Enzyme inhibitor chorismate mutase

The transition state for the enzymatic reaction has been shown to have a chairlike geometry as well [61], and conformationally constrained compounds that mimic this structure, such as the oxabicyclic dicarboxylic acid 1 (Fig. 3.6), are good inhibitors of chorismate mutase enzymes [62 - 64], How a protein might stabilize this high-energy species has been a matter of some debate. Recently, heavy atom isotope effects were used to characterize the structure of the transition state bound to BsCM [65]. A very... 

COMT is, for many of the same reasons as with chorismate mutase, well suited for the study with computational techniques. The reaction mechanism it catalyzes is the same mechanism that operates in the absence of the enzyme, specifically, the S 2 mechanism, facilitating comparison of the bare solution-phase reaction with the catalyzed reaction. The subsfiate and cofactor do not covalendy bind to the enzyme, so that defining the QM region and the MM region should be relatively uncomplicated. Lasdy, the X-ray crystal structure of COMT bound with the inhibitor 3,5-dinitrocatechol has been determined with a resolution of 2 kP An interesting twist to this enzyme is that the active site includes a metal cation, Mg " ". This crystal structure allows for a natural starting point for computational exploration of the means of the catalytic action of COMT. The rate acceleration provided by COMT is substantial the reaction is 10 times faster within the enzyme than in solution. " ... 

Although chorismate mutase does provide a rate enhancement of 2 X 10 (147), this uni-molecular reaction readily occurs without enzyme, under mild conditions. The reaction was expected to pass through a chairlike transition state (59)(Fig. 17.25) but early molecular orbital calculations indicated that the boatlike transition state (60) was not out of the question (147). In an attempt to define the transition-state structure, several compounds, each designed to mimic a putative transition state, were synthesized and tested as chorismate mutase inhibitors (147). The enzyme was found to be inhibited by the exo-carboxy nonane (61), with an apparent value of 3.9 X 10 M Conversely, the endo-carboxy nonane (62) did not inhibit the enzyme. The apparent K- value of the adaman-... 

It has been shown that both the catalyzed and the uncatalyzed reaction proceed through a chairlike transition state, stabilized in polar media59 60-143 262-263. Compound 1, an analog of the transition-state structure, proved to be a potent inhibitor of E. coli chorismate mutase-prephenate dehydrogenase204-255. For a discussion of the mechanism and structural requirements of the enzyme see refs 266 and 267. 

Of the enzymes subsequent to EPSP synthase, only chorismate mutase has been the subject of inhibitor studies (see Chapter 5 by P. Bartlett in this volume). 

In view of the non-linear kinetics frequently observed with chorismate mutases and the resultant difficulty of determining true values, most inhibitor binding constants are reported as I50 values, equal to the concentration of inhibitor which produces 50% inhibition when the chorismate concentration equals Km. In Table 1, the compounds are compared as the ratio of reported Iso/Km values, since they were evaluated against enzymes from different sources and under different conditions. 

The oxabicyclic inhibitor is making a number of other jpearances as well it has starred in two productions already, and its performance in two more is currently being recorded. The X-ray crystal structures of its complexes with the B. subtilis monofunctional chorismate mutase and with the Hilvert catalytic antibody have been reported by William Lipscomb and Yuh-Min Chook at Harvard, and by Ian Wilson and his coworkers at Scripps, respectively, and similar studies with another enzyme and with the Schultz catalytic antibody are underway. When those studies ate completed, we will have an unprecedented oiqx)ttunity to compare the structures of four different proteins that catalyze the same reaction. 

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