Isochorismate synthase

This enzyme [EC 5.4.99.6], also known as isochorismate synthase and isochorismate hydroxymutase, catalyzes the interconversion of chorismate and isochorismate. 

COMPUTER ALGORITHMS SOFTWARE ISOCHORISMATE SYNTHASE ISOCITRATE DEHYDROGENASE ISOCITRATE LYASE "ISO-COOP,"... 

Figure 3-5. Biosynthesis of salicylic acid. The enzymes involved in this pathway are (a) chorismate mutase (E.C. 5.4.99.5), (b) prephenate aminotransferase (E.C. 2.6.1.78 and E.C. 2.6.1.79), (c) arogenate dehydratase (E.C. 4.2.1.91), (d) phenylalanine ammonia lyase (E.C. 4.3.1.5), (e) presumed P-oxidation by a yet to be identified enzyme, (f) benzoic acid 2-hydroxylase, (g) isochorismate synthase (E. C. 5.4.4.2), and (h) a putative plant pyruvate lyase.

Wildermuth M. C., Dewdney, J., Wu, G., and Ausubel, F. M., 2001, Isochorismate synthase is required to synthesize salicylic acid for plant defence, Nature 414 562-565. 

Figure 5 Proposed mechanism for the isomerization of chorismate (4) to isochorismate (5) by the enzyme isochorismate synthase.

In cell cultures of Catharanthus roseus 2,3-DHBA was found to be produced after elicitation of the cells with fungal cell-wall preparations [40,41]. The production of this compound in cell cultures was paralleled by an increase in the activity of the enzyme isochorismate synthase [40]. Although direct evidence for the intermediacy of iso-chorismate is not yet reported, it seems that the isochorismate biosynthetic pathway for benzoic acid derivatives can also be found in plants. 

Isochorismate synthase, that might be involved in the biosynthesis of 2,3-DHBA, has recently been purified from Catharanthus roseus cell-suspension cultures and subsequently its gene was cloned (L. van Tegelen, P. Moreno, A. Croes, G. Wullems and R. Verpoorte, submitted for publication). Two isoforms of the enzyme were purified and characterized. Both have an apparent molecular mass of 65 kD. The Km values for chorismic acid are 558 pM and 319 p.M for isoform I and II respectively. The enzymes are not inhibited by aromatic amino acids and require Mg for enzyme activity. The isolated cDNA encodes a protein of 64 kD with a A-terminal chloroplast targeting signal. The deduced amino acid sequence shares homology with bacterial isochorismate synthases, and also with anthranilate synthases, another chorismate utilizing enzyme. 

The genes isolated from a pyoverdin mutant [82,83] showed two open reading frames (ORFs) pchB, encoding isochorismate pyruvate lyase, and pchA, encoding isochorismate synthase (Table 1) [79]. The TGA stopcodon of pchB overlaps the putative ATG startcodon of pchA and this suggests that these genes are in the same transcription unit. PchA is expressed only when pchB is transcribed and translated simultaneously. 

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... 

Besides the siderophores, the menaquinone pathway also involves isochorismate synthase (review [100]). Menaquinone (vitamin K2) is an electron carrier involved in anaerobic ATP-generating redox reactions. It also plays a role in the anaerobic biosynthesis of pyrimidines, porphyrins and succinyl CoA [101-104]. Menaquinone is detectable in E. call. The accumulation of menaquinone (MK8) is significantly stimulated in the absence of oxygen. 

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]. 

Besides the induction of some of the steps of indole alkaloid biosynthesis, it was found that the enzyme isochorismate synthase was also induced by fungal elicitors (e.g., P. aphanidermatum), at the same time 2,3-dihydroxybenzoic acid (DHBA) was produced (151,163,339). 5-Chloro-sali-cylic acid and 2,6-dichloro-isonicotinic acid enhanced the elicitor effect in... 

Fig. 145. Transformation of chorismic acid to hydroxy and aminobenzoic acids 1 Isochorismate synthase 2 anthranilate synthase...

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