A structural investigation of mechanism in human glutathione transferase omega 1

Abstract

This thesis investigates the structural details of catalysis associated with the human enzyme, Glutathione Transferase Omega 1 (hGSTO1-1), a member of the most recently discovered class of Glutathione Transferase. The human genome contains two Omega class genes (hGSTO1 and 2) and their transcribed products have been found at varied levels within almost every tissue of the human body. The crystal structure of hGSTO1-1 displays a distinguishing cysteine residue in the active site, a feature that undoubtedly plays a role in the novel biochemistry it displays. Of most interest is the hGSTO1-1 mediated reduction of mono-and dimethylated arsenic species that result from human arsenic exposure. The enzyme is also essential for the enzymatic detoxification of alphahaloketones to acetophenones and exhibits a novel dehydroascorbate reductase activity. The following dissertation reports major progress in understanding the details of these chemical mechanisms. By observing these reactions within the crystallised enzyme using X-ray diffraction, several states within these catalytic cycles have been observed in great detail. Subsequently, hypotheses have been drawn about the product inhibition inferred during dehydroascorbate reduction, the existence of glutathione intermediates inherent to its interaction with methylated arsenicals, and the fundamental understanding of Omega mediated catalysis obtained by observing the enzymatic complex of oxidized glutathione and S-(4-Nitrophenacyl) glutathione simultaneously. In addition, adaptation of experimental conditions has led to the discovery of two new crystal forms with which to study the structural dynamics of this protein and its chemistry. The structure of a naturally occurring polymorphism involving the deletion of glutamate 155 has also been solved, revealing pronounced structural plasticity that correlates well with the apparent instability of the mutant enzyme in vivo. Discussions explore the broad implications for interpreting the role of hGSTO1-1 in the pathomechanisms of human disease states such as Alzheimer's disease and the biological mechanisms of arsenics' newly discovered potential as an anti-cancer agent.