Chorismate mutase, aromatic amino acid

The Shikimate pathway is responsible for biosynthesis of aromatic amino acids in bacteria, fungi and plants [28], and the absence of this pathway in mammals makes it an interesting target for designing novel antibiotics, fungicides and herbicides. After the production of chorismate the pathway branches and, via specific internal pathways, the chorismate intermediate is converted to the three aromatic amino acids, in addition to a number of other aromatic compounds [29], The enzyme chorismate mutase (CM) is a key enzyme responsible for the Claisen rearrangement of chorismate to prephenate (Scheme 1-1), the first step in the branch that ultimately leads to production of tyrosine and phenylalanine. 

Schultz and coworkers (Jackson et a ., 1988) have generated an antibody which exhibits behaviour similar to the enzyme chorismate mutase. The enzyme catalyses the conversion of chorismate [49] to prephenate [50] as part of the shikimate pathway for the biosynthesis of aromatic amino acids in plants and micro-organisms (Haslam, 1974 Dixon and Webb, 1979). It is unusual for an enzyme in that it does not seem to employ acid-base chemistry, nucleophilic or electrophilic catalysis, metal ions, or redox chemistry. Rather, it binds the substrate and forces it into the appropriate conformation for reaction and stabilizes the transition state, without using distinct catalytic groups. 

The utilization of evolutionary strategies in the laboratory can be illustrated with proteins that catalyze simple metabolic reactions. One of the simplest such reactions is the conversion of chorismate to prephenate (Fig. 3.3), a [3,3]-sigmatropic rearrangement. This transformation is a key step in the shikimate pathway leading to aromatic amino acids in plants and lower organisms [28, 29]. It is accelerated more than a million-fold by enzymes called chorismate mutases [30],... 

Chorismate mutase (CM) catalyzes the Claisen rearrangement of chorismate to prephenate in the shikimic acid pathway used in the biosynthesis of aromatic amino acids. It represents a reference enzyme to explore the fundamentals of catalysis and has been the subject of extensive experimental and computational research. These have shown both that catalysis proceeds without covalent binding of the substrate to the enzyme, and that the uncatalyzed reaction in water proceeds by the same mechanism. This makes CM a particularly convenient target for QM/MM studies. 

Figure 4. Enzymes of Rhizobium (a) and Lemna (b) proposed as sites of glyphosate inhibition of aromatic amino acid synthesis. Abbreviations CM, chorismate mutase PDH, prephenate dehydrogenase and PD, prephenate dehydratase.

Aromatic amino acid feedback sensitive forms of chorismate mutase have been identified in alfalfa, pea, oak, mung bean, rice, and tissue cultures of tobacco and tomato (Cotton and Gibson, 1968 Gilchrist et al., 1972 Woodin and Nishioka, 1973a Gadal and Bouysson, 1973 Widholm, 1974). In almost all of these plants multiple forms of the enzyme occur, some of which are... 

Fig. 6. Regulation of the synthesis of aromatic amino acids in plants. Each of the sequential arrows represents the enzyme catalyzed steps detected in Figs. 2, 3 and 4. Bold lines indicate inhibition of chorismate mutase by phenylalanine and tyrosine in addition to anthranilate synthase inhibition by tryptophan. The dashed arrow symbolizes the ability of tryptophan to both activate chorismate mutase and antagonize inhibition of this enzyme by either phenylalanine or tyrosine.

In higher plants, phenylalanine seems to be formed in an alternative manner by formation of prephenate (26) and conversion of this intermediate into arogenic acid (32). Chorismate mutase occurs as two isozymes which have been purified to homogeneity from mung bean and sorghum. All chorismate mutase isozymes show allosteric activation by chorismate (Poulsen and Verpoorte, 1991). One chorismate mutase isozyme is inhibited by phenylalanine or tyrosine and activated by tryprophan, whereas the second is not affected by any of the aromatic amino acids. 

Chorismic acid is the key branch point intermediate in the biosynthesis of aromatic amino acids in microorganisms and plants (Scheme 1.1a) [1]. In the branch that leads to the production of tyrosine and phenylalanine, chorismate mutase (CM, chorismate-pyruvate mutase, EC 5.4.99.5) is a key enzyme that catalyzes the isomerization of chorismate to prephenate (Scheme 1.1b) with a rate enhancement of about lO -lO -fold. This reaction is one of few pericyclic processes in biology and provides a rare opportunity for understanding how Nature promotes such unusual transformations. The biological importance of the conversion from chorismate to prephenate and the synthetic value of the Claisen rearrangement have led to extensive experimental investigations [2-43]. 

Fig. 9. Sequential pattern of allosteric control over biosynthesis of aromatic amino acids in the plastid compartment. In the presence of excess aromatic amino acids, L-tyrosine (TYR) inhibits arogenate dehydrogenase, L-phenylalanine (PHE) inhibits arogenate dehydratase and L-tryptophan (TRP) inhibits anthranilate synthase. The three aromatic amino acids exert allosteric inhibition (-) or activation (+) effects upon chorismate mutase-1 as symbolized. However, activation dominates over inhibition. The outcome of these events is to trap L-arogenate (AGN) between the various foci of control in the pathway. As shown symbolically, -arogenate (AGN) then acts to feedback inhibit DAHP synthase-Mn.

The rearrangement of 122 to 123 is a key reaction along the shikimate biosynthetic pathway for generating aromatic amino acids in plant, fungal, and bacterial systems, and it is catalyzed by the enzyme chorismate mutase more than a millionfold. This has stimulated an in-depth investigation of the mechanism of the Claisen rearrangement. ... 

Schneider CZ, Parish T, Basso LA, Santos DS (2008) The two chorismate mutases from both Mycobacterium tuberculosis and Mycobacterium smegmatis biochemical analysis and limited regulation of promoter activity by aromatic amino acids. J Bacteriol 190(1) 122-134. Epub 2007 Oct 26... 

---