Penicillin amidase from E. coli. Enzyme heterogeneity and stability

Ca2+ Is a Cofactor Required for Membrane Transport and Maturation and Is a Yield-Determining Factor in High Cell Density Penicillin Amidase Production

Penicillin amidases (PAs) from E. coli and A. faecalis are periplasmic enzymes that contain one tightly bound Ca(2+) per molecule that does not directly participate in the enzymatic function. This ion may, however, be required for the maturation of the pre-pro-enzyme. The pro-enzyme of homologous PAs are translocated through the Tat- (E. coli PA(EC)) and Sec- (A. faecalis PA(AF)) transport systems, respectively. Cell fractionation, electrophoresis, immunoblotting, and activity staining demonstrated that Ca(2+) binding is required for the membrane transport and maturation of the pro-enzyme to active enzyme. Pro-enzyme without Ca(2+) was targeted to the membrane but not translocated. Influence of Ca(2+) in medium and feed was studied for high cell density cultivations of E. coli expressing these enzymes. Without Ca(2+) in the feed the synthesis of the pre-pro-enzyme was hardly influenced. At optimal Ca(2+) content in the feed the active enzyme amount could be increased by 2 orders of magnitude up to 0.9 g/L (PA(EC)) and 2.3 g/L (PA(AF)) or 4% (PA(EC)) and 8% (PA(AF)) of the cell dry weight. The corresponding specific activities are 1700 U (PA(EC)) and 14000 U (PA(AF)) per gram cell dry weight, respectively. These values are higher than those published previously. Thus, for optimal yields of the studied and other extra- and periplasmic enzymes that require Ca(2+) or other ions as cofactors for membrane transport and maturation, sufficient cofactor must be added in the feed.

Circular dichroism analysis of penicillin G acylase covalently immobilized on silica nanoparticles

Circular dichroism (CD) was used to characterize the secondary structure of penicillin G acylase upon covalent immobilization on silica nanoparticles. Covalent immobilization was achieved by functionalizing the silica nanoparticles with glutardialdehyde and coupling to the free NH(2) groups of the enzyme (lysine and arginine side chains). The loading of the covalently bound enzyme was increased up to saturation, which was reached at 54.6 mg immobilized enzyme per g silica nanobeads. For structural characterization of the commercially available enzyme its exact molecular mass was determined by mass spectrometry in order to enable precise evaluation of the CD data. The fraction of secondary structure elements of the free and immobilized enzyme were estimated from the respective CD spectra using standard algorithms (CONTINLL, CDSSTR, SELCON3). The fractions obtained by the different algorithms for the free enzyme agreed well with one another and also with data from X-ray diffraction described in the literature. Interestingly, the secondary structure fractions found for the immobilized enzyme were very similar to the free enzyme and nearly constant over all experiments. These results indicate that even a loading of up to 55.8 mg/g (enzyme per silica nanoparticles) causes only slight structural changes. However, the specific activity determined by a kinetic assay decreased by around 60%, when increasing the loading from 14.9 to 55.8 mg/g. Because of the fact that we found no major changes in the secondary structure, diffusion limitation seems to be the main reason for the decline of the specific activity.

Pro-sequence and Ca2+-binding: Implications for Folding and Maturation of Ntn-hydrolase Penicillin Amidase from E.coli

Penicillin amidase (PA) is a bacterial periplasmic enzyme synthesized as a pre-pro-PA precursor. The pre-sequence mediates membrane translocation. The intramolecular pro-sequence is expressed along with the A and B chains but is rapidly removed in an autocatalytic manner. In extensive studies we show here that the pro-peptide is required for the correct folding of PA. Pro-PA and PA unfold via a biphasic transition that is more pronounced in the case of PA. According to size-exclusion chromatography and limited proteolysis experiments, the inflection observed in the equilibrium unfolding curves corresponds to an intermediate in which the N-terminal domain (A-chain) still possesses native-like topology, whereas the B-chain is unfolded to a large extent. In a series of in vitro experiments with a slow processing mutant pro-PA, we show that the pro-sequence in cis functions as a folding catalyst and accelerates the folding rate by seven orders of magnitude. In the absence of the pro-domain the PA refolds to a stable inactive molten globule intermediate that has native-like secondary but little tertiary structure. The pro-sequence of the homologous Alcaligenes faecalis PA can facilitate the folding of the hydrolase domain of Escherichia coli PA when added in trans (as a separate polypeptide chain). The isolated pro-sequence has a random structure in solution. However, difference circular dichroism spectra of native PA and native PA with pro-peptide added in trans suggest that the pro-sequence adopts an alpha-helical conformation in the context of the mature PA molecule. Furthermore, our results establish that Ca2+, found in the crystal structure, is not directly involved in the folding process. The cation shifts the equilibrium towards the native state and facilitates the autocatalytic processing of the pro-peptide.

