Biosynthetic pathways penicillin

Valine A branched-chain essential amino add that has stimulant activity. It promotes muscle growth and tissue repair. It is a precursor in the penicillin biosynthetic pathway. [NIH]... 

A MacCabe, H van Liempt, H Palissa, SE Unkles, MBR Riach, E Pfeifer, H von Dohren, J Kinghom. Molecular characterization of the Aspergillus nidulans acvA gene encoding 5-(L-a-aminoadipyl)-L-cysteinyl-D-valine synthetase, the first enzyme of the penicillin biosynthetic pathway. J Biol Chem 266 12646-12654,1991. 

J Nielsen, HS Jorgensen. Metabolic control analysis of the penicillin biosynthetic pathway in a high-yielding strain ofPenicillium chrysogenum. Biotechnol Prog 11 299-305, 1995. 

Penicillin Formation by Penicillium Chrysogenum. The first reactions of the penicillin biosynthetic pathway are identical to the ones in A. chrysogenum (Figure 1.1-1). IPN, however, is not epimerized to penicillin N instead it is converted to 6-aminopenicillanic acid (6-APA) by removal of the L-a-aminoadipic acid side chain, which is substituted by a hydrophobic acyl group. Both steps are catalyzed by the same enzyme, the acyl coenzyme A IPN acyltransferase (IAT). The enzymatic activity of lAT is believed to be the result of the processing of a 40-kD monomeric precursor into a dimeric form consisting of two subunits with MWs of 11 and 29 kD. Due to the broad substrate specifity of lAT, various penicillin derivatives are synthesized naturally by attachment of different acyl-CoA derivatives to the 6-APA-core. For industrial purposes, to facilitate extraction by organic solvents, synthesis usually is directed to the less hydrophilic penicillin V or penicillin G. This is by addition of phenoxyacetic acid or phenylacetic acid, respectively, as precursors to the culture broth. 

Figure 10.4 The penicillin biosynthetic pathway. AcvA, IpnA, and AatA indicate ACV synthase, IPN synthase, and acyl-CoA IPN acyltransferase, respectively. R-COOH represents a large variety of aliphatic and aromatic acid side chains, such as phenylacetic (Penicillin G), phenoxyacetic (V), octanoic (K), hexenoic (DF), and A3-hexenoic (F) acids.

Whereas the elasticity coefficients are properties of the individual enzymes, the FCCs are properties of the system. The FCCs are therefore not fixed but change with the environmental conditions, as illustrated in Fig. 9, which summarize results from analysis of the flux control in the penicillin biosynthetic pathway. The penicillin biosynthetic pathway consists of three enzymatic steps. In the... 

FIGURE 9 MCA of the penicillin biosynthetic pathway. Based on a kinetic model for the enzymes, in this pathway the FCCs were calculated at different stages of fed-batch cultivations. During the first part of the cultivation the flux control was mainly exerted by the first step in the pathway, i.e., the formation of the tripeptide LLD-ACV by ACV synthetase (ACVS), whereas later in the cultivation flux control shifted to the second step in the pathway, i.e., the conversion of LLD-ACV to isopenicillin N by isopenicillin N synthetase (IPNS). This shift in flux control is due to intracellular accumulation of LLD-ACV, which is an inhibitor of ACVS. The initial high isopenicillin N concentration is due to the fact that this sample was taken from the inoculum culture where the side-chain precursor phenoxyacetic acid (POA) was not present in the medium. [The data are taken from Nielsen, J., and Jorgensen, H. S. (1995). Biotechnol. Prog. 11,299-305.]... 

Penicillins and cephalosporins are products of biosynthetic pathways that have many identical enzymatic steps. It is generally accepted that the tripeptide, 8-(L-a-aminoadipyl)-L-cysteinyl-D-valine (LLD-ACV), is the direct precursor to both penicillin and cephalosporin C. 

Figure 6.10 Biosynthetic pathways from isopenicillin N to penicillin G and cephalosporin C. Some strains have the ability to convert deacetylcephalosporin C into cephamycin C.

By the addition of different acyl donors to the medium, different penieillins can be biologically synthesized. For example, penicillin V is made by a similar process to benzylpenieillin, but with phenoxyacetic add as the precursor instead of PAA. In the biosynthetic pathway, the a-aminoadipyl side-chain of isopeniciUin N is replaced by a phenoxyacetyl group. 

In addition to penicillin several other antibiotics (phosphonomycin, bacitracin, and vancomycin) block cell wall synthesis at different locations (see figs. 16.16 and 16.17). In addition to their biological and medical importance, these antibiotics have been very useful in elucidating the biosynthetic pathway. This is because they cause accumulation of the intermediate before the blocked step. This species can frequently be isolated and confirmed as a genuine intermediate in the pathway. 

Figure 1 Biosynthetic pathway for cephalosporin C in C. acremonium, penicillin G in P. chrysogenum, and cephamycin C in S. clavuligerus. Gene designations are shown to the left of the arrows and enzyme designations are shown to the right.

We will begin by giving a brief overview of foe strategies that may be employed to produce desirable antibiotics. Then we will give a brief review of the history of the production of penicillin. We will foen examine foe mode of action of P-lactam antibiotics and briefly describe foe biosynthetic pathways of P-lactam antibiotic production. Subsequently we will examine, in greater depth, the biotransformation of penicillins. A consideration of cephalsporin production will follow and will be ccMnpared with foe production and diversification of penicillins. In foe final part of this chapter we will briefly describe foe new P-lactams. 

The biosynthesis of penicillins has been extensively studied, and the general biosynthetic pathway from a-aminoadipic acid (33), cysteine (34) and valine (35) is shown in Scheme 7. Enzymic removal of the side chain of the product (36) affords 6-APA which may then be chemically modified to form the other semisynthetic penicillins in Figure 5. 

Finally, there are a mixed bag of oxidases, catalysing ethylene formation in plants and many other diverse reactions, illustrated in Figure 13.20, by isopenicillin N-synthase, IPNS, which catalyses the cyclisation of the heterocyclic P-lactam ring. The importance of penicillin- and cephalosporin-related antibiotics in clinical medicine cannot be underestimated and has stimulated the study of their biosynthetic pathways. A key step in the biosynthesis of these antibiotics involves oxidative ring closure reactions of S-(L-a-aminoadipoyl)-L-cysteinyl-D-valine (ACV) to form isopenicillin N, the precursor of penicillins and cephalosporins, catalysed by IPNS (Figure 13.20). The overall reaction utilizes the full oxidative potential of O2, reducing it to two molecules of H2O. As discussed earlier, these enzymes are technically oxidases and the four electrons required for dioxygen reduction come from the substrate. 

Characterization of Biosynthetic Pathways. The productivity of Penicillium chrysogenum could have been augmented impressively during the last decades Penicillin titers have been increased by a factor of 50,000 from a very few milligrams/ Liter in the 1940s to more than several lOg/L by now and also some several lOg/L cephalosporin are currently gained. 

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