Escherichia coli Alkaline Phosphatase

Improved membrane permeability with cetyltrimethylammonium bromide (CTAB) addition for enhanced bidirectional transport of substrate and electron shuttles

The effects of membrane permeability on extracellular electron transfer (EET) and performance of microbial fuel cell (MFC) need to be explored. In this work, cetyltrimethylammonium bromide (CTAB) was chosen to enhance the current generation and bidirectional transport of substrate and electron shuttles by tailoring the cell membrane permeability. Specifically, the peak currents of biofilms treated with CTAB especially at 200 μM were obviously higher than the control biofilm with no CTAB, and the riboflavin mediated electron transfer was promoted prominently. Biomass and viability analyses showed that an appropriate concentration of CTAB had almost no adverse effect on the cell viability of biofilm and could increase the biomass of biofilm. Measurements of the extracellular activity of alkaline phosphatase and UV-vis absorption confirmed the increased membrane permeability and the promoted efficiency of substrates transported into cells. This contribution paves the key step for facilitating EET process by adjusting membrane permeability through CTAB or other surfactants addition.

A method for the transient inhibition of toxicity of secretory recombinant proteins, exemplified by bacterial alkaline phosphatase. Novel protocol for problematic DNA termini dephosphorylation

Genes encoding proteins 'toxic' to recombinant host are difficult for cloning/expression and recombinant clones are unstable. Even tightly controlled inducible T7-lac, PBAD, PL, PR promoters are not totally silent in an uninduced state and thus not adequate for highly toxic proteins. An innovative approach to engineering and expression of the gene, encoding bacterial alkaline phosphatase (BAP) is proposed. The native precursor enzyme contains a signal peptide at the N-terminus and is secreted to the Escherichia coli (E. coli) periplasm. The signal peptide is then removed that allows oxidation and formation of active dimers. To decrease toxicity of the bap gene, its secretion leader coding section was replaced with a N-terminal His6-tag. The gene was expressed in E. coli in a PBAD vector, resulting in the accumulation of soluble His6-BAP in the cytoplasm. The His6-BAP was neutral to the cells, as no maturation was possible in the reducing cytoplasm. The purified homogenous protein was further reactivated in a redox buffer containing the protein structure stabilizing cofactors. The His6-BAP exhibited high activity. A dephosphorylation protocol for all types of DNA termini was developed.The method appears well suited for the industrial production of BAP and can be applied to other problematic proteins.• Efficient toxic gene expression • Novel approach to toxic gene cloning, engineering, expression, purification and reactivation of the transiently inactivated enzyme • Scaled-up production of ultrapure BAP • Improved protocol for all types of DNA termini dephosphorylation.

Boosting toxic protein biosynthesis: transient in vivo inactivation of engineered bacterial alkaline phosphatase

BACKGROUND

The biotechnology production of enzymes is often troubled by the toxicity of the recombinant products of cloned and expressed genes, which interferes with the recombinant hosts' metabolism. Various approaches have been taken to overcome these limitations, exemplified by tight control of recombinant genes or secretion of recombinant proteins. An industrial approach to protein production demands maximum possible yields of biosynthesized proteins, balanced with the recombinant host's viability. Bacterial alkaline phosphatase (BAP) from Escherichia coli (E. coli) is a key enzyme used in protein/antibody detection and molecular cloning. As it removes terminal phosphate from DNA, RNA and deoxyribonucleoside triphosphates, it is used to lower self-ligated vectors' background. The precursor enzyme contains a signal peptide at the N-terminus and is secreted to the E. coli periplasm. Then, the leader is clipped off and dimers are formed upon oxidation.

RESULTS

We present a novel approach to phoA gene cloning, engineering, expression, purification and reactivation of the transiently inactivated enzyme. The recombinant bap gene was modified by replacing a secretion leader coding section with a N-terminal His6-tag, cloned and expressed in E. coli in a P_BAD promoter expression vector. The gene expression was robust, resulting in accumulation of His6-BAP in the cytoplasm, exceeding 50% of total cellular proteins. The His6-BAP protein was harmless to the cells, as its natural toxicity was inhibited by the reducing environment within the E. coli cytoplasm, preventing formation of the active enzyme. A simple protocol based on precipitation and immobilized metal affinity chromatography (IMAC) purification yielded homogeneous protein, which was reactivated by dialysis into a redox buffer containing reduced and oxidized sulfhydryl group compounds, as well as the protein structure stabilizing cofactors Zn²⁺, Mg²⁺ and phosphate. The reconstituted His6-BAP exhibited high activity and was used to develop an efficient protocol for all types of DNA termini, including problematic ones (blunt, 3'-protruding).