Kinetic studies and molecular modelling attribute a crucial role in the specificity and stereoselectivity of penicillin acylase to the pair ArgA145-ArgB263

Kinetic experiments with a substrate series of phenylacetyl-arylamides reveal that at least one polar group in the amine moiety is required for the proper orientation of the substrate in the large nucleophile-binding subsite of penicillin acylase of Escherichia coli. Quantum mechanical molecular modelling of enzyme-substrate interactions in the enzyme active site shows that in the case of substrates lacking local symmetry, the productive binding implies two nonsymmetrical arrangements with respect to the two positively charged guanidinium residues of ArgA145 and ArgB263. This indicates a crucial role of the specified arginine pair in the substrate- and stereoselectivity of penicillin acylase.

Fragments of pro-peptide activate mature penicillin amidase of Alcaligenes faecalis

Penicillin amidase from Alcaligenes faecalis is a recently identified N-terminal nucleophile hydrolase, which possesses the highest specificity constant (kcat/Km) for the hydrolysis of benzylpenicillin compared with penicillin amidases from other sources. Similar to the Escherichia coli penicillin amidase, the A. faecalis penicillin amidase is maturated in vivo from an inactive precursor into the catalytically active enzyme, containing one tightly bound Ca2+ ion, via a complex post-translational autocatalytic processing with a multi-step excision of a small internal pro-peptide. The function of the pro-region is so far unknown. In vitro addition of chemically synthesized fragments of the pro-peptide to purified mature A. faecalis penicillin amidase increased its specific activity up to 2.3-fold. Mutations were used to block various steps in the proteolytic processing of the pro-peptide to obtain stable mutants with covalently attached fragments of the pro-region to their A-chains. These extensions of the A-chain raised the activity up to 2.3-fold and increased the specificity constants for benzylpenicillin hydrolysis mainly by an increase of the turnover number (kcat).

The isopenicillin N acyltransferases of Aspergillus nidulans and Penicillium chrysogenum differ in their ability to maintain the 40-kDa alphabeta heterodimer in an undissociated form

The isopenicillin N acyltransferases (IATs) of Aspergillus nidulans and Penicillium chrysogenum differed in their ability to maintain the 40-kDa proacyltransferase alphabeta heterodimer in an undissociated form. The native A. nidulans IAT exhibited a molecular mass of 40 kDa by gel filtration. The P. chrysogenum IAT showed a molecular mass of 29 kDa by gel filtration (corresponding to the beta subunit of the enzyme) but the undissociated 40-kDa heterodimer was never observed even in crude extracts. Heterologous expression experiments showed that the chromatographic behaviour of IAT was determined by the source of the penDE gene used in the expression experiments and not by the host itself. When the penDE gene of A. nidulans was expressed in P. chrysogenum npe6 and npe8 or in Acremonium chrysogenum, the IAT formed had a molecular mass of 40 kDa. On the other hand, when the penDE gene originating from P. chrysogenum was expressed in A. chrysogenum, the active IAT had a molecular mass of 29 kDa. The intronless form of the penDE gene cloned from an A. nidulans cDNA library and overexpressed in Escherichia coli formed the enzymatically active 40-kDa proIAT, which was not self-processed as shown by immunoblotting with antibodies to IAT. This 40-kDa protein remained unprocessed even when treated with A. nidulans crude extract. In contrast, the P. chrysogenum penDE intronless gene cloned from a cDNA library was expressed in E. coli, and the IAT was self-processed efficiently into its alpha (29 kDa) and beta (11 kDa) subunits. It is concluded that P. chrysogenum and A. nidulans differ in their ability to self-process their respective proIAT protein and to maintain the alpha and beta subunits as an undissociated heterodimer, probably because of the amino-acid sequence differences in the proIAT which affect the autocatalytic activity.

