Effect of pH on high-temperature production of bacterial penicillin acylase in Escherichia coli

High-temperature-oriented production of bacterial penicillin acylase (PAC), which is usually expressed at low temperatures (less than 30 degrees C), was demonstrated in this study via heterologous expression of the Providencia rettgeri (P. rettgeri) pac gene in Escherichia coli (E. coli). While it is possible to produce PAC at a temperature as high as 37 degrees C, the environmental condition (specifically, culture pH) critically affected culture performance. Production of PAC at 37 degrees C was feasible only when culture pH was close to neutral (i.e., 6.5-7.5). Outside this pH range, cell physiology for the host/vector system was seriously affected, resulting in poor culture performance. In acidic culture environments, temperature significantly affected the pac expression level and specific PAC activity decreased with an increase in culture temperature. In basic culture environments, cell growth was seriously inhibited though the pac expression level was minimally affected by temperature. Such unusual types of pH and temperature effects on pac expression were never reported for bacterial PACs. The results suggest that culture pH should be precisely controlled for the current host/vector systems being applied on the overproduction of P. rettgeri PAC in E. coli at high temperatures.     

High-level secretory expression of penicillin amidase from Providencia rettgeri in Saccharomyces cerevisiae: purification and characterization

Heterologous production of the heterodimeric penicillin G amidase (PAC) from Providencia rettgeri was optimized in Saccharomyces cerevisiae. Several factors, including the effect of different growth and induction conditions, were identified to be critical for the enzyme overproduction and secretion. The PAC yield was significantly increased by more than 500-fold compared to that obtained in the native bacterium, and the recombinant enzyme was almost entirely secreted. Electrophoretic characterization of the secreted rPAC(Pr), which was purified over 20-fold by a combination of hydrophobic interaction and ion-exchange chromatography, demonstrated a microheterogeneity of the recombinant enzyme. The recombinant PAC(Pr) was further characterized in terms of specific activity, pH, and temperature profiles and kinetic parameters. The data presented here suggest that by overexpressing rPAC(Pr) in S.cerevisiae and purifying secreted enzyme from culture medium one can readily obtain a large amount of an alternative source of penicillin amidase with properties comparable to that of todays main industrial source of enzyme.     

DegP-coexpression minimizes inclusion-body formation upon overproduction of recombinant penicillin acylase in Escherichia coli

We demonstrated the enhancement of recombinant penicillin acylase (PAC) production in Escherichia coli by increasing the intracellular concentration of the periplasmic protease DegP. Using appropriate host/vector systems (e.g., HB101 harboring pTrcKnPAC2902 or MDDeltaP7 harboring pTrcKnPAC2902) in which the expression of the pac gene was regulated by the strong trc promoter, the overproduction of PAC was often limited by periplasmic processing and inclusion bodies composed of protein aggregates of PAC precursors were formed in the periplasm. The amount of these periplasmic inclusion bodies was significantly reduced and PAC activity was significantly increased upon coexpression of DegP. The specific PAC activity reached an extremely high level of 674 U/L/OD(600) for MDDeltaP7 harboring pTrcKnPAC2902 and pKS12 under optimum culture conditions. However, such improvement in the production of PAC was not observed for the expression systems (e.g., MDDeltaP7 harboring pCLL2902) in which the periplasmic processing was not the step limiting the production of PAC. The results suggest that DegP could in vivo assist the periplasmic processing though the enzyme is shown to be not absolutely required for the formation of active PAC in E. coli. In addition, the steps limiting the production of PAC are identified and the reasons for the formation of PAC inclusion bodies are discussed here.     