CONCLUSIONS

The developed method appears well suited for the industrial production of ultrapure BAP. Further, the method of transient inactivation of secreted toxic enzymes by conducting their biosynthesis in an inactive state in the cytoplasm, followed by in vitro reactivation, can be generally applied to other problematic proteins.

Chloride promotes refolding of active Vibrio alkaline phosphatase through an inactive dimeric intermediate with an altered interface

Most enzymes are homodimers or higher order multimers. Cold‐active alkaline phosphatase from Vibrio splendidus (VAP) transitions into a dimer with very low activity under mild denaturation conditions. The desire to understand why this dimer fails to efficiently catalyse phosphomonoester hydrolysis led us to investigate interfacial communication between subunits. Here, we studied in detail the unfolding mechanism at two pH values and in the presence or absence of sodium chloride. At pH 8.0, the denaturation model had to include an inactive dimer intermediate and follow the pathway: N₂ → I₂ → 2U. At pH 10.5, the model was of a two‐state nature. Enzyme activity was not recovered under several examined refolding conditions. However, in the presence of 0.5 m NaCl, the enzyme was nearly fully reactivated after urea treatment. Thermal inactivation experiments were biphasic where the inactivation could be detected using CD spectroscopy at 190–200 nm. By incorporating a bimane fluorescence probe at the dimer interface, we could monitor inactivation/denaturation at two distinct sites at the dimer interface. A change in bimane fluorescence at both sites was observed during inactivation, but prior to the global unfolding event. Furthermore, the rate of change in bimane fluorescence correlated with inactivation rates at 40 °C. These results indicate and support the hypothesis that the subunits of VAP are only functional in the dimeric state due to the cooperative nature of the reaction mechanism when proper crosstalk between subunits is facilitated.

Challenges and advances in the computational modeling of biological phosphate hydrolysis

Phosphate ester hydrolysis is fundamental to many life processes, and has been the topic of substantial experimental and computational research effort. However, even the simplest of phosphate esters can be hydrolyzed through multiple possible pathways that can be difficult to distinguish between, either experimentally, or computationally. Therefore, the mechanisms of both the enzymatic and non-enzymatic reactions have been historically controversial. In the present contribution, we highlight a number of technical issues involved in reliably modeling these computationally challenging reactions, as well as proposing potential solutions. We also showcase examples of our own work in this area, discussing both the non-enzymatic reaction in aqueous solution, as well as insights obtained from the computational modeling of organophosphate hydrolysis and catalytic promiscuity amongst enzymes that catalyze phosphoryl transfer.

Promiscuity in the Enzymatic Catalysis of Phosphate and Sulfate Transfer

The enzymes that facilitate phosphate and sulfate hydrolysis are among the most proficient natural catalysts known to date. Interestingly, a large number of these enzymes are promiscuous catalysts that exhibit both phosphatase and sulfatase activities in the same active site and, on top of that, have also been demonstrated to efficiently catalyze the hydrolysis of other additional substrates with varying degrees of efficiency. Understanding the factors that underlie such multifunctionality is crucial both for understanding functional evolution in enzyme superfamilies and for the development of artificial enzymes. In this Current Topic, we have primarily focused on the structural and mechanistic basis for catalytic promiscuity among enzymes that facilitate both phosphoryl and sulfuryl transfer in the same active site, while comparing this to how catalytic promiscuity manifests in other promiscuous phosphatases. We have also drawn on the large number of experimental and computational studies of selected model systems in the literature to explore the different features driving the catalytic promiscuity of such enzymes. Finally, on the basis of this comparative analysis, we probe the plausible origins and determinants of catalytic promiscuity in enzymes that catalyze phosphoryl and sulfuryl transfer.

Membrane permeabilization by conjugated oligoelectrolytes accelerates whole-cell catalysis

Conjugated oligoelectrolytes (COE) increase outer membrane permeability inEscherichia coli,improve transport of small molecules through the cell envelope and thus accelerate whole-cell catalysis.

