Comparative characterization of new glutaryl 7-ACA and cephalosporin C acylases

Engineering of a CPC acylase using a facile pH indicator assay

Cephalosporin C (CPC) acylase is important for the one-step production of 7-aminocephalosporanic acid (7-ACA), a key intermediate for cephalosporin antibiotics. However, its application is hampered by the low activity, substrate inhibition, and product inhibition. In this study, two rounds of combinatorial active-site saturation testing (CASTing) were carried out on the CPC acylase acyII from Pseudomonas SE83, and one mutant H57βA/H70βY with no substrate inhibition was obtained. For further engineering to reduce the product inhibition, a quick pH indicator assay was developed, allowing for real-time monitoring of the product inhibition in the presence of added 7-ACA. The utility of the assay was demonstrated by screening six libraries of site-directed saturation mutagenesis libraries of H57βA/H70βY. A new mutant H57βA/H70βY/I176βN was obtained, which showed a k  cat 3.26-fold and a K  IP 3.08-fold that of the wild type, respectively. Given the commercial value of the enzyme, both this pH indicator assay and the triple mutant should be useful for further engineering of the enzyme to increase the specific activity and to decrease the product inhibition.

Cephalosporin C acylase: dream and(/or) reality

Cephalosporins currently constitute the most widely prescribed class of antibiotics and are used to treat diseases caused by both Gram-positive and Gram-negative bacteria. Cephalosporins contain a 7-aminocephalosporanic acid (7-ACA) nucleus which is derived from cephalosporin C (CephC). The 7-ACA nucleus is not sufficiently potent for clinical use; however, a series of highly effective antibiotic agents could be produced by modifying the side chains linked to the 7-ACA nucleus. The industrial production of higher-generation semi-synthetic cephalosporins starts from 7-ACA, which is obtained by deacylation of the naturally occurring antibiotic CephC. CephC can be converted to 7-ACA either chemically or enzymatically using D-amino acid oxidase and glutaryl-7-aminocephalosporanic acid acylase. Both these methods show limitation, including the production of toxic waste products (chemical process) and the expense (the enzymatic one). In order to circumvent these problems, attempts have been undertaken to design a single-step means of enzymatically converting CephC to 7-ACA in the course of the past 10 years. The most suitable approach is represented by engineering the activity of a known glutaryl-7-aminocephalosporanic acid acylase such that it will bind and deacylate CephC more preferentially over glutaryl-7-aminocephalosporanic acid. Here, we describe the state of the art in the production of an effective and specific CephC acylase.

Kinetics of hydrolysis and mutational analysis of N,N-diethyl-m-toluamide hydrolase from Pseudomonas putida DTB

The initial step in the biodegradation pathway of N,N-diethyl-m-toluamide (DEET) in Pseudomonas putida strain DTB is catalyzed by DEET hydrolase (DthA), which hydrolyzes the amide bond to yield 3-methylbenzoic acid and diethylamine. In order to extend our understanding of DthA, the enzyme was purified and characterized. The enzyme is most active at pH 7.9, and is probably a tetramer in its native state. The kinetic parameters of the wild-type enzyme are K(m) = 10.2 ± 0.8 μm, k(cat) = 5.53 ± 0.09 s(-1) , and k(cat) /K(m) = (5.4 ± 0.4) × 10(5) m(-1) ·s(-1) . Mild substrate inhibition was observed with DEET concentrations over 500 μm. A homology model of DthA was used to guide mutational analysis of the active site, confirming that the catalytic triad is formed by Ser166, Ap292, and His320. The oxyanion hole is formed by the side chain OH of Tyr84 and the backbone amide of Trp167, with the Tyr84 OH being essential for enzyme activity. The DthA model also revealed a hydrophobic substrate-binding pocket comprosed of Trp167, Met170, and Trp214. W167A and M170A mutations decreased enzymatic activity and exacerbated substrate inhibition, whereas Trp214, which probably plays a role in substrate recognition, was essential for enzymatic activity. The pH rate profile of DthA was fitted to two ionizable groups (pK(a1) = 6.1 and pK(a2) = 9.9) that probably correspond to Nε of His320 and the OH of Tyr84, respectively. In addition to catalyzing the hydrolysis of DEET, DthA hydrolyzed a variety of esters and amides.

