Enzyme reaction engineering: effect of methanol on the synthesis of antibiotics catalyzed by immobilized penicillin G acylase under isothermal and non-isothermal conditions

The effect of methanol on the kinetically controlled synthesis of cephalexin by free and immobilized penicillin G acylase (PGA) was investigated. Catalytic and hydrophobic membranes were obtained by chemical grafting, activation, and PGA immobilization on hydrophobic nylon supports. Butyl methacrylate (BMA) was used as graft monomer. Increasing concentrations of methanol were found to cause a greater deleterious effect on the activity of free than on that of the immobilized enzyme. Methanol, however, improved the kinetic stability of cephalexin synthesized by free PGA, resulting in higher maximum yields. By contrast, immobilized PGA reached 100% yields even in the absence of the cosolvent. Cephalexin synthesis by the catalytic membrane was also performed in a non-isothermal bioreactor. Under these conditions, a 94% increase of the synthetic activity and complete conversion of the limiting substrate to cephalexin were obtained. The addition of methanol reduced the non-isothermal activity increase. The physical cause responsible for the non-isothermal behavior of the hydrophobic catalytic membrane was identified in the process of thermodialysis.

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.

A DNA oligonucleotide-hemin complex cleaves t-butyl hydroperoxide through a homolytic mechanism

Both electron paramagnetic resonance (EPR) and electronic absorption spectroscopy have been employed to investigate the reaction of a guanine-rich DNA nucleotide-hemin complex (PS2.M-hemin complex) and organic peroxide (t-Bu-OOH). Incubation of the PS2.M-hemin complex with t-Bu-OOH resulted in the time-dependent decrease in the heme Soret with concomitant changes to the visible bands of the electronic absorbance spectrum for the PS2.M-hemin complex. Parallel EPR studies using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) combined with spectral simulation demonstrated the presence of tert-butyloxyl, carbon-centered methyl, and methyl peroxyl radicals as well as a simple nitroxide (triplet) signal. Experiments, performed by maintaining a constant ratio of t-Bu-OOH/PS2.M-hemin complex ( approximately 35 mol/mol) while varying DMPO concentration, indicated that the relative contributions of each radical adduct to the composite EPR spectrum were significantly influenced by the DMPO concentration. For example, at DMPO/PS2.M-hemin of 10-50 mol/mol, a complex mixture of radicals was consistently detected, whereas at high trapping efficiency (i.e., DMPO/PS2.M-hemin of approximately 250 mol/mol) the tert-butyloxyl-DMPO adduct was predominant. In contrast, at relatively low DMPO/PS2.M-hemin complex ratios of < or =5 mol/mol, a simple nitroxide three-line EPR signal was detected largely in the absence of all other radicals. Together, these data indicate that tert-butyloxyl radical is the primary radical likely formed from the homolytic cleavage of the O-O peroxy bond of t-Bu-OOH, while methyl and methyl peroxyl radicals result from beta-scission of the primary tert-butyloxyl radical product.

The peroxidase activity of a hemin--DNA oligonucleotide complex: free radical damage to specific guanine bases of the DNA

A specific DNA oligonucleotide--hemin complex (PS2.M--hemin complex) that exhibits DNA-enhanced peroxidative activity was studied by EPR and UV--visible spectroscopy and by chemical probing analysis. EPR data obtained from low-temperature experiments on the PS2.M--hemin complex showed both a low-field g approximately 6 and a high-field g approximately 2 signal. These EPR signals are typical of high-spin ferric heme with axial symmetry as judged by the EPR spectrum of six-coordinate heme iron in acidic Fe(III)-myoglobin. This similarity is consistent with the presence of two axial ligands to the heme iron within the PS2.M--hemin complex, one of which is a water molecule. Optical analyses of the acid-base transition for the hemin complex yielded a pK(a) value for the water ligand of 8.70 +/- 0.03 (mean +/- SD). Low-temperature EPR analysis coupled with parallel spin-trapping investigations following the reaction of the PS2.M--hemin complex and hydrogen peroxide (H(2)O(2)) indicated the formation of a carbon-centered radical, most likely on the PS2.M oligonucleotide. Chemical probing analysis identified specific guanine bases within the PS2.M sequence that underwent oxidative damage upon reaction with H(2)O(2). These and other experimental findings support the hypothesis that the interaction of specific guanines of PS2.M with the bound hemin cofactor might contribute to the superior peroxidative activity of the PS2.M--hemin complex.

