Periplasmic aggregation limits the proteolytic maturation of the Escherichia coli penicillin G amidase precursor polypeptide

Biotechnological advances on penicillin G acylase: pharmaceutical implications, unique expression mechanism and production strategies

In light of unrestricted use of first-generation penicillins, these antibiotics are now superseded by their semisynthetic counterparts for augmented antibiosis. Traditional penicillin chemistry involves the use of hazardous chemicals and harsh reaction conditions for the production of semisynthetic derivatives and, therefore, is being displaced by the biosynthetic platform using enzymatic transformations. Penicillin G acylase (PGA) is one of the most relevant and widely used biocatalysts for the industrial production of β-lactam semisynthetic antibiotics. Accordingly, considerable genetic and biochemical engineering strategies have been devoted towards PGA applications. This article provides a state-of-the-art review in recent biotechnological advances associated with PGA, particularly in the production technologies with an emphasis on using the Escherichia coli expression platform.

Effect of heat-shock proteins for relieving physiological stress and enhancing the production of penicillin acylase inEscherichia coli

High-level expression of recombinant penicillin acylase (PAC) using the strong trc promoter system in Escherichia coli is frequently limited by the processing and folding of PAC precursors (proPAC) in the periplasm, resulting in physiological stress and inclusion body formation in this compartment. Periplasmic heat-shock proteins with protease or chaperone activity potentially offer a promise for overcoming this technical hurdle. In this study, the effect of the two genes encoding periplasmic heat-shock proteins, that is degP and fkpA, on pac overexpression was investigated and manipulation of the two genes to enhance the production of recombinant PAC was demonstrated. Both DeltadegP and DeltafkpA mutants showed defective culture performance primarily due to growth arrest. However, pac expression level was not seriously affected by the mutations, indicating that the two proteins were not directly involved in the pathway for periplasmic processing of proPAC. The growth defect caused by the two mutations (i.e., DeltadegP and DeltafkpA) was complemented by either one of the wild-type proteins, implying that the function of the two proteins could partially overlap in cells overexpressing pac. The possible role that the two heat-shock proteins played for suppression of physiological stress caused by pac overexpression is discussed.

Characterization of the T7 promoter system for expressing penicillin acylase in Escherichia coli

The pac gene encoding penicillin acylase (PAC) was overexpressed under the regulation of the T7 promoter in Escherichia coli. PAC, with its complex formation mechanism, serves as a unique target protein for demonstration of several key strategies for enhancing recombinant protein production. The current T7 system for pac overexpression was fraught with various technical hurdles. Upon the induction with a conventional inducer of isopropyl-beta-D-thiogalactopyranoside (IPTG), the production of PAC was limited by the accumulation of PAC precursors (proPAC) as inclusion bodies and various negative cellular responses such as growth inhibition and cell lysis. The expression performance could be improved by the coexpression of degP encoding a periplasmic protein with protease and chaperone activities. In addition to IPTG, arabinose was shown to be another effective inducer. Interestingly, arabinose not only induced the current T7 promoter system for pac expression but also facilitated the posttranslational processing of proPAC for maturation, resulting in significant enhancement for the production of PAC. Glycerol appeared to have an effect similar to, but not as significant as, arabinose for enhancing the production of PAC. The study highlights the importance of developing suitable genetically engineered strains with culture conditions for enhancing recombinant protein production in E. coli.

Chaperone-mediated folding and maturation of the penicillin acylase precursor in the cytoplasm of Escherichia coli