Strain improvement to enhance the production of recombinant penicillin acylase in high-cell-density Escherichia coli cultures

Using fed-batch operation for high-cell-density cultivation, efforts are frequently made for optimization of culture parameters, particularly feeding strategy. The current study also emphasized the importance of selecting strains for the production of recombinant proteins in high-cell-density cultures. With Escherichia coli penicillin acylase (PAC) as a target protein, the host/vector system of MDdeltaP7 harboring pTrcKnPAC2902 and pKS12 was designed for optimization of fed-batch cultivation for recombinant protein production. The host, MDdeltaP7, potentially had a high translational and periplasmic processing efficiency for pac expression. On the other hand, the vector, pTrcKnPAC2902, was genetically constructed for pac overexpression. Coexistence of the other vector, pKS12, significantly enhanced PAC production by improving cell physiology and reducing the amount of inclusion body formation upon pac overexpression. An extremely high volumetric PAC activity at 37,500 U/L was obtained with the use of the developed host/vector system under optimum fed-batch culture conditions.     

Synthesis and secretion of Providencia rettgeri and Escherichia coli heterodimeric penicillin amidases in Saccharomyces cerevisiae

The Providencia rettgeri and Escherichia coli pac genes encoding heterodimeric penicillin G amidases (PAC) were successfully expressed in Saccharomyces cerevisiae. Furthermore, these recombinant enzymes are secreted from the yeast cell into the medium which is in contrast to bacterial hosts, where the enzymes are retained in the periplasm. Contrary to the P. rettgeri PAC-encoding gene, the E. coli pac is poorly expressed in yeast. The highest yield of P. rettgeri PAC was obtained with a multi-copy plasmid, resulting in of 1500units per liter. This yield is higher by an order of magnitude than that obtained in the best recombinant bacterial expression system. The recombinant P. rettgeri enzyme is only partially and selectively O-glycosylated. Only every sixth or seventh alpha-subunit is glycosylated, while the beta-subunit is not glycosylated at all. N-Glycosylation has not been detected.     

雷氏普罗威登斯菌青霉素G酰化酶基因在大肠杆菌中的克隆与表达

利用PCR和分子克隆技术从雷氏普罗威登斯菌（Prouidencia rettgeri)(ATCC29944)的基因组DNA中获得一个青霉素G酰化酶（penicillinGacylase,PGA)基因并将其装入表达质粒pET24a。携带有重组质粒pETPGA的Escherichia coli基因工程菌BL21（DE3）/pETPGA实现了PGA的高效表达,对发酵条件的研究表明基因工程菌在24℃,添加5g/L甘油条件下以1.0mmol/LIPTG诱导1.5h酶活力即达到993.4U/L,比野生菌酶活力（15U/L）提高了66倍。     

High-level gene expression for recombinant penicillin acylase production using the araB promoter system in Escherichia coli

The pac gene encoding penicillin acylase (PAC) was overexpressed under the regulation of the araB promoter (ParaB, also known as PBAD) in Escherichia coli (E. coli). The current ParaB expression system exhibited minimum leaking pac expression in the absence of arabinose as well as fast and high-level pac expression upon induction with arabinose in a wide concentration range. The production of PAC was limited by the accumulation of PAC precursors (i.e., proPAC in both soluble and insoluble forms) and various negative cellular responses, such as growth arrest and cell lysis. The culture performance could be improved by degP coexpression and the individual contribution of DegP protease and chaperone activities to the enhancement on the production of PAC was characterized. The study highlights the importance of identifying the step(s) limiting high-level gene expression and subsequent design and construction of the host/vector system for enhancing recombinant protein production in E. coli.     

大肠杆菌中青霉素G酰化酶成熟的限制性因素

为了研究大肠杆菌中青霉素G酰化酶 (PenicillinGacylase ,PAC)成熟的限制性步骤 ,分别构建了PAC表达质粒pKKpacSP ,pETpacSP ,并将它们在大肠杆菌宿主中表达。通过酶活性的测定及Westernblotting分析 ,分别在PAC自身表达系统 ,Tac ,T7及氧调控表达系统中 ,研究PAC前体蛋白加工成α亚基、β亚基的效率及亚基折叠组装形成活性酶的能力。结果表明 :PAC成熟过程中的限制性步骤因宿主 载体系统而异 ;PAC本身表达系统中 ,前体肽加工成α亚基、β亚基的效率为 57.2 % ,亚基组装能力为 0.72;Tac启动子表达调控系统中α亚基的折叠和稳定成为限制性步骤 ;T7及氧调控表达系统中 ,PAC前体肽加工成α亚基、β亚基的效率达 90 %左右 ,亚基组装成活性酶的能力分别为 1 82 ,2 ;低氧调控系统表达PAC时 ,成熟的效率最高 ,PAC表达的单位重量活性提高 10倍.     

