Characterisation of the active site of Kynurenine 3-Monooxygenase
Bell, Helen Barbara
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Kynurenine 3-monooxygenase (KMO) is a flavoprotein which has been implicated in Huntington’s disease, Alzheimer’s disease and acute pancreatitis. Recently there has been important research published about this enzyme including the structure of a truncated Saccharomyces cerevisiae KMO enzyme and KMO inhibition studies in animal models of disease. In previous work from this research group the complete Pseudomonas fluorescens KMO enzyme has been successfully crystallised both with and without the substrate, L-kynurenine, from which significant insights were gained into function and the potential role of domain movement. To examine substrate binding in KMO and to consolidate previous structural studies, key residues in the active site have been investigated using site directed mutagenesis, crystallography and kinetic analysis using steady-state techniques. This analysis has identified the interactions between the enzyme and the substrate and provides a basis for inhibitor design. The residues implicated in substrate binding are N369, Y404 and R84. For N369 and Y404, minor changes to the amino acid in the mutations N369S and Y404F were shown to cause a decrease in binding affinity of the substrate but the enzyme remained active. For the mutations Y404A and R84K enzyme activity was significantly affected. Crystal structures of N369S, Y404F and R84K were also obtained. Another residue in the active site studied was H320 which is the only amino acid to differ in the active sites of the human and Pseudomonas fluorescens enzymes. This residue was therefore of interest to determine whether the bacterial enzyme used in this work is likely to be a good model for the human enzyme, which has not yet been successfully isolated in significant quantities for in vitro research. Modifying this residue to obtain H320F KMO revealed that this residue does not have a significant role in substrate binding. Potent inhibitor molecules have been studied with this enzyme and shown in kinetic assays to have nanomolar Ki values. These inhibitors are the most potent inhibitors studied with Pseudomonas fluorescens to date and continue previous inhibitor studies carried out with this enzyme. This group of inhibitors contain different substituents in the part of the molecule shown to bind closest to the C-terminal domain of the protein. These novel inhibitors do not allow the flavin to be reduced by NADPH (which results in unwanted peroxide production) unlike a number of previously studied molecules and therefore have the potential to be clinically useful. This research therefore answers many questions about this enzyme, in particular about the role of particular residues in the active site, substrate recognition and inhibition of this important drug target.