Methylsalicylate synthase

Methylsalicylate synthase (D 3.3.1), for instance, shares most of the enzyme activities of the fatty acid synthase system (D 3.2), a complex of two polyfunctional enzymes of primary metabolism, and specific antibodies show it to be a very similar protein (Packter 1973). Hence it is likely that 6-methylsalicylate synthase evolved from fatty acid synthase. 

Once the active 6-methylsalicylate synthase has been formed it is meta-bolically stable, and persists independent of new protein synthesis. In this it differs from some of the later enzymes on the route to patulin. These are only formed under even more stringent growth restriction. In addition, being metabolically labile (i.e., subject to turnover), they disappear quite quickly either when protein synthesis is stopped (with inhibitors) or when fresh nutrients are made available. As a result, such cultures accumulate 6-methylsalicylate. The regulation of these later enzymes is a clear instance of control by nutrient limitation, and some of them may also be controlled by specific induction mechanisms. In well-phased laboratory cultures they are formed in a regular succession (Bu Lock et ai, 1969 Forrester and Gaucher, 1972 Murphy et ai, 1974). 

SPENCER, J.B., JORDAN, P.M., Purification and properties of 6-methylsalicylic acid synthase from Penicilliumpatulum, Biochem. J., 1992,288,839-846. 

Cell-free systems capable of in vitro synthesis of 6-methylsalicylic acid (6-MS A) and a related tetraketide, orsellinic acid, were developed long before the advent of recombinant DNA technologies in the field of natural product biosynthesis [113-115] (Fig. 5). Since then, the biosynthetic mechanisms and molecular recognition features of 6-methylsalicylic acid synthase (6-MSAS) have been extensively studied. 6-MSAS initiates synthesis with an acetyl group derived from acetyl Co A, extends the polyketide chain to a tetraketide via three decar-boxylative condensations of malonyl CoA-derived extender units, and uses NADPH to specifically reduce one of resulting carbonyls to a hydroxyl group. In its natural producer, Penicillium patulum, the product, 6-MSA is subsequently glycosylated to form the antibiotic patulin [116]. 

Dimroth P, Greull G, Seyfferet R, Lynen F (1972) Hoppe Syler s Z Physiol Chem 353 126 Dimroth P,Ringelmann E, Lynen F (1976) Eur J Biochem 68 591 Bhogal P, Shoolingin-Jordan P. Incorporation of novel substrates into 6-methylsalicylic acid and triacetic acid lactone analogues by 6-methyl salicylic acid synthase. In Polyketides chemistry, biochemistry, molecular genetics. 1996. Royal Society of Chemistry, Bristol, UK, p 19... 

Beck J, Ripka S, Siegner A, Schiltz E, Schweizer. The multifunctional 6-methylsalicylic acid synthase gene of Penicillium patulum. Its gene structure relative to that of other polyketide synthases. Eur. J. Biochem. 1990 192 487-498. 

Child CJ, Spencer JB, Bhogal P, Shoolingin-Jordan PM. Structural similarities between 6-methylsalicylic acid synthase from Penicillium patulum and vertebrate type 1 fatty acid synthase evidence from thiol modification studies. Biochemistry 1996 35 12267-12274. 

Figure I. Schematic representation of domain architecture in Jungal Type I PKSs based on reported gene sequences. KS. ketoacyl, AT, acyltransferase DH, def dratase KR. ketoreductase ER, enoyl reductase MT, methyltransferase CLC, Claisen-iike cyclase MSAS, methylsalicylic acid synthase THN, tetrahydronaphthalene AF, ST, ( atoxin, sterigmatocystin...

For more complex metabolites (phenols and tetraacetic acid lactone), further condensation of the triacetyl residue must take place and can only proceed if the priming acetyl residue, with its C-5 oxygen function, is stabilized away from the thioester region of the acyl-enzyme intermediate at this time. Possibly, the presence of NADPH assists in this process, in appropriate enzymes, since the ability to form triacetic acid lactone has been demonstrated by fatty acid and 6-methylsalicylate synthetases when deprived of this nucleotide, but other synthetases have not been tested for this effect. Moreover, the analogous formation of the styrylpyrone bisnoryangonin by flavan-one synthase (Kreuzaler and Hahlbrock, 1975a,b) confirms that the potential for synthesis of stable products with shorter chains does exist. 

The polyketide synthases and peptide synthetases catalyze an unusually large number of reactions. 6-Methylsalicylic acid synthase, which produces 6-methylsalicylic acid from acetyl Co A, malonyl Co A and NADPH, for instance, carries out a total of thirteen reactions. The peptide antibiotic synthetases activate each of the amino acids involved in the formation of the peptides by a two step mechanism at a specific peripheral domain of the enzyme protein and catalyze also linkage of the activated amino acids by sequential transpeptidation. In the case of gramicidin S synthetase for example, there are twenty-one different reactions carried out by two multifunctional enzyme proteins. 

Monodictyphenone has been previously isolated from a marine fungus Monodictys putredinis as well as an engineered strain of A mdidans. This strain of A. nidulans expressed the (3area lozoyensis polyketide synthase gene involved for 6-methylsalicylic acid (15) biosynthesis, and the authors could not determine whether the monodictyphenone produced in addition to 6-methydsalicylic acid was due... 

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