Production of glycosylated thermostable Providencia rettgeri penicillin G amidase in Pichia pastoris

Penicillin G amidase from Providencia rettgeri is a heterodimer of 92 kDa. We have previously expressed the Pr. rettgeri pac gene coding for this enzyme in Saccharomyces cerevisiae, and now we report the expression and characterization in the methylotrophic yeast Pichia pastoris. The recombinant catalytically active enzyme (rPAC(Pr)) was secreted from shake flask-grown P. pastoris cells into the medium at a level of approximately 0.18 U ml(-1). This yield of rPAC(Pr) was higher, by two orders of magnitude, than that obtained using a single-copy expression plasmid in S. cerevisiae. In addition, the secreted recombinant enzyme was entirely N-glycosylated. The recombinant PAC(Pr) was further characterized in terms of specific activity, kinetic parameters and thermostability. Except the significantly higher thermostability of the glycosylated rPAC(Pr) produced in P. pastoris, the other parameters were very similar to those of the corresponding non-glycosylated enzymes produced in bacteria or in S. cerevisiae. The higher thermostability of this recombinant enzyme has a clear industrial advantage.     

Repression of penicillin G acylase of Proteus rettgeri by tricarboxylic acid cycle intermediates.

The regulation of the penicillin acylase in proteus rettgeri ATCC 31052 was compared with that of the enzyme in Escherichia coli ATCC 9637. Unlike the E. coli acylase, the P. rettgeri enzyme was not induced by phenylacetic acid, nor was it subject to catabolite repression by glucose. The P. rettgeri acylase appears to be expressed constitutively but is subject to repression by the C4-dicarboxylic acids of the tricarboxylic acid cycle, succinate, fumarate, and malate.     

Purification and some properties of beta-lactamases from Proteus rettgeri and Proteus inconstans

Two beta-lactamases were isolated from strains of Proteus species and purified, one from a strain of P. rettgeri and the other from a strain of P. inconstans. Each enzyme preparation gave a single protein band on polyacrylamide gel electrophoresis. Molecular weights of P. rettgeri and P. inconstans enzymes were found to be 42,000 and 43,000, and their isoelectric points pH 8.7 and 8.6, respectively. The two enzymes presented typical cephalosporinase profiles. Cefmetazole (CS-1170) and cefoxitin, both cephamycin antibiotics, not only resisted hydrolysis by both of the enzymes, but also inhibited their activities competitively. Rabbit antiserum against purified P. rettgeri enzyme inhibited the activity of both purified and crude enzyme preparations from other strains of P. rettgeri so far tested. None of the beta-lactamases produced by other species of Proteus including P. inconstans was inhibited by the antiserum, thus showing that the purified cephalosporinase was of the species-specific types. The enzymological properties of the preparations were compared with those of beta-lactamases derived from other gram-negative enteric bacteria.     

(R)-phenylacetylcarbinol production in aqueous/organic two-phase systems using partially purified pyruvate decarboxylase from Candida utilis