Improvement of posttranslational bottlenecks in the production of penicillin amidase in recombinant Escherichia coli strains

Using periplasmic penicillin amidase (PA) from Escherichia coli ATCC 11105 as a model recombinant protein, we reviewed the posttranslational bottlenecks in its overexpression and undertook attempts to enhance its production in different recombinant E. coli expression hosts. Intracellular proteolytic degradation of the newly synthesized PA precursor and translocation through the plasma membrane were determined to be the main posttranslational processes limiting enzyme production. Rate constants for both intracellular proteolytic breakdown (k(d)) and transport (k(t)) were used as quantitative tools for selection of the appropriate host system and cultivation medium. The production of mature active PA was increased up to 10-fold when the protease-deficient strain E. coli BL21(DE3) was cultivated in medium without a proteinaceous substrate, as confirmed by a decrease in the sum of the constants k(d) and k(t). The original signal sequence of pre-pro-PA was exchanged with the OmpT signal peptide sequence in order to increase translocation efficiency; the effects of this change varied in the different E. coli host strains. Furthermore, we established that simultaneous coexpression of the OmpT pac gene with some proteins of the Sec export machinery of the cell resulted in up to threefold-enhanced PA production. In parallel, we made efforts to increase PA flux via coexpression with the kil gene (killing protein). The primary effects of the kil gene were the release of PA into the extracellular medium and an approximately threefold increase in the total amount of PA produced per liter of bacterial culture.

The relative importance of intracellular proteolysis and transport on the yield of the periplasmic enzyme penicillin amidase in Escherichia coli*

Intracellular proteolysis is an important mechanism for regulating the level of the periplasmic enzyme penicillin amidase in Escherichia coli. Evidence is presented that the active enzyme is localized in the periplasmic space and maturation of pro-enzyme occurs during transport through the cytoplasmic membrane or rapidly after its entrance in the periplasm. The rate constants of the transport through cytoplasmic membrane and of the intracellular proteolysis were estimated to be 0.01 h and 0.5 h, respectively. This indicates that more than 90% of the synthesized pre-pro-enzyme is lost by intracellular proteolysis occurring in the cytoplasm.

Intramolecular autoproteolysis initiates the maturation of penicillin amidase from Escherichia coli

The penicillin amidase (PA) from Escherichia coli belongs to a group of proteolytically processed bacterial enzymes. The mechanism of the maturation of the single polypeptide proenzyme has been studied for the PA from E. coli using a slowly processing mutant proenzyme. The mutant proenzyme was constructed by replacing Thr with Gly in the Thr(263)-Ser(264) bond that must be hydrolysed in active PA. The mutant proenzyme was purified by biospecific affinity chromatography using an immobilized monoclonal antibody against PA. The maturation of the free and covalently immobilized purified proenzyme was studied in vitro. For the free proenzyme the same products with PA activity as observed in homogenates of wild-type PA-producing E. coli cells were found to be formed during this process. A kinetic analysis of the possible inter- and intramolecular processes involved in the maturation demonstrated that unambiguous evidence for the existence of intramolecular processes can only be obtained in systems where intermolecular processes are excluded. The Gly(263)-Ser(264) bond was found to be hydrolysed first in the free and immobilized mutant proenzyme, based on determinations of mass spectra, N-terminal sequences and active site concentrations. In the system with immobilized proenzyme intermolecular processes are excluded, demonstrating that this bond is hydrolysed by intramolecular autoproteolysis. Based on the known three-dimensional structure of the PA from E. coli the same maturation mechanism should apply for the wild-type proenzyme.

pH dependence of penicillin amidase enantioselectivity for charged substrates

The pH dependence of E (enantiomeric ratio or enantioselectivity, a quantitative measure for enzyme stereospecificity) was studied for penicillin amidase catalysed hydrolysis of charged enantiomeric substrates. Theoretical analysis shows that a pH dependence can only be observed around the pK values of groups in the active site whose ionisation control the enzyme activity. For charged substrates that may perturb these pK values, a pH dependence of E is also expected. This was experimentally verified around these pK values. The S'(1)-stereospecificity of penicillin amidase was studied for the hydrolysis of the enantiomeric phenylacetyl-S/R-Phe and for the racemic phenylacetyl-S,R-PhG. The S(1)-stereospecificity was investigated for the hydrolysis of the enantiomeric S/R-PhG-NH(2). The observed pH modulation of E (more than 3-fold for the studied substrates in the pH range 4.5-9) was found to be a result of compensatory effects for binding and catalysis. The ratios k(cat, S)/k(cat,R) and K(m,S)/K(m,R) for the hydrolysis of the enantiomeric phenylacetyl-Phe were found to decrease from 1000 to 10 and from 0.1 to 0.01, respectively in the pH range 5-8. The dependence was stronger for the S'(1)- than for the S(1)-subsite. This is probably due to the stronger influence of the substrate carboxyl group in the S'(1)-subsite than that of the substrate amino group in the S(1)-subsite on the pK of the N-terminal Ser B1 that is essential for the activity. The observed pH dependence of E was used to discuss the importance of ground-state interactions for discrimination between enantiomers and for enzyme catalysis in general. The experimental results conform to the split site model according to which a better binding must not be fundamentally inhibitory.

