatic regulation in which the target proteins might be stabilised by the bound antibody, extending the enzyme half-life and consequently enhancing the signal–background ratio. In addition, the tested sensors are differently responsive to sera from immune farm animals, depending on the antigenic similarity between the B-cell epitopes in the immunising virus and those in the peptide used as sensing element on the enzyme surface. Altogether, these results point out the utility of these enzymatic biosensors for a simple diagnosis of foot-and-mouth disease in an extremely fast homogeneous assay.