Crystallization and preliminary diffraction analysis of an engineered cephalosporin acylase

Crystallization conditions are reported for an engineered cephalosporin acylase based on the sequence of glutaryl-7-aminocephalosporanic acid acylase from Pseudomonas strain N176. Initial crystals were grown using polyethylene glycol as a crystallizing agent; however, these crystals diffracted poorly and exhibited high mosaicity. A dehydration procedure in which crystals were transferred to a solution containing a higher concentration of polyethylene glycol as well as glycerol improved the diffraction quality such that a 1.57 A diffraction data set could be obtained.

Enzymatic Modifications of Cephalosporins by Cephalosporin Acylase and Other Enzymes

Semisynthetic cephalosporins are important antibacterials in clinical practice. Semisynthetic cephalosporins are manufactured by derivatizing 7-aminocephalosporanic acid (7-ACA) and its desacetylated form. Microbial enzymes such as D-amino acid oxidase, glutaryl-7-ACA acylase and cephalosporin esterase are being used as biocatalysts for the conversion of cephalosporin C (CEPH-C) to 7-ACA and its desacetylated derivatives. Recent developments in the field of enzymatic modifications of cephalosporin with special emphasis on group of enzymes called as cephalosporin acylase is discussed in this review. Aspects related to screening methods, isolation and purification, immobilization, molecular cloning, gene structure and expression and protein engineering of cephalosporin acylases have been covered. Topics pertaining to enzymatic modifications of cephalosporin by D-amino acid oxidase, cephalosporin methoxylase and beta-lactamase are also covered.

Affinity labeled glutaryl-7-amino cephalosporanic acid acylase C130 can hydrolyze the inhibitor during crystallization

7Beta-bromoacetyl amino cephalosporanic acid (BA-7-ACA), an analog of glutaryl-7-amino cephalosporanic acid (GL-7-ACA), can inhibit and specifically alkylate GL-7-ACA acylase (C130) from Pseudomonas sp.130, forming a carbon-carbon bond between BA-7-ACA and the C-2 on indole ring of Trp-beta4 residue of C130. Here we reported that BA-7-ACA labeled C130 (BA-C130) could self-catalyze the hydrolysis of BA-7-ACA during crystallization process. The hydrolysis was confirmed to be a reaction analogous to the one of GL-7-ACA by comparative matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) spectrometry analysis. BA-C130 was inactive at room temperature, but in the process of crystallization at 18 degrees C it catalyzed the hydrolysis of BA-7-ACA, and thus made the latter become a substrate. Meanwhile, in crystals, 7-ACA was released but the acetic acid still bound with Trp-beta4, and as a result, the enzyme remained to be inactive. These results demonstrated that Trp-beta4 in the alphabetabetaalpha motif was critical and sensitive for the activity of C130 and also suggested that there was a conformational change induced by deacylation during the process of crystallization.

Deacylation activity of cephalosporin acylase to cephalosporin C is improved by changing the side-chain conformations of active-site residues

Semisynthetic cephalosporins are primarily synthesized from 7-aminocephalosporanic acid (7-ACA), mainly by environmentally toxic chemical deacylation of cephalosporin C (CPC). Thus, the enzymatic conversion of CPC to 7-ACA by cephalosporin acylase (CA) would be very interesting. However, CAs use glutaryl-7-ACA (GL-7-ACA) as a primary substrate and the enzymes have low turnover rates for CPC. The active-site residues of a CA were mutagenized to various residues to increase the deacylation activity of CPC, based on the active-site conformation of the CA structure. The aim was to generate sterically favored conformation of the active-site to accommodate the D-alpha-aminoadipyl moiety of CPC, the side-chain moiety that corresponds to the glutaryl moiety of GL-7-ACA. A triple mutant of the CA, Q50betaM/Y149alphaK/F177betaG, showed the greatest improvement of deacylation activity to CPC up to 790% of the wild-type. Our current study is an efficient method for improving the deacylation activity to CPC by employing the structure-based repetitive saturation mutagenesis.