Kinetic study of penicillin acylase from Alcaligenes faecalis

Penicillin acylase from Alcaligenes faecalis has a very high affinity for both natural (benzylpenicillin, Km = 0.0042 mM) and colorimetric (6-nitro-3-phenylacetamidobenzoic acid, Km = 0.0045 mM) substrates as well as the product of their hydrolysis, phenylacetic acid (Ki = 0.016 mM). The enzyme is partially inhibited at high benzylpenicillin concentrations but the triple SES complex formed still retains 43% of the maximal catalytic activity; the affinity of benzylpenicillin for the second substrate molecule binding site is much lower (K(S)' = 54 mM) than for the first one. Phenylmethylsulfonyl fluoride was shown to be a very effective irreversible inhibitor, completely inactivating the penicillin acylase from A. faecalis in a few minutes at micromolar concentrations; this compound was used for enzyme active site titration. The absolute values of the determined kinetic parameters for enzymatic hydrolysis of 6-nitro-3-phenylacetamidobenzoic acid (k(cat) = 95 s(-1) and k(cat)/Km = 2.1 x 10(-7) M(-1) s(-1)) and benzylpenicillin (k(cat) = 54 s(-1) and k(cat)/Km = 1.3 x 10(-7) M(-1) s(-1)) by penicillin acylase from A. faecalis were shown to be highest of all the enzymes of this family that have so far been studied.

Purification of monoclonal antibodies by simulated moving-bed chromatography

A simulated moving bed (SMB) system has been developed for the biospecific purification of monoclonal antibodies. Adsorption and desorption of the desired product is performed under different conditions. To increase the purity and yield of the antibodies, two purge steps have to be introduced. The steady-state performance of the SMB system was modelled by solving the governing differential equations using a linear driving force approximation. The model parameters were determined independently in batch experiments. They were used to determine the operating conditions of the SMB system for the purification of monoclonal antibodies from cell culture supernatant. The antibodies could be isolated with a yield of > or = 90. SDS gel electrophoresis of the feed and product stream showed that more than 99% of the contaminating proteins were removed in a single step by SMB chromatography.

Perfusible and non-perfusible supports with monoclonal antibodies for bioaffinity chromatography of Escherichia coli penicillin amidase within its pH stability range

Several monoclonal antibodies (mABs) have been prepared and immobilized for the biospecific isolation of penicillin amidase (PA) from Escherichia coli (EC 3.5.1.11), an enzyme without S-S bridges and a pH stability range of 4-9. During the immobilization the fluorescence emission maxima of the mABs were found to change from 336 nm to ca. 350 nm. Only one of these mABs was found to be suitable for preparative bioaffinity chromatography of PA within the pH stability range. This mAB was immobilized on different spherical supports (Eupergit C 250 L and Sepharose) and one perfusible support (Knauer Quick Disc) and used for analytical and preparative bioaffinity chromatography. Under isocratic conditions the plate height for the perfusible biospecific adsorbent was found to be an order of magnitude lower than for the other supports. The different forms of this proteolytically processed bacterial enzyme could not be separated, however, by the biospecific adsorbents. At the mAB density used in the adsorbents (10-30 microM), less than 30% of the theoretical binding capacity of the immobilized mABs could be used to adsorb the enzyme.

Rapid protein purification using phenylbutylamine-Eupergit: a novel method for large-scale procedures

Electrophoretic desorption was used to compare the protein binding capacities of some hydrophobic adsorbents [the phenylbutylamine (PBA) derivatives of Eupergit C and agarose and Phenyl-Sepharose] for low-pressure chromatography. The highest capacity was observed for the bifunctional adsorbent PBA-Eupergit. The hydrophobically adsorbed proteins can be selectively desorbed by decreasing the pH of the eluent due to electrostatic repulsion between positive charges on the adsorbed proteins and positively charged secondary amines on the adsorbent. This was used to purify 1500 U penicillin amidase from E. coli homogenates per gram wet weight of PBA-Eupergit in 50 adsorption-desorption cycles without organic solvents (greater than 90% yield, purification factor = 5.3).