Penicillin amidase-catalysed transfer of low specific acyl moiety. Synthesis of 7-benzoxazolonylacetamido desacetoxycephalosporanic acid

6-aminopenicillanic acid production by intact cells of E. coli containing penicillin G acylase (PGA)

The aim of present study was to optimize conditions for conversion of penicillin G into 6-APA using intact crude cells of locally collected PGA producing bacterial strains as biocatalyst. Corn steep liquor medium supplemented with phenylacetic acid was used for PGA production. For enzymatic conversion of penicillin G into 6-APA by PGA impregnated bacterial cells, a maximum reaction time of 4 h was found adequate. The procedure for extraction and crystallization of 6-APA from the enzyme reaction mixture was standardized. Isolation process was carried out under controlled pH conditions and 6-APA crystals were recovered from the reaction mixture via filtration, concentration and drying. The maximum PGA activity was observed in Escherichia coli strain BDCS-N-FMu12 (6.4 mg 6-APA h(-1) mg(-1) wet cells) whereas Bacillus megaterium (ATCC 14945 used as check) exhibited only 2.4 mg 6-APA h(-1) mg(-1) wet cells. The overall yield of 6-APA crystals obtained after enzymatic conversion of penicillin G ranged between 37-55 and 47-68% in foreign and local strains, respectively. BDCS-N-FMu12 was identified as the best PGA producer with 68% 6-APA conversion whereas ATCC 14945 showed the lowest conversion (37%). The recovery of 6-APA (68%) obtained by using crude intact cells as cheap biocatalyst appeared promising. The process of enzyme fermentation and 6-APA crystallization optimized during this study seems cost-effective and environment-friendly. However, further studies are required to scale up the 6-APA biosynthesis reaction for achieving 80-90% conversion of penicillin G into 6-APA by PGA hyper-producing locally collected strains of E. coli.

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.

Advantages of using non-isothermal bioreactors for the enzymatic synthesis of antibiotics: the penicillin G acylase as enzyme model

A new hydrophobic and catalytic membrane was prepared by immobilizing Penicillin G acylase (PGA, EC.3.5.1.11) from E. coli on a nylon membrane, chemically grafted with butylmethacrylate (BMA). Hexamethylenediamine (HMDA) and glutaraldehyde (Glu) were used as a spacer and coupling agent, respectively. PGA was used for the enzymatic synthesis of cephalexin, using D(-)-phenylglycine methyl ester (PGME) and 7-amino-3-deacetoxycephalosporanic acid (7-ADCA) as substrates. Several factors affecting this reaction, such as pH, temperature, and concentrations of substrates were investigated. The results indicated good enzyme-binding efficiency of the pre-treated membrane, and an increased stability of the immobilized PGA towards pH and temperature. Calculation of the activation energies showed that cephalexin production by the immobilized biocatalyst was limited by diffusion, resulting in a decrease of enzyme activity and substrate affinity. Temperature gradients were employed as a way to reduce the effects of diffusion limitation. Cephalexin was found to linearly increase with the applied temperature gradient. A temperature difference of about 3 degrees C across the catalytic membrane resulted into a cephalexin synthesis increase of 100% with a 50% reduction of the production times. The advantage of using non-isothermal bioreactors in biotechnological processes, including pharmaceutical applications, is also discussed.

Synthesis of beta-lactam antibiotics containing alpha-aminophenylacetyl group in the acyl moiety catalyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase

D-(-)-Phenylglycyl-beta-lactamide amidohydrolase was isolated from Xanthomonas sp., purified, and characterized. A characteristic feature of the enzyme is its high specificity for substrates containing an alpha-aminophenylacetic group in the acyl moiety. Cephalexin and D-C-(-)-phenylglycine methyl ester (MEPG), being nonspecific penicillin acylase (EC 3.5.1.11) substrates, have the highest values of bimolecular constant kcat/Km (2.8 x 10(5) and 2.0 x 10(5) M-1 x s-1, respectively) in the case of amidohydrolase. On the contrary, benzylpenicillin is not hydrolyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase. The other peculiarity of the enzyme is its affinity for the charged forms of substrates. Using the amidohydrolase, it was found that the values of delta Go'pH7.0 for hydrolysis of the amide bond in cephalexin and ampicillin are -3.3 and -2.3 kJ mol-1, respectively. They are less by a minimum of 2.7 kJ mol-1 than those for other beta-lactam antibiotics. Detailed thermodynamic and kinetic studies of the synthesis of cephalexin from MEPG and 7-aminodeacetoxycephalosporanic acid (7-ADCA) catalyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase were undertaken. A kinetic scheme is proposed which describes well the experimental curves. The value of conversion of "nucleus" was found to be 76% when the synthesis was carried out from a 31.5 mM solution of 7-ADCA and an 88.5 mM solution of MEPG at pH 6.2 (optimum conditions). A 75% conversion of 7-aminocephalosporanic acid (7-ACA) was achieved in the synthesis of cephaloglycine catalyzed by D-(-)-phenylglycyl-beta-lactamide amidohydrolase.