Perspectives in liquid membrane extraction of cephalosporin antibiotics

In this paper an overview of the developments in liquid membrane extraction of cephalosporin antibiotics has been presented. The principle of reactive extraction via the so-called liquid-liquid ion exchange extraction mechanism can be exploited to develop liquid membrane processes for extraction of cephalosporin antibiotics. The mathematical models that have been used to simulate experimental data have been discussed. Emulsion liquid membrane and supported liquid membrane could provide high extraction flux for cephalosporins, but stability problems need to be fully resolved for process application. Non-dispersive extraction in hollow fiber membrane is likely to offer an attractive alternative in this respect. The applicability of the liquid membrane process has been discussed from process engineering and design considerations.

Active site residues of cephalosporin acylase are critical not only for enzymatic catalysis but also for post-translational modification

Cephalosporin acylase (CA) is a recently identified N-terminal hydrolase. It is also a commercially important enzyme in producing 7-aminocephalosporanic acid (7-ACA), a backbone chemical in synthesizing semi-synthetic cephalosporin antibiotics. CA is translated as an inactive single chain precursor, being post-translationally modified into an active enzyme. The post-translational modification takes place in two steps. The first intramolecular autocatalytic proteolysis takes place at one end of the spacer peptide by a nucleophilic Ser or Thr, which in turn becomes a new N-terminal Ser or Thr. The second intermolecular modification cleaves off the other end of the spacer peptide by another CA. Two binary structures in complex with glutaryl-7-ACA (the most favored substrate of CAs) and glutarate (side chain of glutaryl-7-ACA) were determined, and they revealed the detailed interactions of glutaryl-7-ACA with the active site residues (Y. Kim and W. G. J. Hol (2001) Chem. Biol., in press). In this report: 1) we have mutated key active site residues into nonfunctional amino acids, and their roles in catalysis were further analyzed; 2) we performed mutagenesis studies indicating that secondary intermolecular modification is carried out in the same active site where deacylation reaction of CA occurs; and 3) the cleavage site of secondary intermolecular modification by another CA was identified in the spacer peptide using mutational analysis. Finally, a schematic model for intermolecular cleavage of CA is proposed.

Structure of cephalosporin acylase in complex with glutaryl-7-aminocephalosporanic acid and glutarate: insight into the basis of its substrate specificity

BACKGROUND

Semisynthetic cephalosporins are primarily synthesized from 7-aminocephalosporanic acid (7-ACA), which is obtained by environmentally toxic chemical deacylation of cephalosporin C (CPC). Thus, the enzymatic conversion of CPC to 7-ACA by cephalosporin acylase (CA) would be of great interest. However, CAs use glutaryl-7-ACA (GL-7-ACA) as a primary substrate and the enzyme has low turnover rates for CPC.

RESULTS

The binary complex structures of CA with GL-7-ACA and glutarate (the side-chain of GL-7-ACA) show extensive interactions between the glutaryl moiety of GL-7-ACA and the seven residues that form the side-chain pocket. These interactions explain why the D-alpha-aminoadipyl side-chain of CPC yields a poorer substrate than GL-7-ACA.

CONCLUSIONS

This understanding of the nature of substrate specificity may be useful in the design of an enzyme with an improved performance for the conversion of CPC to 7-ACA. Additionally, the catalytic mechanism of the deacylation reaction was revealed by the ligand bound structures.

The 2.0 A crystal structure of cephalosporin acylase

BACKGROUND

Semisynthetic cephalosporins are primarily synthesized from 7-aminocephalosporanic acid (7-ACA), which is usually obtained by chemical deacylation of cephalosporin C (CPC). The chemical production of 7-ACA includes, however, several expensive steps and requires thorough treatment of chemical wastes. Therefore, an enzymatic conversion of CPC to 7-ACA by cephalosporin acylase is of great interest. The biggest obstacle preventing this in industrial production is that cephalosporin acylase uses glutaryl-7ACA as a primary substrate and has low substrate specificity for CPC.

