Stimulatory effect of growth in the presence of alcohols on biotransformation of penicillin G into cephalosporin-type antibiotics by resting cells of Streptomyces clavuligerus NP1

Engineering deacetoxycephalosporin C synthase as a catalyst for the bioconversion of penicillins

7-aminodeacetoxycephalosporanic acid (7-ADCA) is a key intermediate of many clinically useful semisynthetic cephalosporins that were traditionally prepared by processes involving chemical ring expansion of penicillin G. Bioconversion of penicillins to cephalosporins using deacetoxycephalosporin C synthase (DAOCS) is an alternative and environmentally friendly process for 7-ADCA production. Arnold Demain and co-workers pioneered such a process. Later, protein engineering efforts to improve the substrate specificity and catalytic efficiency of DAOCS for penicillins have been made by many groups, and a whole cell process using Escherichia coli for bioconversion of penicillins has been developed.

Reconstitution of TCA cycle with DAOCS to engineer Escherichia coli into an efficient whole cell catalyst of penicillin G

Many medically useful semisynthetic cephalosporins are derived from 7-aminodeacetoxycephalosporanic acid (7-ADCA), which has been traditionally made by the polluting chemical method. Here, a whole-cell biocatalytic process based on an engineered Escherichia coli strain expressing 2-oxoglutarate-dependent deacetoxycephalosporin C synthase (DAOCS) for converting penicillin G to G-7-ADCA is developed. The major engineering strategy is to reconstitute the tricarboxylic acid (TCA) cycle of E. coli to force the metabolic flux to go through DAOCS catalyzed reaction for 2-oxoglutarate to succinate conversion. Then the glyoxylate bypass was disrupted to eliminate metabolic flux that may circumvent the reconstituted TCA cycle. Additional engineering steps were taken to reduce the degradation of penicillin G and G-7-ADCA in the bioconversion process. These steps include engineering strategies to reduce acetate accumulation in the biocatalytic process and to knock out a host β-lactamase involved in the degradation of penicillin G and G-7-ADCA. By combining these manipulations in an engineered strain, the yield of G-7-ADCA was increased from 2.50 ± 0.79 mM (0.89 ± 0.28 g/L, 0.07 ± 0.02 g/gDCW) to 29.01 ± 1.27 mM (10.31 ± 0.46 g/L, 0.77 ± 0.03 g/gDCW) with a conversion rate of 29.01 mol%, representing an 11-fold increase compared with the starting strain (2.50 mol%).

Enhancement of validamycin A production by addition of ethanol in fermentation of Streptomyces hygroscopicus 5008

The effect of ethanol on the production of the important agro-antibiotic validamycin A (Val-A) in medium containing agricultural by-products was investigated. Under the optimal condition of ethanol addition, the maximal Val-A production titer reached 18 g/L, which increased by 60% compared to the control. To provide an insight into cell response to ethanol, the intracellular reactive oxygen species (ROS), gene transcription and enzyme activity were determined. Intracellular ROS as the molecular signal was increased in the ethanol condition. Global regulators afsR and glnR were involved in regulation of Val-A biosynthesis, and the transcription of eight Val-A structural genes was enhanced. The activity of glucose-6-phosphate dehydrogenase (G6PD) was enhanced while glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was inhibited. A signal transduction cascade from cell signal response to activated transcription of Val-A biosynthetic genes and enhanced antibiotic production is proposed. The information can be helpful for the improvement of large-scale fermentation.

Expandase-like activity mediated cell-free conversion of ampicillin to cephalexin by Streptomyces sp. DRS I

Cell-free extracts of Streptomyces sp. DRS I converted ampicillin to cephalexin, presumably due to the activity of the enzyme, expandase. The extract was fractionated and characterized by colorimetric and chromatographic measurements coupled with disc-agar diffusion bioassay against an ampicillin-resistant, cephalexin-sensitive E. coli strain. Though expandase could not be identified, the presence of a hitherto unreported expandase in Streptomyces sp. DRS I is suggested.

