Shikimic acid synthetase

Several studies of the biosynthesis of chloramphenicol have led to the conclusion that it is formed via the shikimic acid pathway, specifically from chorismic acid. An arylamine synthetase promotes formation of p-amino-L-phenyl alanine (1 ) 50>51. This product is converted to chloramphenicol (15) by oxidation of the amine function to a nitro group, by hy-droxylation of the benzylic methylene group, reduction of the carboxyl... 

A highly specialized synthesis converted (-)-shikimic acid 7.169) to 7.170. Reaction with azide and catalytic hydrogenation gave 5-aminoshikimate 7.171, methyl 5-amino-3,4-dihydroxy-l-cyclohexene carboxylate) in 21% overall yield. Amino acid 7.171 is an inhibitor of EPSP synthetase, and can be used as an herbicide,... 

Lack of Repression of the Shildmic Acid Pathway.—It is well established that a control mechanism on the shikimic acid pathway is by repression of enzymes early in the pathway by the end products, e.g. by phenylalanine. Cases where this type of control appear to have broken down have now been reported. Lowe and Westlake, in studying the biosynthesis of the phenolic antibiotic chloramphenicol in Streptomyces sp. 3022a, examined several enzymes of the pathway, notably chorismate mutase and anthranilate synthetase, but could find no evidence that end-product control on chloramphenicol synthesis was operating in this organism. Similarly, Chu and Widholm fed phenylalanine and tyrosine to a range of tissue cultures of higher plants, but were not able to observe any evidence of feedback control on chorismate mutase levels. 

The 3-deoxy-D-ara6mo-2-heptulosonic acid 7-phosphate (DAHP) synthetase (EC 4.1.2.15) is an enzyme involved in the shikimic pathway of aromatic amino acids biosynthesis in bacteria and plants, where catalyzes the construction of 3-deoxy-D-ara6/ o-2-heptulosonic acid 7-phosphate from phosphoenolpyruvate and D-erythrose 4-phosphate [6]. Although 3-deoxy-D-ara6/H0-2-heptulosonic acid 7-phosphate (DAHP) synthetase has not been widely investigated it has been employed for the DAHP synthesis on preparative scale from D-fructose in multienzyme system [68], This one-pot synthesis was subsequently even more simplified by the results of further studies which indicated that it was more efficient and economical to use the whole cells containing a DAHP synthetase plasmid [69]. 

In B. subtilis the single DAHPS enzyme is repressed most strongly by tyrosine, but maximal repression required the presence of both tyrosine and phenylalanine [121]. Tryptophan appeared to play no role in repression eontrol of synthesis of the enzyme. Nester et al. [121] also reported that a mixture of the three aromatic amino acids repressed synthesis of DAHPS, dehydroquinate synthetase, and shikimate kinase noncoordinately, but had no effect on the level of dehydroquinase. 

The carbon flow from 3-phosphoglycerate, phosphoenolpyruvate, pyruvate and acetyl-CoA. Even if the synthesis of aromatic amino acids by shikimate pathway /28,29,30,31/ and also prenyl-PP synthesis via mevalonate /32,33,34/ has been established in chloroplasts by identification of respective plastidic enzymes, it is still a matter of discussion from where PEP origins to supply DAHP synthesis of the shikimate pathway and from where pyruvate is delivered to supply the plastidic pyruvate dehydrogenase complex (for isolation see Treede and Heise, this Conference). Because phosphoglycerate mutase (PGM) to form 2-PGA from 3-PGA could not be detected in chloroplasts /35/ and acetyl-CoA is preferably synthesized from added acetate by the actetyl-CoA synthetase /36/, particularly in spinach chloroplasts, it was argued that chloroplasts are dependent on import of these substrates from the external site. Evidence for PEP formation from 3-PGA within the chloroplast could be obtained by three different approaches (D. Schulze-Siebert, A. Heintze and G. Schultz, in preparation D. Schulze-Siebert and G. Schultz, in preparation, for plastidic isoenzyme of PGM in Ricinus see /37/ and in Brassica /38/). 

Salicylic Acid.—It has recently been demonstrated (see last year s Report ) that whereas 6-methylsalicylic acid in both micro-organisms and higher plants is acetate-derived, salicylic add itself is formed in both groups of organisms by the shikimic add pathway. Further confirmation of this has come from Marshall and Ratledge, who have studied the enzymology of biosynthesis of salicylic acid in Mycobacterium smegmatis. These authors isolated the enzyme salicylate synthetase, which catalyses the last step (see Scheme 1) in synthesis of salicylic acid (1) from isochorismic add (2), formed in turn from chorismic (3) and shikimic adds (4). The enzyme has no cofactor requirements and converts (2) directly into (1). No evidence could be obtained for the presence in bacterial cultures of the possible intermediate 2,3-dihydroxy-2,3-dihydrobenzoic add. 

In considering the commercially successful herbicides known to be inhibitors of amino acid biosynthesis, three aspects will be considered the kinetic description of the inhibition, the molecular interactions involved, and the relevance of the proposed target site to the observed physiological effects. Because of the extensive body of research published on the shikimate pathway and the herbicide glyphosate, this example will be taken as a paradigm of the inhibitor-enzyme relationship. The other cases considered will be the branched-chain amino acid family, histidine biosynthesis, and glutamine synthetase. 

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