RESULTS

We have solved the first crystal structure of a cephalosporin acylase from Pseudomonas diminuta at 2.0 A resolution. The overall structure looks like a bowl with two "knobs" consisting of helix- and strand-rich regions, respectively. The active site is mostly formed by the distinctive structural motif of the N-terminal (Ntn) hydrolase superfamily. Superposition of the 61 residue active-site pocket onto that of penicillin G acylase shows an rmsd in Calpha positions of 1.38 A. This indicates structural similarity in the active site between these two enzymes, but their overall structures are elsewhere quite different.

CONCLUSION

The substrate binding pocket of the P. diminuta cephalosporin acylase provides detailed insight into the ten key residues responsible for the specificity of the cephalosporin C side chain in four classes of cephalosporin acylases, and it thereby forms a basis for the design of an enzyme with an improved conversion rate of CPC to 7-ACA. The structure also provides structural evidence that four of the five different classes of cephalosporin acylases can be grouped into one family of the Ntn hydrolase superfamily.

In vivo post-translational processing and subunit reconstitution of cephalosporin acylase from Pseudomonas sp. 130

Cephalosporin acylases are a group of enzymes that hydrolyze cephalosporin C (CPC) and/or glutaryl 7-amino cephalosporanic acid (GL-7ACA) to produce 7-amino cephalosporanic acid (7-ACA). The acylase from Pseudomonas sp. 130 (CA-130) is highly active on GL-7ACA and glutaryl 7-aminodesacetoxycephalosporanic acid (GL-7ADCA), but much less active on CPC and penicillin G. The gene encoding the enzyme is expressed as a precursor polypeptide consisting of a signal peptide followed by alpha- and beta-subunits, which are separated by a spacer peptide. Removing the signal peptide has little effect on precursor processing or enzyme activity. Substitution of the first residue of the beta-subunit, Ser, results in a complete loss of enzyme activity, and substitution of the last residue of the spacer, Gly, leads to an inactive and unprocessed precursor. The precursor is supposed to be processed autocatalytically, probably intramolecularly. The two subunits of the acylase, which separately are inactive, can generate enzyme activity when coexpressed in Escherichia coli. Data on this and other related acylases indicate that the cephalosporin acylases may belong to a novel class of enzymes (N-terminal nucleophile hydrolases) described recently.

Extraction and purification of cephalosporin antibiotics

The biologically active natural and semisynthetic cephalosporin antibiotics require proper methods of extraction and purification for their isolation and subsequent pharmacological studies. This article reviews the various methods useful for extraction and purification of individual compounds as well as the enzymes involved in their biosynthesis. Applicability of the methods for downstream processing of the spent medium has been critically analysed. Adsorption chromatography, particularly with reverse phase materials, in combination with membrane separation is the most successful technique for extraction as well as purification of most of the enzymes and individual compounds. Techniques such as reactive extraction in liquid membrane, non-dispersive extraction in hollow fiber membrane and aqueous two-phase extraction are likely to emerge in new generation processes. Finally, some aspects of process design and scale-up have been discussed, highlighting the research needs of pragmatic importance.

Oxidative modification of a cephalosporin C acylase from Pseudomonas strain N176 and site-directed mutagenesis of the gene