Aqueous/organic two-phase systems have been evaluated for enhanced production of (R)-phenylacetylcarbinol (PAC) from pyruvate and benzaldehyde using partially purified pyruvate decarboxylase (PDC) from Candida utilis. In a solvent screen, octanol was identified as the most suitable solvent for PAC production in the two-phase system in comparison to butanol, pentanol, nonanol, hexane, heptane, octane, nonane, dodecane, methylcyclohexane, methyl tert butyl ether, and toluene. The high partitioning coefficient of the toxic substrate benzaldehyde in octanol allowed delivery of large amounts of benzaldehyde into the aqueous phase at a concentration less than 50 mM. PDC catalyzed the biotransformation of benzaldehyde and pyruvate to PAC in the aqueous phase, and continuous extraction of PAC and byproducts acetoin and acetaldehyde into the octanol phase further minimized enzyme inactivation, and inhibition due to acetaldehyde. For the rapidly stirred two-phase system with a 1:1 phase ratio and 8.5 U/mL carboligase activity, 937 mM (141 g/L) PAC was produced in the octanol phase in 49 h with an additional 127 mM (19 g/L) in the aqueous phase. Similar concentrations of PAC could be produced in the slowly stirred phase separated system at this enzyme level, although at a much slower rate. However at lower enzyme concentration very high specific PAC production (128 mg PAC/U carboligase at 0.9 U/mL) was achieved in the phase separated system, while still reaching final PAC levels of 102 g/L in octanol and 13 g/L in the aqueous phase. By comparison with previously published data by our group for a benzaldehyde emulsion system without octanol (50 g/L PAC, 6 mg PAC/U carboligase), significantly higher PAC concentrations and specific PAC production can be achieved in an octanol/aqueous two-phase system.     

High-level production and covalent immobilization of Providencia rettgeri penicillin G acylase (PAC) from recombinant Pichia pastoris for the development of a novel and stable biocatalyst of industrial applicability

A complete, integrated process for the production of an innovative formulation of penicillin G acylase from Providencia rettgeri(rPAC(P.rett))of industrial applicability is reported. In order to improve the yield of rPAC, the clone LN5.5, carrying four copies of pac gene integrated into the genome of Pichia pastoris, was constructed. The proteinase activity of the recombinant strain was reduced by knockout of the PEP4 gene encoding for proteinase A, resulting in an increased rPAC(P.rett) activity of approximately 40% (3.8 U/mL vs. 2.7 U/mL produced by LN5.5 in flask). A high cell density fermentation process was established with a 5-day methanol induction phase and a final PAC activity of up to 27 U/mL. A single step rPAC(P.rett) purification was also developed with an enzyme activity yield of approximately 95%. The novel features of the rPAC(P.rett) expressed in P.pastoris were fully exploited and emphasized through the covalent immobilization of rPAC(P.rett). The enzyme was immobilized on a series of structurally correlated methacrylic polymers, specifically designed and produced for optimizing rPAC(P.rett) performances in both hydrolytic and synthetic processes. Polymers presenting aminic functionalities were the most efficient, leading to formulations with higher activity and stability (half time stability >3 years and specific activity ranging from 237 to 477 U/g (dry) based on benzylpenicillin hydrolysis). The efficiency of the immobilized rPAC(P.rett) was finally evaluated by studying the kinetically controlled synthesis of beta-lactam antibiotics (cephalexin) and estimating the synthesis/hydrolysis ratio (S/H), which is a crucial parameter for the feasibility of the process.     

Expression of the Aspergillus fumigatus phytase gene in Pichia pastoris and characterization of the recombinant enzyme

Aspergillus fumigatus phytase is a heat-stable enzyme of great potential. Our objective was to determine if a high level of functional expression of the A. fumigatus phytase gene could be produced in Pichia pastoris and how the recombinant phytase reacted to different substrates, heating conditions, and proteases. A 1.4-kb DNA fragment containing the coding region of the gene was inserted into the expression vector pPICZalphaA and expressed in P. pastoris as an active, extracellular phytase (r-Afp). The yield was 729 mg of purified protein per liter of culture, with a specific activity of 43 units/mg of protein. The enzyme r-Afp shared similar pH and temperature optima, molecular size, glycosylation extent, and specificity for p-nitrophenyl phosphate and sodium phytate to those of the same enzyme expressed in A. niger. Given 20 min of exposure to 65 to 90 degrees C, the enzyme retained 20 to 39% higher residual activity in 10 and 200 mM sodium acetate than that in sodium citrate. The enzyme seemed to be resistant to pepsin digestion, but was degraded by high levels of trypsin. In conclusion, P. pastoris is a potential host to express high levels of A. fumigatus phytase and the thermostability of the recombinant enzyme is modulated by the specificity of buffer used in the heat treatment.     

