Designing arsenic based inhibitors of redox-active enzymes

Sapra, Aparna
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University of Delaware
The surprising efficacy of arsenic trioxide in the treatment of acute promyelocytic leukemia has renewed interest in the synthesis and testing of arsenicals as chemotherapeutic agents for other types of cancers. The biological effects of arsenicals largely reflect coordination of As(III) species to vicinal thiols. Several enzymes of oxidative protein folding are both up-regulated in certain cancer cells, and contain catalytically essential vicinal thiols. Here, I sought to develop arsenical-based inhibitors that would capture these redox-active motifs. The work in this dissertation introduces synthesis of simple multivalent arsenicals that could take advantage of the chelate effect leading to more effective inhibition of enzymes with multiple CxxC motifs like q uiescin s ulfhydryl ox idase (QSOX) and p rotein d isulfide i somerase (PDI). The results from this study showed that the small molecule multivalent arsenicals were able to inhibit the oxidative folding pathways of the reduced unfolded proteins, riboflavin binding protein (RfBP) and ribonuclease (RNase). These compounds were effective even in the presence of millimolar concentration of glutathione, which is an effective competitor for the arsenic functionality. However, the main targets of these small molecule arsenicals turned out to be the reduced unfolded protein substrates and not the redox-active enzymes. Next, to increase the specificity towards the more structured CxxC motifs, an As(III)-containing maleimide (As-Mal) was synthesized which could be readily conjugated to exposed cysteine residues in peptides and proteins. The conjugation provided a scaffold for directing the As(III) species to the target proteins for their inhibition. Experiments with thioredoxin reductase and protein disulfide isomerase show that their CxxC motifs can be efficiently captured by their cognate arsenical-carrying substrates. These peptide-based arsenicals as well as small molecule multivalent arsenicals could be used to inhibit cell-surface redox-active enzymes where the extracellular concentration of reduced GSH is in low micromolar range. The As-Mal reagent was also used to generate two new arsenic-labeled affinity purification resins by reaction with thiol-activated Sepharose beads. These resins were subsequently used for purifying thiol-containing peptides from yeast extracts and proteins containing redox active CxxC motifs. A new technique for regenerating these resins was also developed that makes this affinity purification method highly cost effective.