Regulation of ACV synthetase activity in the beta-lactam biosynthetic pathway by carbon sources and their metabolites

Cephalosporins as key lead generation beta-lactam antibiotics

Abstract

Antibiotics are antibacterial compounds that interfere with bacterial growth, without harming the infected eukaryotic host. Among the clinical agents, beta-lactams play a major role in treating infected humans and animals. However, the ever-increasing antibiotic resistance crisis is forcing the pharmaceutical industry to search for new antibacterial drugs to combat a range of current and potential multi-resistant bacterial pathogens. In this review, we provide an overview of the development, innovation, and current status of therapeutic applications for beta-lactams with a focus on semi-synthetic cephalosporins. Cephalosporin C (CPC), which is a natural secondary metabolite from the filamentous fungus Acremonium chrysogenum, plays a major and demanding role in both producing modern antibiotics and developing new ones. CPC serves as a core compound for producing semi-synthetic cephalosporins that can control infections with different resistance mechanisms. We therefore summarize our latest knowledge about the CPC biosynthetic pathway and its regulation in the fungal host. Finally, we describe how CPC serves as a key lead generation source for the in vitro and better, in vivo synthesis of 7-aminocephalosporanic acid (7-ACA), the major core compound for the pharmaceutical synthesis of current and future semi-synthetic cephalosporins.

Key points

•Latest literature on cephalosporin generations

•Biotechnical production of cephalosporins

•In vivo production of 7-ACA

Comparison of the secondary metabolites in two scales of cephalosporin C (CPC) fermentation and two different post-treatment processes

Cephalosporin C (CPC) is the precursor of a class of antibiotics that were more effective than traditional penicillins. CPC production is performed mainly through fermentation by Acremonium chrysogenum, whose secondary metabolism was sensitive to the environmental changes. In the present work, secondary metabolites were measured by ion-pair reversed-phase liquid chromatography tandemed with hybrid quadrupole time-of-flight mass spectrometry, and the disparity of them from two scales of CPC fermentations (pilot and industrial) and also two different post-treatment processes (oxalic acid and formaldehyde added and control) were investigated. When fermentation size was enlarged from pilot scale (50 l) to industrial scale (156,000 l), the remarkable disparities of concentrations and changing trends of the secondary metabolites in A. chrysogenum were observed, which indicated that the productivity of CPC biosynthesis was higher in the large scale of fermentation. Three environmental factors were measured, and the potential reasons that might cause the differences were analyzed. In the post-treatment process after industrial fermentation, the changes of these secondary metabolites in the tank where oxalic acid and formaldehyde were added were much less than the control tank where none was added. This indicated that the quality of the final product was more stable after the oxalic acid and formaldehyde were added in the post-treatment process. These findings provided new insight into industrial CPC production.

Production of microbial secondary metabolites: regulation by the carbon source

Microbial secondary metabolites are low molecular mass products, not essential for growth of the producing cultures, but very important for human health. They include antibiotics, antitumor agents, cholesterol-lowering drugs, and others. They have unusual structures and are usually formed during the late growth phase of the producing microorganisms. Its synthesis can be influenced greatly by manipulating the type and concentration of the nutrients formulating the culture media. Among these nutrients, the effect of the carbon sources has been the subject of continuous studies for both, industry and research groups. Different mechanisms have been described in bacteria and fungi to explain the negative carbon catabolite effects on secondary metabolite production. Their knowledge and manipulation have been useful either for setting fermentation conditions or for strain improvement. During the last years, important advances have been reported on these mechanisms at the biochemical and molecular levels. The aim of the present review is to describe these advances, giving special emphasis to those reported for the genus Streptomyces.

Secondary metabolite scale-up to minimize homolog impurity levels

A mutant strain of Streptomyces hygroscopicus was found to produce up to 9.0 units/L of an immunoregulant precursor, immunomycin, with up to 3.5% of a lower homolog impurity under either dual fed-batch or batch conditions. Glycerol and valine were key nutrients influencing productivity and impurity levels. Soybean oil was successfully substituted for glycerol as a carbon source to minimize shot additions to batch culture. The remainder of the production medium was composed largely of defined components with the exception of yeast extract. Valine limitation increased lower homolog formation while decreasing higher homolog formation; excess valine decreased lower homolog formation below 2-3% while increasing higher homolog formation. Higher homolog formation in the presence of valine seemed to be slower than lower homolog formation in the absence of valine. Valine was believed to be the major butyrate precursor; consequently its availability influenced the impurity profile. A preliminary cost analysis suggests that elimination of added valine from the cultivation and replacement of glycerol with soybean oil can result in a 6.6-fold reduction in media costs relative to the original fed-batch process. Copyright 1998 John Wiley & Sons, Inc.