Expression of the leaderless pac gene (LL pac), which lacks the coding region for the signal peptide of penicillin acylase (PAC), in Escherichia coli was conducted. It was demonstrated that the PAC precursor, proPAC, can be produced and even processed to form mature PAC in the cytoplasm, indicating that the posttranslational processing steps for PAC maturation can occur in both the periplasm and the cytoplasm of E. coli. The outcome of proPAC folding and PAC maturation could be affected by several factors, such as inducer type, proPAC formation rate, and chaperone availability. Misfolding of proPAC in the cytoplasm could be partially resolved through the coexpression of cytoplasmic chaperones, such as trigger factor, GroEL/ES, or DnaK/J-GrpE. The three chaperones tested showed different extents of the effect on proPAC solublization and PAC maturation, and trigger factor had the most prominent one. However, the chaperone-mediated solublization of proPAC did not guarantee its maturation, which is usually limited by the first autoproteolytic step. It was observed that arabinose could act as an effective inducer for the induction of LL pac expression regulated by the lac-derived promoter system of trc. In addition, PAC maturation could be highly facilitated by arabinose supplementation and coexpression of trigger factor, suggesting that the coordination of chaperone systems with proper culture conditions could dramatically impact recombinant protein production. This study suggests that folding/misfolding of proPAC could be a major step limiting the overproduction of PAC in E. coli and that the problem could be resolved through the search for appropriate chaperones for coexpression. It also demonstrates the analogy in the issues of proPAC misfolding as well as the expression bottleneck occurring in the cytoplasm (i.e., LL pac expression) and those occurring in the periplasm (i.e., wild-type pac expression).

Optimization of the host/vector system and culture conditions for production of penicillin acylase in Escherichia coli

Culture performance for the production of penicillin acylase (PAC) in a bioreactor was investigated using HB101 or ATCC11105 as the host and pCLL2902, pCLL3201 or pTrcKnPAC2902 as the expression plasmid. We observed that the production of PAC by HB101 harboring pCLL3201 was, similar to ATCC11105, induced by phenyl acetic acid (PAA) and catabolicaily repressed by glucose, whereas the production of PAC by HB101 harboring pCLL2902 did not require PAA for induction and was not repressed by glucose. PAC activity of HB101 harboring pCLL2902 was significantly higher than that of HB101 harboring pCLL3201. There was no significant effect of host or carbon source on the production of PAC using pCLL2902. The production of PAC by HB101 harboring pTrcKnPAC2902, in which the pac gene expression was controlled by the trc promoter system, was about the same as that by HB101 harboring pCLL2902, when the culture was appropriately induced with isopropyl beta-d-thiogalactopyranoside (IPTG). Therefore, the use of both pCLL2902 and pTrcKnPAC2902 could be expected to be feasible for industrial applications. However, optimization of IPTG induction for HB101 harboring pTrcKnPAC2902 might be required, since formation of inclusion bodies tends to limit the production of PAC in some cases.

Regulation of penicillin G acylase gene expression in Escherichia coli by repressor PaaX and the cAMP-cAMP receptor protein complex

The pga gene of Escherichia coli W ATCC11105 encodes a penicillin G acylase whose expression is regulated at both the transcriptional and post-transcriptional level. In this work we have shown that PaaX is the repressor of pga expression, and we have identified its binding consensus as TGATTC(N27)GAATCA. We conclude that the process of "PAA induction" actually involves relief of pga from repression by PaaX. Other features of the pga promoter have also been characterized. (i) It has a native class III cAMP-receptor protein (CRP)-dependent promoter with two CRP-binding sites. (ii) The downstream CRP-binding site II has higher affinity. (iii) Binding of cAMP-CRP to both sites (I + II) is required for maximal expression. We have also shown that the PaaX-binding site overlaps with the CRP-binding site I. This implies that PaaX and the cAMP-CRP compete for binding to the region around the CRP-binding site I and therefore have antagonistic effects on pga expression.

Recent biotechnological interventions for developing improved penicillin G acylases

Penicillin G acylase (PAC; EC 3.5.1.11) is the key enzyme used in the industrial production of beta-lactam antibiotics. This enzyme hydrolyzes the side chain of penicillin G and related beta-lactam antibiotics releasing 6-amino penicillanic acid (6-APA), which is the building block in the manufacture of semisynthetic penicillins. PAC from Escherichia coli strain ATCC 11105, Bacillus megaterium strain ATCC 14945 and mutants of these two strains is currently used in industry. Genes encoding for PAC from various bacterial sources have been cloned and overexpressed with significant improvements in transcription, translation and post-translational processing. Recent developments in enzyme engineering have shown that PAC can be modified to gain conformational stability and desired functionality. This review provides an overview of recent advances in the production, stabilization and application of PAC, highlighting the recent biotechnological approaches for the improved catalysis of PAC.