Biphasic aqueous/organic biotransformation of acetaldehyde and benzaldehyde by Zymomonas mobilis pyruvate decarboxylase

Zymomonas mobilis pyruvate decarboxylase (PDC) transformed acetaldehyde and benzaldehyde into (R)-phenylacetylcarbinol (PAC), the precursor for the synthesis of ephedrine and pseudoephedrine. Organic solvents were screened for a biphasic biotransformation with the enzyme in an aqueous phase and the toxic substrates delivered through the organic phase. In the absence of substrates a second phase of 1-pentanol, hexadecane or MTBE (methyl tertiary-butyl ether) stabilized the PDC activity in comparison to a control without added solvent. Organic phase solvents for optimal PAC production had partitioning coefficient (log P) values between 0.8 and 2.8 (production of more than 8 mg PAC/ U PDC), however there was no correlation between enzyme stability and log P. Best PAC formation was observed with the eight tested alcohols, which in contrast to the other solvents allowed lower initial concentrations of toxic acetaldehyde (54-81 mM) in the aqueous phase. 1-pentanol, 1-hexanol, and isobutanol resulted in the highest specific PAC production of 11 mg PAC /U PDC. Without the addition of an organic phase, only 1.2 mg/U was formed.     

pPAC-ResQ: A yeast-bacterial shuttle vector for capturing inserts from P1 and PAC clones by recombinogenic targeted cloning

We have developed a method to capture inserts from P1 and P1 artificial chromosome (PAC) clones into a yeast-bacteria shuttle vector by using recombinogenic targeting. We have engineered a vector, pPAC-ResQ, a derivative of pClasper, which was previously used to capture inserts from yeast artificial chromosome clones. pPAC-ResQ contains DNA fragments flanking the inserts in P1 and PAC vectors as recombinogenic ends. When linearized pPAC-ResQ vector and P1 or PAC DNA are cotransformed into yeast, recombination between the two leads to the transfer of inserts into pPAC-ResQ. pPAC-ResQ clones thus obtained can be further modified in yeast for functional analysis and shuttled to Escherichia coli to produce large quantities of cloned DNA. This approach provides a rapid method to modify P1/PAC clones for functional analysis.     

Crystal structure of penicillin G acylase from the Bro1 mutant strain of Providencia rettgeri.

Penicillin G acylase is an important enzyme in the commercial production of semisynthetic penicillins used to combat bacterial infections. Mutant strains of Providencia rettgeri were generated from wild-type cultures subjected to nutritional selective pressure. One such mutant, Bro1, was able to use 6-bromohexanamide as its sole nitrogen source. Penicillin acylase from the Bro1 strain exhibited an altered substrate specificity consistent with the ability of the mutant to process 6-bromohexanamide. The X-ray structure determination of this enzyme was undertaken to understand its altered specificity and to help in the design of site-directed mutants with desired specificities. In this paper, the structure of the Bro1 penicillin G acylase has been solved at 2.5 A resolution by molecular replacement. The R-factor after refinement is 0.154 and R-free is 0.165. Of the 758 residues in the Bro1 penicillin acylase heterodimer (alpha-subunit, 205; beta-subunit, 553), all but the eight C-terminal residues of the alpha-subunit have been modeled based on a partial Bro1 sequence and the complete wild-type P. rettgeri sequence. A tightly bound calcium ion coordinated by one residue from the alpha-subunit and five residues from the beta-subunit has been identified. This enzyme belongs to the superfamily of Ntn hydrolases and uses Ogamma of Ser beta1 as the characteristic N-terminal nucleophile. A mutation of the wild-type Met alpha140 to Leu in the Bro1 acylase hydrophobic specificity pocket is evident from the electron density and is consistent with the observed specificity change for Bro1 acylase. The electron density for the N-terminal Gln of the alpha-subunit is best modeled by the cyclized pyroglutamate form. Examination of aligned penicillin acylase and cephalosporin acylase primary sequences, in conjunction with the P. rettgeri and Escherichia coli penicillin acylase crystal structures, suggests several mutations that could potentially allow penicillin acylase to accept charged beta-lactam R-groups and to function as a cephalosporin acylase and thus be used in the manufacture of semi-synthetic cephalosporins.