Engineering Streptomyces clavuligerus deacetoxycephalosporin C synthase for optimal ring expansion activity toward penicillin G

The deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus was engineered with the aim of enhancing the conversion of penicillin G into phenylacetyl-7-aminodeacetoxycephalosporanic acid, a precursor of 7-aminodeacetoxycephalosporanic acid, for industrial application. A single round of random mutagenesis followed by the screening of 5,500 clones identified three mutants, G79E, V275I, and C281Y, that showed a two- to sixfold increase in the k(cat)/K(m) ratio compared to the wild-type enzyme. Site-directed mutagenesis to modify residues surrounding the substrate resulted in three mutants, N304K, I305L, and I305M, with 6- to 14-fold-increased k(cat)/K(m) values. When mutants containing all possible combinations of these six sites were generated to optimize the ring expansion activity for penicillin G, the double mutant, YS67 (V275I, I305M), showed a significant 32-fold increase in the k(cat)/K(m) ratio and a 5-fold increase in relative activity for penicillin G, while the triple mutant, YS81 (V275I, C281Y, I305M), showed an even greater 13-fold increase in relative activity toward penicillin G. Our results demonstrate that this is a robust approach to the modification of DAOCS for an optimized DAOCS-penicillin G reaction.

Performance of a recombinant strain of Streptomyces lividans for bioconversion of penicillin G to deacetoxycephalosporin G

We examined the performance of Streptomyces lividans strain W25 containing a hybrid expandase (deacetoxycephalosporin C synthase; DAOCS) gene, obtained by in vivo recombination between the expandase genes of S. clavuligerus and Nocardia lactamdurans for resting-cell bioconversion of penicillin G to deacetoxycephalosporin G. Strain W25 carried out a much more effective level of bioconversion than the previously used strain, S. clavuligerus NP1. The two strains also differed in the concentrations of FeSO(4) and alpha-ketoglutarate giving maximal activity. Whereas NP1 preferred 1.8 mM FeSO(4 )and 1.3 mM alpha-ketoglutarate, recombinant W25 performed best at 0.45 mM FeSO(4) and 1.9 mM alpha-ketoglutarate.

Secondary metabolism in simulated microgravity

We have studied microbial secondary metabolism in a simulated microgravity (SMG) environment provided by NASA rotating-wall bioreactors (RWBs). These reactors were designed to simulate some aspects of actual microgravity that occur in space. Growth and product formation were observed in SMG in all cases studied, i.e., Bacillus brevis produced gramicidin S (GS), Streptomyces clavuligerus made beta-lactam antibiotics, Streptomyces hygroscopicus produced rapamycin, and Escherichia coli produced microcin B17 (MccB17). Of these processes, only GS production was unaffected by SMG; production of the other three products was inhibited. This was determined by comparison with performance in an RWB positioned in a different mode to provide a normal gravity (NG) environment. Carbon source repression by glycerol of the GS process, as observed in shaken flasks, was not observed in the RWBs, whether operated in the SMG or NG mode. The same phenomenon occurred in the case of MccB17 production, with respect to glucose repression. Thus, the negative effects of carbon source on GS and beta-lactam formation are presumably dependent on shear, turbulence, and/or vessel geometry, but not on gravity. Stimulatory effects of phosphate and the precursor L-lysine on beta-lactam antibiotic production, as observed in flasks, also occurred in SMG. An almost complete shift in the localization of produced MccB17 from cells to extracellular medium was observed when E. coli was grown in the RWB under SMG or NG. If a plastic bead was placed in the RWB, accumulation became cellular, as it is in shaken flasks, indicating that sheer stress favors a cellular location. In the case of rapamycin, the same type of shift was observed, but it was less dramatic, i.e., growth in the RWB under SMG shifted the distribution of produced rapamycin from 2/3 cellular:1/3 extracellular to 1/3 cellular:2/3 extracellular. Stress has been shown to induce or promote secondary metabolism in a number of other microbial systems. RWBs provide a low stress SMG environment, which, however, supports only poor production of MccB17, as compared to production in shaken flasks. We wondered whether the poor production in RWBs under SMG is due to the low level of stress, and whether increasing stress in the RWBs would raise the amount of MccB17 formed. We found that increasing shear stress by adding a single Teflon bead to the RWB improved MccB17 production. Although shear stress seems to have a marked positive effect on MccB17 production in SMG, addition of various concentrations of ethanol to RWBs (or to shaken flasks) failed to increase MccB17 production. Ethanol stress merely decreased production and, at higher concentrations, inhibited growth. Interestingly, cells growing in the RWB were much more resistant to the growth- and production-inhibitory effects of ethanol than cells growing in shaken flasks. With respect to S. hygroscopicus, addition of Teflon beads to the RWB reversed the inhibition of growth, but rapamycin production was still markedly inhibited, and the distribution did not revert back to a preferential cellular site.