Molecular regulation of beta-lactam biosynthesis in filamentous fungi

The most commonly used beta-lactam antibiotics for the therapy of infectious diseases are penicillin and cephalosporin. Penicillin is produced as an end product by some fungi, most notably by Aspergillus (Emericella) nidulans and Penicillium chrysogenum. Cephalosporins are synthesized by both bacteria and fungi, e.g., by the fungus Acremonium chrysogenum (Cephalosporium acremonium). The biosynthetic pathways leading to both secondary metabolites start from the same three amino acid precursors and have the first two enzymatic reactions in common. Penicillin biosynthesis is catalyzed by three enzymes encoded by acvA (pcbAB), ipnA (pcbC), and aatA (penDE). The genes are organized into a cluster. In A. chrysogenum, in addition to acvA and ipnA, a second cluster contains the genes encoding enzymes that catalyze the reactions of the later steps of the cephalosporin pathway (cefEF and cefG). Within the last few years, several studies have indicated that the fungal beta-lactam biosynthesis genes are controlled by a complex regulatory network, e. g., by the ambient pH, carbon source, and amino acids. A comparison with the regulatory mechanisms (regulatory proteins and DNA elements) involved in the regulation of genes of primary metabolism in lower eukaryotes is thus of great interest. This has already led to the elucidation of new regulatory mechanisms. Furthermore, such investigations have contributed to the elucidation of signals leading to the production of beta-lactams and their physiological meaning for the producing fungi, and they can be expected to have a major impact on rational strain improvement programs. The knowledge of biosynthesis genes has already been used to produce new compounds.

Induction of L-lysine epsilon-aminotransferase by L-lysine in Streptomyces clavuligerus, producer of cephalosporins

L-Lysine epsilon-aminotransferase (LAT) catalyzes the first reaction in the two-step conversion of L-lysine (Lys) to 1-alpha-aminoadipic acid (Aaa), a direct precursor of cephalosporins (including cephamycin C) in Streptomyces clavuligerus. Previous work showed that addition of Lys to chemically defined medium improved antibiotic production. We show that in S. clavuligerus cultures supplemented with high concentrations of Lys, Lys enhances antibiotic production by a dual effect, i.e. as a substrate of LAT thus providing Aaa and also as an inducer of LAT yielding even more Aaa. On the other hand, LAT is not induced by Aaa.

Analysis of the regulation of the Aspergillus nidulans penicillin biosynthesis gene aat (penDE), which encodes acyl coenzyme A:6-aminopenicillanic acid acyltransferase

The regulation of the Aspergillus nidulans penicillin biosynthesis gene aat (penDE), which encodes acyl coenzyme A:6-aminopenicillanic acid acyltransferase (AAT), was analysed. Major transcriptional start sites map within 100 nucleotides upstream from the aat initiation codon. To study the regulation of aat expression, various aat-lacZ gene fusions were constructed, in which the aat promoter region was fused in frame with the Escherichia coli lacZ reporter gene. A. nidulans strains carrying recombinant plasmids integrated as single copies at the chromosomal argB locus were identified. In both fermentation and minimal media, aat-lacZ expression was maximal during the first 24 h of a fermentation run. Compared with minimal medium, aat-lacZ expression was increased two-fold in fermentation medium. Although AAT specific activity was reduced in mycelia grown on glucose instead of lactose, expression of aat-lacZ gene fusions was not repressed on glucose, suggesting that the glucose effect is mediated posttranscriptionally. The effect of glucose on AAT activity was reversed by further incubation of glucose-grown mycelia on lactose. Neither the inclusion of the first intron of the aat gene in the aat-lacZ fusion integrated at the chromosomal argB locus, nor the disruption of the acvA gene had any regulatory effect on aat-lacZ expression. In the heterologous, nonpenicillin producer A. niger, basal expression of aat-lacZ gene fusions was observed at about the same level as in A. nidulans.

Production kinetics and stability properties of delta(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase from Streptomyces clavuligerus

Delta-(L-alpha-Aminoadipyl)-L-cysteinyl-D-valine (ACV)-synthetase is a key enzyme that channels primary metabolites to a tripeptide common to cephalosporin and cephamycin biosynthesis in Streptomyces clavuligerus. Time-course studies indicated that the S. clavuligerus ACV-synthetase was stable during the cephamycin C fermentation: the enzyme was produced early in the growth phase and its activity remained high up to 96 h of growth. The detection of crude ACV-synthetase activity in older cultures was best achieved with an assay medium supplemented with 5 mM phosphoenolpyruvate, at lower ATP concentrations. During storage at 4 degrees C, a progressive decrease in the stability of crude ACV-synthetase was observed with increasing culture age. Although a proteinolytic activity with a pH optimum at 8.2 was detected in crude cell-free extracts, no significant variation was observed in its activity with increasing culture age to account for the instability of ACV-synthetase in vitro. Addition of proteinase inhibitors did not improve the stability of the enzyme. However, a stabilization cocktail containing dithiothreitol, MgCl2, the three substrate amino acids, and glycerol increased the stability of the enzyme isolated from cultures grown for 30-40 h, which was shortly after the appearance of antibiotics in the culture fluid. This stabilized enzyme retained half of its initial activity after 6 days at 4 degrees C.