Chorismate rearrangement

Under conditions in which all of (-)-chorismate rearranges, only half of the racemic substrate is converted to prephenate by 1F7 (37). The kcat value for ( )-chorismate is the same as that measured for the pure (-)-isomer, but its apparent is twice larger. Because the kcat value determined for the racemate is unchanged relative to the optically pure material, (+)-chorismate can be treated as a competitive inhibitor. From our data, the term O.SKn/Ki must be much less than 1, indicating that binding of the (+)-isomer to the antibody is at least one or two orders of magnitude weaker than that of (-)-chorismate. 

As to why the chorismate rearrangement occurs so rapidly and with a dissociative transition state was addressed by Jurayj who prepared a series chorismate esters successively stripped of substituents. In the absence of both the hydroxyl and the carboalkoxy group on the ring, the rearrangement in aqueous methanol occurred rapidly at - 15°C (Scheme 7.95). 

Now, as shown in Scheme 12.21, chorismate rearranges (chorismate mutase, EC 5.4.99.5) in what might be an internal Cope-type process (see Scheme 11.84 and the... 

Chorismate Mutase catalyzed Claisen Rearrangement- 10 rate enhancement over non-enzymatic reaction... 

Figure 30.14 Pathway for the bacterial biosynthesis of phenylalanine from chorismate, involving a Claisen rearrangement.

The enzyme chorismate mutase was found to accelerate the Claisen rearrangement of chorismic acid.147 For many years, the origin of the acceleration perplexed and intrigued chemists and biochemists. Polar... 

The differences in the rate constant for the water reaction and the catalyzed reactions reside in the mole fraction of substrate present as near attack conformers (NACs).171 These results and knowledge of the importance of transition-state stabilization in other cases support a proposal that enzymes utilize both NAC and transition-state stabilization in the mix required for the most efficient catalysis. Using a combined QM/MM Monte Carlo/free-energy perturbation (MC/FEP) method, 82%, 57%, and 1% of chorismate conformers were found to be NAC structures (NACs) in water, methanol, and the gas phase, respectively.172 The fact that the reaction occurred faster in water than in methanol was attributed to greater stabilization of the TS in water by specific interactions with first-shell solvent molecules. The Claisen rearrangements of chorismate in water and at the active site of E. coli chorismate mutase have been compared.173 It follows that the efficiency of formation of NAC (7.8 kcal/mol) at the active site provides approximately 90% of the kinetic advantage of the enzymatic reaction as compared with the water reaction. 

In this contribution, we describe work from our group in the development and application of alternatives that allow the explicit inclusion of environment effects while treating the most relevant part of the system with full quantum mechanics. The first methodology, dubbed MD/QM, was used for the study of the electronic spectrum of prephenate dianion in solution [18] and later coupled to the Effective Fragment Potential (EFP) [19] to the study of the Claisen rearrangement reaction from chorismate to prephenate catalyzed by the chorismate mutase (CM) enzyme [20]. 

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. 

Besides the obvious biological interest, chorismate mutase is important for being a rare example of an enzyme that catalyses a pericyclic reaction (the Claisen rearrangement), which also occurs in solution without the enzyme, providing a unique... 

Chorismate mutase catalyzes the Claisen rearrangement of chorismate to prephenate at a rate 106 times greater than that in solution (Fig. 5.5). This enzyme reaction has attracted the attention of computational (bio)chemists, because it is a rare example of an enzyme-catalyzed pericyclic reaction. Several research groups have studied the mechanism of this enzyme by use of QM/MM methods [76-78], It has also been studied with the effective fragment potential (EFP) method [79, 80]. In this method the chemically active part of an enzyme is treated by use of the ab initio QM method and the rest of the system (protein environment) by effective fragment potentials. These potentials account... 

The conversion of [49] into [50] involves a Claisen rearrangement. Once this was realized it was less surprising that no specific catalytic groups on the enzyme are involved. Support for the Claisen-type mechanism comes from the inhibition shown by the bicyclic dicarboxylate [51], prepared by Bartlett and Johnson (1985) as an analogue of the presumed transition state [52], This same structure [51], coupled through the hydroxyl group to a small protein, was used as a hapten to induce antibodies, one (out of eight) of which mimics the behaviour of chorismate mutase, albeit less efficiently (Table 7). 

Claisen rearrangement chorismic acid to prephenic acid... 

Figure 11 Chorismate-prephenate rearrangement catalyzed by antibody 1F7 raised against a bicyciic hapten that mimics the chair conformation of the transition state of the reaction and X-ray structure of the corresponding antibody 1 F7-hapten compiex.

The hydrophobic effects between the apolar gronps involved in the Diels-Alder reaction also occnr when the apolar gronps belong to the same molecules, and thus should also be beneficial to the Claisen rearrangement. The nonenzymatic rearrangement of chorismate to prephenate occurs 100 times faster in water than in methanol (Copley and Knowles, 1987 Grieco et al., 1989). 

Claisen rearrangement plays an important part in the biosynthesis of several natural products. For example, the chorismate ion is rearranged to the prephenate ion by the Claisen rearrangement, which is catalysed by the enzyme chorismate mutase. This prephenate ion is a key intermediate in the shikimic acid pathway for the biosynthesis of phenylalanine, tyrosine and many other biologically important natural products. 

FIGURE 22-19 Biosynthesis of phenylalanine and tyrosine from chorismate in bacteria and plants. Conversion of chorismate to prephenate is a rare biological example of a Claisen rearrangement. 

Chorismate mutase (CM) catalyzes conversion of chorismate into prephenate in a Claisen rearrangement prephenate is a precursor in the biosynthesis of both i-phenylalanine (i-Phe) and i-tyrosine (i-Ttyr). CM occurs as a dimer for structural studies, the monomer was needed (MacBeath, 1998). The assay for CM activity was based on growth of the colonies in the absence of i-Phe and L-Tyr and thus was based on selection, not screening. One resulting mutant was found to be a monomer and to contain a somewhat polar Ala-Arg-Trp-Pro-Trp-Ala sequence. 

A thoroughly investigated reaction on the biosynthetic pathway to aromatics is the [3+3]-sigmatropic Claisen rearrangement from chorismic acid to prephenic acid (Figure 18.8). 

Figure 18.8 Claisen rearrangement from chorismate to prephenate catalyzed by antibodies.

Returning to the main course of the shikimate pathway, a singular rearrangement process occurs transforming chorismic acid into prephenic acid... 

Figure 4.11). This reaction, a Claisen rearrangement, transfers the PEP-derived side-chain so that it becomes directly bonded to the carbocycle, and so builds up the basic carbon skeleton of phenylalanine and tyrosine. The reaction is catalysed in nature by the enzyme chorismate mutase, and, although it can also occur thermally, the rate increases some 106-fold in the presence of the enzyme. The enzyme achieves this by binding the pseudoaxial conformer of chorismic acid, allowing a transition state with chairlike geometry to develop. 

Competitive experiments with 2H-, 13C- and 180-labelled chorismate derivatives and three different chorismate mutase enzymes have shown that in all catalysed and non-catalysed Claisen rearrangements a non-synchronous, concerted, pericyclic transition state is involved, with C-O bond cleavage considerably in advance of C-C bond formation. Some evidence has suggested that the ionic active site of the enzymes may polarize the transition state more than occurs in solution. Similar findings apply to the retro-ene fragmentation of chorismate to 4-hydroxybenzoate.17... 

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