Improvement in the resting-cell bioconversion of penicillin G to deacetoxycephalosporin G by addition of catalase

AIMS

To improve the resting cell bioconversion of penicillin G to deacetoxycephalosporin G (DAOG) by elimination of an oxidizing intermediate which inactivates the enzyme during the reaction.

METHODS AND RESULTS

Resting cells of Streptomyces clavuligerus strain NP1 were incubated with penicillin G, required co-factors and decane in the presence of catalase or superoxide dismutase, and production of DAOG was measured. Catalase stimulated the bioconversion but superoxide dismutase did not.

CONCLUSIONS

Production of hydrogen peroxide during the ring expansion reaction is at least partially responsible for enzyme inactivation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Catalase addition improves the bioconversion and will contribute to the eventual replacement of the current multi-step, expensive and environmentally-unfriendly chemical ring expansion by a biological route.

Mutation of N304 to leucine in Streptomyces clavuligerus deacetoxycephalosporin C synthase creates an enzyme with increased penicillin analogue conversion

Superimposition of deacetoxycephalosporin C synthase (DAOCS) and isopenicillin N synthase (IPNS) structures revealed that R74, R160, R266 and N304 are strategically located in the catalytic cavity of Streptomyces clavuligerus DAOCS (scDAOCS) and are crucial for orchestrating different substrates. Substitutions at these sites to a hydrophobic leucine residue were expected to stabilize the hydrophobic substrate bound state. Substantial improvements in the biotransformation of penicillin G, ampicillin and amoxicillin to their respective cephalosporin moieties were observed using the N304L mutant scDAOCS. Thus, our results have demonstrated the enhancement of scDAOCS activity via critical computational analysis and site-directed mutagenesis of endogenous ligands.

Environmentally safe production of 7-aminodeacetoxycephalosporanic acid (7-ADCA) using recombinant strains of Acremonium chrysogenum

Medically useful semisynthetic cephalosporins are made from 7-aminodeacetoxycephalosporanic acid (7-ADCA) or 7-aminocephalosporanic acid (7-ACA). Here we describe a new industrially amenable bioprocess for the production of the important intermediate 7-ADCA that can replace the expensive and environmentally unfriendly chemical method classically used. The method is based on the disruption and one-step replacement of the cefEF gene, encoding the bifunctional expandase/hydroxylase activity, of an actual industrial cephalosporin C production strain of Acremonium chrysogenum. Subsequent cloning and expression of the cefE gene from Streptomyces clavuligerus in A. chrysogenum yield recombinant strains producing high titers of deacetoxycephalosporin C (DAOC). Production level of DAOC is nearly equivalent (75-80%) to the total beta-lactams biosynthesized by the parental overproducing strain. DAOC deacylation is carried out by two final enzymatic bioconversions catalyzed by D-amino acid oxidase (DAO) and glutaryl acylase (GLA) yielding 7-ADCA. In contrast to the data reported for recombinant strains of Penicillium chrysogenum expressing ring expansion activity, no detectable contamination with other cephalosporin intermediates occurred.