Cu/Zn-superoxide dismutase and glutathione are involved in response to oxidative stress induced by protein denaturing effect of alachlor in Saccharomyces cerevisiae

Alachlor is a widely used pre-emergent chloroacetanilide herbicide which has been shown to have many harmful ecological and environmental effects. However, the mechanism of alachlor-induced oxidative stress is poorly understood. We found that, in Saccharomyces cerevisiae, the intracellular levels of reactive oxygen species (ROS) including superoxide anions were increased only after long-term exposure to alachlor, suggesting that alachlor is not a pro-oxidant. It is likely that alachlor-induced oxidative stress may result from protein denaturation because alachlor rapidly induced an increased protein aggregation, leading to upregulation of SSA4 and HSP82 genes encoding heat shock proteins (Hsp) of Hsp70 and Hsp90 family, respectively. Although only SOD1 encoding Cu/Zn-superoxide dismutase (SOD), but not SOD2 encoding Mn-SOD, is essential for alachlor tolerance, both SODs play a crucial role in reducing alachlor-induced ROS. We found that, after alachlor exposure, glutathione production was inhibited while its utilization was increased, suggesting the role of glutathione in protecting cells against alachlor, which becomes more important when lacking Cu/Zn-SOD. Based on our results, it seems that alachlor primarily causes damages to cellular macromolecules such as proteins, leading to an induction of endogenous oxidative stress, of which intracellular antioxidant defense systems are required for elimination.

Pyrococcus abyssi alkaline phosphatase: the dimer is the active form

Alkaline phosphatases (APs), E.C. 3.1.3.1, are non-specific phosphomonoesterases optimally active under alkaline conditions. They are classically known to be homodimeric metalloenzymes. This quaternary structure has been considered necessary for activity, although the relationship between quaternary structure and activity is not well understood. Recombinant Pyrococcus abyssi AP was previously isolated and characterized, appearing to have two active quaternary structures on native polyacrylamide gel electrophoresis: a monomer and a homodimer. The purpose of the present work was to determine the actual quaternary structure of P. abyssi AP in solution, by isolating each of the two quaternary forms and establishing the parameters governing the assembly and dissociation of the dimer. pH appeared to be an important parameter: in acidic media, the monomer/dimer ratio shifted towards monomer. Buffer composition also affected the quaternary structure: at the same pH, in potassium phosphate buffer, the two quaternary structures were observed, whereas in tris(hydroxymethyl)aminomethane hydrochloride buffer, only the dimer was observed. Metals bound to the enzyme were found to be involved in the stability of the quaternary structure. Indeed, the P. abyssi AP obtained upon removal of the metals was monomeric. Reactivation of the latter was achieved with variable efficiency. From these experiments, no active monomer could be isolated, leading the conclusion that the active form of P. abyssi AP is the homodimer.

Roles of disulfide bonds in bacterial alkaline phosphatase

Alkaline phosphatase of Escherichia coli (a homodimeric protein found in the periplasmic space) contains two intramolecular disulfide bonds (Cys-168-Cys-178 and Cys-286-Cys-336) that are formed after export to the periplasmic space. The location-specific folding character of this enzyme allowed its wide usage as a reporter of protein localization in prokaryotic cells. To study the roles of disulfide bonds in alkaline phosphatase, we eliminated each of them by Cys to Ser mutations. Intracellular stability of alkaline phosphatase decreased in the absence of either one or both of the disulfide bonds. The mutant proteins were stabilized in a DegP protease-deficient strain, allowing accumulation at significant levels and subsequent characterization. A mutant protein that lacked the N-terminally located disulfide bond (Cys-168-Cys-178) was found to have Cys-286 and Cys-336 residues disulfide-bonded, to have a dimeric structure, and to have almost full enzymatic activity. Nevertheless, the mutant protein lost the trypsin-resistant conformation that is characteristically observed for the wild-type enzyme. In contrast, mutants lacking Cys-286 and Cys-336 were monomeric and inactive. These results indicate that the Cys-286-Cys-336 disulfide bond is required and is sufficient for correctly positioning the active site region of this enzyme, but such an active conformation is still insufficient for the conformational stability of the enzyme. Thus, a fully active state of this enzyme can be formed without full protein stability, and the two disulfide bonds differentially contribute to these properties.

