Penicillic acid biosynthesis

Penicillic acid (82 in Figure 17) is a toxic compound produced by many Penicillum and some Aspergillus species.166 It is biosynthesized from orsellinic acid through the pathway shown in Figure 17.167 A candidate of a gene encoding a PKS for penicillic acid biosynthesis was obtained from A. ochraceus 6t... 

Axberg,. K., and S. Gatenbeck Intermediates in the Penicillic Acid Biosynthesis in Penicillium cyclopium. Acta. Chem. Scand. B29, 749 (1975). 

The finding of phyllostine as a precursor to patulin induces the prediction of a similar intermediate in penicillic acid biosynthesis (Fig. 42). Therefore, I would predict that a plausible intermediate after the 3-methoxytoluquinone step would be the epoxyquinone 67 (Fig. 42). Specific reduction at C(6) catalyzed by a dehydrogenase and NADPH would rearrange the epoxide 68 (Fig. 42). Hydrolysis of 68 will yield the seven-membered ring lactone, 69. This lactone (69) (Fig. 42) differs from the one proposed (22) (Fig. 14) as it... 

Fig. 42. Postulated mechanism for the steps beyond 2-methoxy-6-methylbenzoquinone in penicillic acid biosynthesis.

Two interesting metabolites of Penicillium patulum are patulin and penicillic acid. Their biosynthesis involves the cleavage of an aromatic ring. These substances are mycotoxins and their activity in this context is discussed in Chapter 9. 

The salt-water culture of Aspergillus ochraceus separated from the Indo-Pacific sponge Jaspis coriacea has yielded two new chlorine containing polyketides, chlorocarolide A (183) and B (184). These compounds have an overall structural analogy to penicilic acid whose biosynthesis has been intensely studied. The structures and stereochemical features of the chlorocarolides were reported [214]. 

Zamir, L. O., The biosynthesis of patulin and penicillic acid, in The Biosynthesis of Mycotoxins (P. Steyn, ed.), 223-268, Academic Press, New York, 1980. 

The fungal metabolites patulin (10) and multicolic acid (79) are derived in nature via oxidative cleavages of polyketide derived aromatic intermediates a similar pathway has been established for the tetronic acid penicillic acid (185). By contrast, the biosynthesis of carolic acid (76a) which is found with dehydrocarolic acid (75) in P. charlesii, has been shown to occur from Krebs cycle intermediates. 

The biosynthesis of the fungal tetronic acids penicillic acid (185) and carolic acid (76 a), which are closely related structurally to patulin and multicolic acid, have also been studied extensively. Early work with " C-labelled precursors has clearly demonstrated the polyketide origin of penicillic acid, and also the intermediacy of orsellinic acid... 

The biosynthesis of the fungal tetronic acid carolic acid (76 a) in Penicillium charlesii is markedly different from that of patulin, multicolic acid and penicillic acid. Carbon-14 work has demonstrated that the carbon sub-unit C-3, C-4, C-9 in the acid is derived from a C-4-dicarboxylic acid such as succinate, whereas the remaining six carbons (C-1, C-2, C-5, C-6, C-7, C-8) are derived from two malonate units (C-5, C-6, C-7, C-8) and just one acetate unit (C-1, C-2) (Scheme 19) (775). It is proposed then that the biosynthesis of carolic acid proceeds from a C4-dicarboxylic acid first to either y-carboxymethyltetronic acid (196) or to carlosic acid (77 b) hydroxylation of carlosic acid then gives rise to carlic acid (77 a) which on decarboxylation produces carolic acid 176). 

Bentley, R., and J. G. Keil Tetronic Acid Biosynthesis in Molds II. Formation of Penicillic Acid in Penicillium cyclopium. J. Biol. Chem. 237, 867 (1962). 

Fig. 5- Biosynthesis of penicillic acid from orsellinic acid (Mosbach, I960 Bentley...

Birch, A. J., G. E. Blance, and H, Smith Studies in relation to biosynthesis. Part. XVIII. Penicillic acid. J. Chem. Soc. 1958a, 4582. 

The technique of tritium NMR was reported by Bloxsidge et at., (1971). The first application of this method in biosynthesis involved penicillic acid (Al-Rawi et at., 91A Elvidge et at., 1977). The triton has a spin of j, and its sensitivity to detection is even higher than the proton s. Bloxsidge et at. 

Whole-cell feeding experiments imply adding labeled putative precursors (radioactive or stable isotope label) to the whole organism, which is grown on a synthetic media. The products and intermediates are then isolated, purified, and analyzed by the various methods. This type of experiment enables one to propose a plausible biosynthetic pathway. Isolation and purification of the enzymes responsible for each step of the pathway enable verification at the enzymatic level. The in vitro conversion of a putative precursor by the pure enzyme to the product constitutes unambiguous proof of the biosynthetic reaction. The ultimate proof of a biosynthetic pathway depends, therefore, on the characterization of the enzymes involved in each step of the pathway. The only major drawback is that the procedure is extremely difficult, and the detailed enzymology for polyketide-derived eompounds deserves further study. The problems are (a) instability of the enzymes, (b) lack of reproducibility, and (c) variability of activity in different enzymatic preparations (some of these enzymes are membrane bound and easily deactivated by isolation). Despite the difficulties involved, some of the enzymes involved in the biosynthesis of patulin and penicillic acid have been partially purified and characterized. 

The biosynthesis of patulin, as well as that of penicillic acid, can be summarized as acetate aromatic compound -> product. In this section we will concentrate on the first step the origin of the aromatic precursor. 

Early studies (Birch et al., 1958) of the biosynthesis of penicillic acid in P. cyclopium with [2-acetate have postulated the intermediacy of orsellinic acid (17). Orsellinic acid is the classic example of an aromatic compound produced by the polyketide pathway. Indeed, it is formed by aldol condensation of a tetraacetyl unit with no further modifications (Fig. 19). 

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