Roles of DegP in prevention of protein misfolding in the periplasm upon overexpression of penicillin acylase in Escherichia coli

Enhancement of the production of soluble recombinant penicillin acylase in Escherichia coli via coexpression of a periplasmic protease/chaperone, DegP, was demonstrated. Coexpression of DegP resulted in a shift of in vivo penicillin acylase (PAC) synthesis flux from the nonproductive pathway to the productive one when pac was overexpressed. The number of inclusion bodies, which consist primarily of protein aggregates of PAC precursors in the periplasm, was highly reduced, and the specific PAC activity was highly increased. DegP was a heat shock protein induced in response to pac overexpression, suggesting that the protein could possibly suppress the physiological toxicity caused by pac overexpression. Coexpression of DegP(S210A), a DegP mutant without protease activity but retaining chaperone activity, could not suppress the physiological toxicity, suggesting that DegP protease activity was primarily responsible for the suppression, possibly by degradation of abnormal proteins when pac was overexpressed. However, a shortage of periplasmic protease activity was not the only reason for the deterioration in culture performance upon pac overexpression because coexpression of a DegP-homologous periplasmic protease, DegQ or DegS, could not suppress the physiological toxicity. The chaperone activity of DegP is proposed to be another possible factor contributing to the suppression.

Secretion of active recombinant human tissue plasminogen activator derivatives in Escherichia coli

The DNA fragment coding for kringle 2 plus serine protease domains (K2S) of tissue plasminogen activator (tPA) was inserted into a phagemid vector, pComb3HSS. In the recombinant vector, pComb3H-K2S, the K2S gene was fused to gpIII of PhiM13 and linked to the OmpA signal sequence. The resulting gene, rK2S-gpIII, was inducibly expressed in Escherichia coli XL-1 Blue. The protein was presented on the phage particle. To stop the expression of gpIII, a stop codon between K2S and the gpIII gene was inserted by site-directed mutagenesis. This mutated vector, MpComb3H-K2S, was transformed in XL-1 Blue. After induction with IPTG (isopropyl-beta-D-thiogalactopyranoside), rK2S was found both in the periplasm as an inactive form of approximately 32% and in the culture supernatant as an active form of approximately 68%. The secreted form of rK2S was partially purified by ammonium sulfate (55%) precipitation. The periplasmic form was isolated from whole cells by chloroform extraction. The fibrin binding site of kringle 2 was demonstrated in all expressed versions (phage-bound, periplasmic, and secreted forms) using the monoclonal anti-kringle 2 antibody (16/B). Only the secreted form of rK2S revealed a fibrinogen-dependent amidolytic activity with the specific activity of 236 IU/microg. No amidolytic activity of rK2S was observed in either the periplasmic or the phage-bound form. The secretion of rK2S as an active enzyme offers a novel approach for the production of the active-domain deletion mutant tPA, rK2S, without any requirements for bacterial compartment preparation and in vitro refolding processes. This finding is an important technological advance in the development of large-scale, bacterium-based tPA production systems.

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.

Manipulation of carbon assimilation with respect to expression of the pac gene for improving production of penicillin acylase in Escherichia coli

A strategy of genetically manipulating carbon assimilation with respect to expression of the pac gene was employed for overproduction of recombinant penicillin acylase (PAC). Two expression plasmids of pCLL2902 and pCLL3201, which contain the pac coding region but differ in the pac regulatory region, were constructed for the production experiments. Expression of the pac gene was subjected to phenyl acetic acid (PAA-) induction and glucose catabolite repression for pCLL3201, whereas it was subjected to neither of the two transcriptional regulations for pCLL2902. The specific PAC activity for strains harboring pCLL2902 was significantly higher than that for strains harboring pCLL3201 due to an improved transcription efficiency. In addition, no inclusion bodies were observed upon production of PAC using the current expression systems. The results suggest that using the native pac promoter instead of a strong promoter such as tac for regulation is a feasible approach for production of PAC. The impact of the current expression systems is also significant from a process viewpoint since, using strains harboring pCLL2902, not only could glucose replace PAA as a carbon source of Escherichia coli cultures for production of PAC but also the volumetric PAC activity was highly improved.