| Ricin and Shiga toxin type 1 (Stx1) are potent cytotoxins known as ribosome inhibiting proteins, or RIPs. Due to its exquisite cytotoxicity, the plant toxin ricin has been used as a biological warfare agent. There is no effective vaccine against ricin despite U.S. military efforts, and therefore, there is interest in the identification of ricin inhibitors. Ricin inhibitors may also be useful against the homologous bacterial proteins Shiga toxin (ST) and Shiga toxin type 1 (Stx1), which are responsible for dysentery, and diseases related to food poisoning, including hemolytic uremic syndrome.; The identification of ricin inhibitors is carried out using structure-based drug design (SBDD). The RTA X-ray structure has been determined and the active site residues have been identified (Katzin et al., 1991; Kim and Robertus, 1992). In addition, the X-ray structures of substrate analogs bound to ricin were solved (Monzingo and Robertus, 1992). The pterin-based compound pteroic acid (PTA) was shown to inhibit RTA with an apparent K i of 0.6 mM. A 2.3 Å crystal structure of the RTA·PTA complex revealed the mode of binding and has been the basis for further drug-design investigations (Robertus et al., 1996, Yan et al ., 1997, 1998).; Despite the favorable interactions observed between the pterin ring of pteroic acid and RTA ‘specificity site’, the limited solubility of pterin, and various derivative compounds, posed a synthetic and experimental problem. Therefore, recent efforts have focused on developing a more soluble lead compound with the same or improved potency. Calculations suggested guanine-like compounds could interact strongly with the RTA specificity pocket, and served as a basis for the design and synthesis of a series of inhibitors described in this thesis.; In addition to using the specificity site for inhibitor binding, graphical displays of the various inhibitor-protein complexes revealed an extended cleft near the adenine specificity site. It is our belief that significant improvements in inhibitor efficacy will be achieved from ligands with additive contributions to binding in this region. (Abstract shortened by UMI.)... |