Recombinant antibody-alkaline phosphatase conjugates for diagnosis of human IgGs: application to anti-HBsAg detection

We have designed an expression vector permitting the production in the periplasm of Escherichia coli of a fusion protein comprising a dimer of bacterial alkaline phosphatase (PhoA) and two Fab or scFv fragments of a monoclonal antibody directed against human IgG. Each hybrid protein expressed both high specificity for the antigen and full PhoA activity. We show that crude periplasmic extracts containing these conjugates can be used as such in enzyme immunoassays for the detection of human IgG, as exemplified in the case of anti-hepatitis B immunoglobulin.

Affinity-based reverse micellar extraction and separation (ARMES): a facile technique for the purification of peroxidase from soybean hulls

A new technique for the purification of proteins has been developed which combines the high selectivity of affinity interaction with the scalability and ease of operation of liquid-liquid extraction. The approach is called affinity-based reverse micellar extraction and separation (ARMES). The salient features of ARMES include the following: (1) intraphasic interaction between the ligand and ligate which provides for high ligand utilization; (2) no chemical modification of the ligand is needed; and (3) ease of operation and inherent scalability due to the use of liquid-liquid extraction. This technique has been used to purify the peroxidase from soybean hulls using the lectin concanavalin A (con A) as a sugar-binding affinity ligand. A purification factor of 30 is achieved to provide a nearly pure peroxidase solution (as determined by HPLC and SDS-PAGE) with nearly complete regeneration of the con A ligand. We propose that ARMES will be useful in the facile purification of complex biomolecules such as glycoform protein variants using lectins as affinity ligands and proteins of therapeutic importance using antibodies as affinity ligands.

Naturally-occurring pituitary growth hormone is phosphorylated

The results of this communication show that ovine growth hormone (oGH) contains organically-bound phosphorous. The phosphorous content of growth hormone, lot S-11, is 1:3 (mol/mol) and that of lot S-12 is 1:6 (mol/mol). Results of 31P NMR studies suggest that the phosphorous exists in two chemical forms: as a monophosphoryl ester and as a phosphodiester. Evidence is provided which demonstrates that growth hormone can be phosphorylated in vitro with the catalytic subunit of protein kinase.

Molecular asymmetry in alkaline phosphatase of Escherichia coli

Thermal inactivation of alkaline phosphatase of Escherichia coli has been studied at different temperatures (45 to 70 degrees C) and pHs (7.5, 9.0, and 10.0) for the commercial, buffer-dialyzed (pH 9.0) and EDTA-dialyzed (pH 9.0) enzymes. In each case, the inactivation exhibits biphasic kinetics consistent with the rate equation, (formula; see text) where A0 and A are activities at time zero and t, and k1 and k2 are first-order rate constants for the fast and slow phase, respectively. Values of k1 and k2 change independently with temperature, pH, and pretreatment (dialysis) of the enzyme. Time course of inactivation of the enzyme with excess EDTA and effect of Zn2+ ion concentration on the activity of EDTA-dialyzed enzyme have been investigated. The data suggest that the dimeric enzyme protein has two types of catalytic sites which have equal catalytic efficiency (or specific activity) but differ in several other properties. Structural implications of these results have been discussed.

Characterization of the properties of the multiple metal binding sites in alkaline phosphatase by carbon-13 nuclear magnetic resonance

Carbon-13 nuclear magnetic resonance (13C NMR) of Escherichia coli alkaline phosphatase labeled biosynthetically with beta,beta-[gamma-13C]dideuteriohistidine has been used to determine the number and identity of the histidine residues that participate in metal ion coordination at the three classes of binding sites in this dimeric Zn2+ metalloenzyme. Detailed 13C NMR titrations of the apoenzyme with 113Cd2+ and Mg2+, in conjunction with parallel 13 Cd NMR measurements [Otvos, J.D., & Armitage, I.M. (1980) Biochemistry (third of three papers in this issue)], permitted the assignment of four histidine residues as ligands to the "catalytic", or A site, metal ions, two coordinated via their N pi imidazole nitrogens and two via N pi. In addition, a fifth histidyl ligand, coordinated through N pi, was shown to be located at the "structural", or B, sites on the dimer. The "regulatory", or C, sites do not contain histidyl metal ligands. Unambiguous identification of the three histidines coordinated to metal ion via N pi was provided by the observation of resolved 113Cd-13C spin-spin coupling (3J = 12-19 Hz) in their gamma-carbon resonances. Once assigned, the 13C resonances of the five histidyl metal ligands were used to monitor the relative affinities of the A, B, and C sites for Cd2+ and Zn2+. At pH 6.3, Cd2+ was found to bind to the A sites at least 10 times tighter than to the B or C sites, which have roughly equal affinities. In marked contrast, Zn2+ was found to have similar affinities for the A and B sites at both pH 6.3 and 8.0. The affinity of the C sites for Zn2+ and Mg2+ was shown to be at least an order of magnitude lower. The binding constants of all three sites for Cd2+ and Zn2+ are greater than 10(5) M-1. Evidence is also presented that suggests that the A, B, and C sites may be located in close proximity to one another in the monomers.

