The yeast (Saccharomyces cerevisiae) multidrug transporter Pdr5p effluxes a broad range of substrates that are variable in structure and mode of action. Previous work suggested that molecular size and ionization could be important parameters. In this study, we compared the relative sensitivity of isogenic PDR5 and pdr5 strains toward putative substrates that are similar in chemical structure. Three series were used: imidazolecontaining compounds, trialkyltin chlorides, and tetraalkyltin compounds. We demonstrate that the Pdr5p transporter is capable of mediating transport of substrates that neither ionize nor have electron pair donors and that are much simpler in structure than those transported by the human MDR1-encoded P-glycoprotein. Furthermore, the size of the substrate is critical and independent of any requirement for hydrophobicity. Substrates have surface volumes greater than 90 Å 3 with an optimum response at ϳ200 -225 Å 3 as determined by molecular modeling. Assays measuring the efflux from cells of [ 3 H]chloramphenicol and [ 3 H]tritylimidazole were used. A concentration-dependent inhibition of chloramphenicol transport was observed with imidazole derivatives but not with either the organotin compounds or the antitumor agent doxorubicin. In contrast, several of the organotin compounds were potent inhibitors of tritylimidazole efflux, but the Pdr5p substrate tetrapropyltin was ineffective in both assays. This argues for the existence of at least three substrate-binding sites on Pdr5p that differ in behavior from those of the mammalian P-glycoprotein. Evidence also indicates that some substrates are capable of interacting at more than one site. The surprising observation that Pdr5p mediates resistance to tetraalkyltins suggests that one of the sites might use only hydrophobic interactions to bind substrates.In Saccharomyces cerevisiae, broad spectrum resistance to structurally and mechanistically diverse inhibitors is mediated by several members of the ATP-binding cassette (ABC) 1 superfamily of transporters, including Pdr5p, a 160-kDa ABC transporter found in the plasma membrane. Pdr5p is essential for basal levels of resistance to a broad array of substrates, including antifungal and antitumor agents, hormones, and ionophores (1-4). Compared with isogenic wild-type strains, pdr5 loss-of-function mutants exhibit hypersensitivity because of their inability to efflux inhibitors (3, 5).Although an important study (3) provided strong indirect evidence for the existence of more than one substrate-binding site, the chemical basis of Pdr5p substrate specificity is unclear. One reason is that many of the compounds analyzed to date are complex in chemical structure. As an alternative approach, we searched for a series of relatively simple compounds that were systematically related to each other in structure but varied in their ability to serve as Pdr5p substrates. By comparing several series, we hoped to identify shared properties that are used in the Pdr5p-substrate interaction. In the initial study, we comp...

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The use of computational molecular modeling to enhance the teaching of chemical concepts is becoming commonplace. Incorporation of this technique into the curriculum, however, typically requires financial investment. This reality poses a problem for institutions where funding and associated resources are scarce and has a potential negative impact on student learning. To address this situation, a free, universally accessible web resource designed to make computational chemistry available to community colleges and similar institutions that lack sufficient resources or expertise to design their own computational modules is presented. The website enables students to view, manipulate, and analyze Gaussian 09 outputs of organic molecules in the front-end user interface WebMO. The associated exercises guide students through key concepts in understanding the structure, bonding, and reactivity of organic molecules. The website and exercises have been used at two different two-year institutions as part of their second-semester organic chemistry courses.

Enzyme-catalysed hydration of the N-acetylcysteamine thioester of (E)-dec-2-enoic acid to the corresponding thioester of (R)-3-hydroxydecanoic acid proceeds in a syn fashion, with protonation on the si face at substrate C-2. P-Hydroxydecanoylthioester dehydrase , I the key enzyme in the biosynthesis of unsaturated fatty acids in bacteria under anaerobic conditions, mediates the interconversion of thioesters of (R)-3-hydroxydecanoic acid, (Q-dec-2-enoic acid, and (Z)-dec-3-enoic acid, (l), (2), and (3), respectively (R = acyl carrier protein, in vivo). We present evidence herein that the dehydrase-catalysed hydration of (2) to give (1) is a syn

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