Isochorismic acid

The quinone ring is derived from isochorismic acid, formed by isomerization of chorismic acid, an intermediate in the shikirnic acid pathway for synthesis of the aromatic amino acids. The first intermediate unique to menaquinone formation is o-succinyl benzoate, which is formed by a thiamin pyrophosphate-dependent condensation between 2-oxoglutarate and chorismic acid. The reaction catalyzed by o-succinylbenzoate synthetase is a complex one, involving initially the formation of the succinic semialdehyde-thiamin diphosphate complex by decarboxylation of 2-oxoglutarate, then addition of the succinyl moiety to isochorismate, followed by removal of the pyruvoyl side chain and the hydroxyl group of isochorismate. 

The first intermediate in the biosynthesis of 2,3-DHBA in K. pneumoniae is isochorismic acid. The conversion of chorismic acid to isochorismic acid requires Mg ... 

Experiments on the rate of 2,3-DHBA formation from chorismic acid and the observation that chorismic acid, in the absence of NAD was converted to a compound which could serve as a substrate for 2,3-DHBA formation, indicated that at least two steps were concerned in the conversion of chorismic acid to 2,3-DHBA [84,86]. The first intermediate is isochorismic acid, which is converted to the second intermediate... 

Young, Batterham and Gibson [87] firmly established isochorismate as an intermediate in 2,3-DHBA biosynthesis. Isochorismic acid is an unstable compound and at room temperature, in aqueous solution at pH 7, it decomposes readily to a mixture of SA and... 

Enzymes and genes involved in the 2,3-DHBA biosynthesis When in 1968 the intermediates of 2,3-DHBA biosynthesis were identified [85], it became clear that isochorismate synthase (isochorismate hydroxy mutase) is the enzyme that converts chorismic acid to isochorismic acid. 2,3-Dihydro-2,3-DHBA synthase (or isochorismatase) converts isochorismic acid to 2,3-dihydro-2,3-DHBA and finally... 

The biosynthesis of menaquinones in E. coli (Fig. 5) starts with the conversion of isochorismic acid and a-ketoglutaric acid in the presence of thiamine pyrophosphate [105-107] to 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid (SHCHC) catalyzed by SHCHC synthase, and a-ketoglutarate decarboxylase, both encoded by the menD gene [104,108,109]. SHCHC is converted to o-succinylbenzoic acid by dehydration, catalyzed by a protein encoded by menC (Table 1) [110]. Palaniappan et al., [111] showed for the first time the biosynthesis of menaquinones via o-succinyl benzoic acid in B. subtilis including the activity of the enzymes. 

In contrast to E. coli, Flavobacterium [112] is unable to grow anaerobically and does not seem to produce a catechol siderophore from isochorismic acid. Regulation of isochorismic acid synthesis is therefore likely to be completely different. The relative high K , value of isochorismate synthase for chorismic acid, compared to E. coli, may prevent drainage of substrates into isochorismic acid-utilizing reactions. This may indicate that isochorismate synthesis is controlled not only at the level of transcription, as is the case in E. coli, but also at enzyme level [112]. 

The biosynthesis starts from isochorismic acid, 2-oxoglutaric acid, and mevalonic acid lactone, see an-thranoids. 

CjiHioOs, Mr 222.20, mp. 135-137 °C. An intermediate in the biosynthesis of 1,4-dihydroxynaphthalene-2-carboxylic acid, which in turn is a precursor of alizarin(e), menaquinone (vitamin K3), and vitamin K] (phylloquinone). The biosynthesis proceeds from isochorismic acid and 2-oxoglutaric acid. 

Anthraquinones derived from shikimic and isochorismic acid are known to occur only in certain dicotyledonous families Bignoniaceae, Gesneriaceae, Rubiaceae, Scrophulariaceae, and Verbenaceae (Gentianales, Lamiales, and Scro-phulariales). These families also characteristically accumulate iridoid monoterpenes (Dahlgren et al., 1981). 

In some fungi, salicylic acid (65) and related compounds, such as 6-methylsalicylic acid, are derived from acetate-ma-lonate pathways. In bacterial systems, similar compounds are derived from chorismic acid via isochorismic acid see Chapter 7). Salicylic acid often is found in species of Salix... 

Isochorismic acid, salicylic acid, p-hydroxybenzoic acid, anthranilic acid, p-aminobenzoic acid ... 

D 8.2 Hydroxy and Aminobenzoic Acids Formed from Chorismic Acid and Isochorismic Acid... 

With the exception of isochorismic acid the above mentioned acids are aromatic compounds, derived from benzoic acid by an additional hydroxy or amino group in o- or p-position. 

Chorismic acid is transformed to p-hydroxybenzoic acid by loss of the enol-pyruvate side chain. Salicylic acid may be synthesized in bacteria after rearrangement to isochorismic acid by a similar reaction. (Formation of salicylic acid from o-coumaric acid in plants, fungi and animals, cf. D 22.2.5.) The amino groups of anthranilic acid and of p-aminobenzoic acid originate from glutamine. Intermediates in the transformation of chorismis acid to these acids are unknown and probably do not occur in the free state. 

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