A cephalosporanic acid acylase from Pseudomonas strain N176 catalyzes hydrolysis of both glutarylcephalosporanic acid and cephalosporin C to 7-amino-cephalosporanic acid. Chemical modification of the enzyme with acidic hydrogen peroxide was performed to investigate residues which play important roles in enzymatic activity. The activity of the enzyme was reduced to 76% of the original by oxidation. From protein chemical analysis combined with site-directed point mutagenesis, modification of Met-164 was found to correspond to the reduction in activity. To study the effect of Met-164 on the enzymatic character, we prepared mutant acylases in which Met-164 was replaced with several other amino acids and obtained the following data: (i) there existed a trend of mutation to noncharged hydrophilic residues, resulting in an increase of activity against glutarylcephalosporanic acid; (ii) the mutation of Met-164 to Gly and Ala resulted in the lowering of both Km values and the optimal pHs against glutarylcephalosporanic acid; (iii) the mutation to Leu enhanced cephalosporin C acylase activity; and (iv) the mutation to Gln improved the k(cat) value for glutarylcephalosporanic acid. In particular, the mutation to Gln resulted in a high rate of conversion of glutarylcephalosporanic acid to 7-amino-cephalosporanic acid under conditions similar to those of a bioreactor system. These results may indicate that Met-164 is located in or near the cephalosporin compound binding pocket on the enzyme.

Production of cephalosporin intermediates by feeding adipic acid to recombinant Penicillium chrysogenum strains expressing ring expansion activity

We demonstrate a novel and efficient bioprocess for production of the cephalosporin intermediates, 7-aminocephalosporanic acid (7-ACA) or 7-amino deacetoxycephalosporanic acid (7-ADCA). The Streptomyces clavuligerus expandase gene or the Cephalosporium acremonium expandase-hydroxylase gene, with and without the acetyltransferase gene, were expressed in a penicillin production strain of Penicillium chrysogenum. Growth of these transformants in media containing adipic acid as the side chain precursor resulted in efficient production of cephalosporins having an adipyl side chain, proving that adipyl-6-APA is a substrate for either enzyme in vivo. Strains expressing expandase produced adipyl-7-ADCA, whereas strains expressing expandase-hydroxylase produced both adipyl-7-ADCA and adipyl-7-ADAC (aminodeacetylcephalosporanic acid). Strains expressing expandase-hydroxylase and acetyltransferase produced adipyl-7-ADCA, adipyl-7-ADAC and adipyl-7-ACA. The adipyl side chain of these cephalosporins was easily removed with a Pseudomonas-derived amidase to yield the cephalosporin intermediates.

Biochemical characterization of a glutaryl-7-aminocephalosporanic acid acylase from Pseudomonas strain BL072

Pseudomonas strain BL072 produces an acylase enzyme active in hydrolyzing glutaryl-7-aminocephalosporanic acid to 7-aminocephalosporanic acid. This acylase was purified by column chromatography and gel electrophoresis. The native acylase was composed of two subunits of approximately 65 and 24 kDa, though some heterogeneity was seen in both the native acylase and its small subunit. The isoelectric point of the acylase is approximately 8.5, and it has Km of 1.6 mM for glutaryl desacetoxy aminocephalosporanic acid. The acylase hydrolyzes the desacetoxy and desacetyl derivatives of glutaryl-7-aminocephalosporanic acid at rates similar to that of glutaryl-7-aminocephalosporanic acid. Cephalosporin C was hydrolyzed at a reduced rate. The pH optimum was found to be 8.0, and an activation energy of 9 kcal/mol (ca. 38 kJ/mol) was observed. The acylase has transacylase activity 10 times that of its hydrolytic activity. Eupergit C-immobilized acylase had a half-life of greater than 400 h.

Isolation and Characterization of a Pseudomonas Strain Producing Glutaryl-7-Aminocephalosporanic Acid Acylase

Several screening methods were developed for the selection of Pseudomonas strains capable of hydrolyzing glutaryl-7-aminocephalosporanic acid to 7-aminocephalosporanic acid. An isolate exhibiting high acylase activity, designated BL072, was identified as a strain of Pseudomonas diminuta . It grew optimally at pH 7 to 8 and at a temperature of 32 to 40°C, but acylase activity was highest when the strain was grown at 28°C. Mutants of BL072 were generated by nitrosoguanidine treatment and screened for increased production of glutaryl-7-aminocephalosporanic acid acylase. A superior mutant gave a fourfold increase in acylase titer. The cell-associated acylase had similar activities against various glutaryl-cephems but had undetectable activity against cephalosporin C. This acylase may prove useful for the conversion of cephalosporin C to 7-aminocephalosporanic acid.