A ribozyme and a catalytic DNA with peroxidase activity: active sites versus cofactor-binding sites

BACKGROUND

An 18-nucleotide DNA oligomer, PS2.M, derived using an in vitro selection method was previously reported to bind hemin (Fe(III)-protoporphyrinIX) with submicromolar affinity. The DNA-hemin complex exhibited DNA-enhanced peroxidative activity. PS2. M is guanine-rich and requires potassium ions to fold to its active conformation, consistent with its forming a guanine-quaduplex. In investigating the specific catalytic features of PS2.M we tested the peroxidative properties of its RNA version (rPS2.M) as well as that of an unrelated DNA guanine-quadruplex, OXY4.

RESULTS

The hemin-binding affinity of rPS2.M was found to be 30-fold weaker than that of PS2.M. The UV-visible spectra and kinetics of enzymatic peroxidation of the RNA-hemin complex, however, were nearly identical to those of its DNA counterpart. Both displayed peroxidase activity substantially greater than those of heme proteins such as catalase and Fe(III)-myoglobin. Kinetic analysis suggested that PS2. M and rPS2.M catalyzed the breakdown of the hemin-hydrogen peroxide covalent complex to products. The hemin complex of folded OXY4 (which bound hemin as strongly as did rPS2.M) had a distinct absorption spectrum and only a minor peroxidase activity above the background level.

CONCLUSIONS

The results indicated that it is possible for RNA and DNA of the same sequence to fold to form comparable cofactor-binding sites, and to show comparable catalytic behavior. The results further suggest that only a subset of cofactor-binding sites formed within folded nucleic acids might be able to function as active sites, by providing the appropriate chemical environments for catalysis.

DNA-enhanced peroxidase activity of a DNA-aptamer-hemin complex

BACKGROUND

In vitro selection (SELEX) previously identified short single-stranded DNAs that specifically bound N-methylmesoporphyrin IX (NMM), a stable transition-state analogue for porphyrin-metallation reactions. Interestingly, iron(III)-protoporphyrin (hemin) was a good competitive inhibitor for the DNA-catalyzed metallation reaction, and appeared to bind strongly to the NMM-binding DNA aptamers. We investigated the peroxidase activity of the aptamer-hemin complexes to see if the DNA component of the complex, like the apoenzymes in protein peroxidases, could enhance the low intrinsic peroxidatic activity of hemin.

RESULTS

Two porphyrin-binding DNA aptamers bound hemin with submicromolar affinity. The aptamer-hemin complexes had significantly higher peroxidase activity than hemin alone, under physiological conditions. The Vobs of the PS2.M-hemin complex was 250 times greater than that of hemin alone, and significantly superior to a previously reported hemin-catalytic-antibody complex. Preliminary spectroscopic evidence suggests the coordination of the hemin iron in the complex changes, such that the complex more closely resembles horseradish peroxidase and other heme proteins rather than hemin.

CONCLUSIONS

A new class of catalytic activity for nucleic acids is reported. The aptamer-hemin complexes described are novel DNA enzymes and their study will help elucidate the structural and functional requirements of peroxidase enzymes in general and the ways that a nucleic acid 'apoenzyme' might work to enhance the intrinsic peroxidatic ability of hemin. These aptamer-hemin complexes could be regarded as prototypes for redox-catalyzing ribozymes in a primordial 'RNA world'.