Characterization of the histidine residues in alkaline phosphatase by carbon-13 nuclear magnetic resonance

beta,beta-[gamma-13C]Dideuteriohistidine has been biosynthetically incorporated into alkaline phosphatase from Escherichia coli and utilized as a nonperturbing 13C nuclear magnetic resonance (NMR) probe of the environments of the histidine residues in this zinc metalloenzyme. The 13C NMR spectrum of the labeled enzyme exhibits 9 separate resonances arising from the 10 histidine residues located in each of the symmetrically disposed subunits of the dimer. The excellent resolution and large chemical shift range (14 ppm) displayed by these signals are direct consequences of the sensitivity of the histidine gamma-carbon chemical shift to the ionization state and tautomeric form of the imidazole side chains and the coordination of several of these to metal ion. The environments of the individual histidine residues were inferred by investigating the chemical shift responses of their 13C resonances to enzyme metal composition, pH, and inhibitor binding. Additional information concerning their motional freedom was obtained from spin relaxation measurements which were analyzed in terms of the contributions expected from intramolecular 13C-1H and 13C-14N dipolar relaxation and chemical shift anisotropy. The combined results indicate that 4 of the 10 histidines, the only ones that titrate with pH, are surface residues located relatively remote from the active site. Of the six nontitrating residues, one appears to be buried in a solvent-inaccessible region of the protein. Three others are almost certainly involved in metal ion ligation to active-site metal ion(s), two via their N pi nitrogen atoms and the other via N pi. The spectral characteristics of the remaining two histidine residues strongly suggest they are also located at or near the active site. One or both may also participate in metal ion coordination, although the current evidence for this is inconclusive.

Zinc effects on LDH, MDH and alkaline phosphatase from cultures of fathead minnow cells

1. Three purported zinc metalloenzymes have been investigated from cell cultures of the fathead minnow (Pimephales promelas). 2. With the addition of increasingly higher concentrations of zinc to the tissue culture medium, the specific activity of LDH increased. 3. The results with MDH were equivocal. 4. The specific activity of alkaline phosphatase decreased in the presence of increasing amounts of zinc in the growth medium. 5. Zinc exogenously added to the LDH enzyme assay did not alter the LDH enzyme activity of cells grown without zinc.

Zinc stoichiometry in Escherichia coli alkaline phosphatase. Studies by 31P NMR and ion-exchange chromatography

31P nuclear magnetic resonance spectra and enzymatic activities are compared for alkaline phosphatase (orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1) species with different zinc contents. The enzyme containing two Zn2+ per protein dimer exists in two forms; one, prepared by dialysis of native enzyme, has full enzymatic activity and a 31P magnetic resonance spectrum similar to but distinguishable from that of the native enzyme containing four or more Zn2+. The other form, prepared by restoring two Zn2+ to apoenzyme, has low enzymatic activity and a 31P magnetic resonance spectrum that indicates stoichiometric binding of phosphate, but otherwise altered properties. Reconstituted enzyme with four Zn2+ is similar to but distinguishable from native enzyme with four Zn2+. Chromatography on DEAE-cellulose can separate apoenzyme and enzyme containing two Zn2+ and suggests that the binding of a pair of Zn2+ is cooperative.

Induction of alkaline phosphatase in Escherichia coli. Effect of phenethyl alcohol

Induction of alkaline phosphatase, an enzyme located in the periplasmic region of Escherichia coli, was inhibited by phenethyl alcohol, an agent believed to alter the cell membrane structure. Studies to elucidate mechanism of this inhibition showed that while phenethyl alcohol arrested the incorporation of [3H]leucine into active alkaline phosphatase, it did allow substantial incorporation of the label into inactive monomer subunits of the enzyme. These results suggest that phenethyl alcohol may not interfere with the de novo synthesis of monomer subunits of the enzyme but arrest conversion of these into active dimer enzyme presumably by its primary action on the cell membrane structure.