Binding Site for AHAS-inhibiting Herbicides

The IMI herbicides also exhibit complex interactions with AHAS. When enzyme activity was measured over an extended period in the presence of various concentrations of imazapyr, inhibition increased with time, thereby suggesting that the equilibrium between the herbicide and AHAS was reached slowly, a characteristic of tight-binding inhibitors [51]. In contrast to SUs, substrate-inhibitor studies suggested that inhibition by imazapyr is uncompetitive with respect to pyruvate, which implies that the synthetic molecule binds to AHAS only after formation of the ternary enzyme-pyruvate-ThDP complex [52]. However, noncompetitive binding has also been reported for the IMIs, which underscores the complexity of the kinetics of AHAS inhibition [49]. 

One noteworthy difference between the crystal structures of the AHAS SU complex and the free enzyme is that the volume of the protein in the region in 

Twenty-two years after the introduction of the first commercial AHAS inhibiting herbicide, a preliminary crystal structure at 3.0-A resolution of the AHAS catalytic subunit from a plant, A. thaliana, was published by Pang et al. [58]. This was followed two years later by crystal structures of the same enzyme complexed with five SUs and one IMI at 2.5- and 2.8-A resolution, respectively [59]. The latter achievement by McCourt et al. represented the first reported X-ray crystal 

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