The role of sulfhydryl sites in metal binding onto bacterial cells.

摘要

Bacteria can adsorb a wide range of metals through interactions with their abundant cell envelope binding sites, thereby strongly affecting the speciation, distribution, bioavailability and mobility of metals in these systems. Besides carboxyl, phosphoryl, and amine groups, the sulfhydryl sites have also been identified recently as potential metal binding sites, but the exact role of sulfhydryl sites has not yet been extensively studied. The research presented in this dissertation focuses on the role played by sulfhydryl sites in metal adsorption onto bacteria. Chapter 2 presents an approach to determine the concentrations and acidity constant values of sulfhydryl sites within bacterial cell envelopes, and the approach is applied to a range of bacterial species. Chapter 3 focuses on understanding the adsorption and desorption of Cd onto bacterial sulfhydryl sites, and the work results in the first determination of a thermodynamic stability constant value for a metal-sulfhydryl bacterial complex. Chapter 4 presents work that determines the distribution of sulfhydryl sites within cell envelopes, with a focus on the concentrations of sulfhydryl sites on EPS molecules.;The results in Chapters 2 and 3 demonstrate that the measured concentration of sulfhydryl sites is lower than those of the more abundant carboxyl and phosphoryl sites within cell envelopes, but that the concentration of sulfhydryl sites is high enough to control the binding of chalcophilic metals onto bacteria under low metal-loading conditions due to the high affinity of these sites to bind chalcophilic metals. In Chapter 3, I developed a surface complexation model based on the calculated stability constants of Cd-sulfhydryl and Cd-non-sulfhydryl bacterial complexes that successfully accounts for the adsorption of Cd onto Shewanella oneidensis as a function of pH and metal loading. The distributions of sulfhydryl sites within cell envelopes determined in Chapter 4 vary significantly between Shewanella oneidensis and Pseudomonas putida, likely resulting in the different Cd resistance that was measured between these two bacterial species. The research presented in this dissertation significantly improves our understanding of the controls on metal adsorption onto bacteria, especially under environmentally-relevant low metal loading conditions, and thereby can lead to improved models of the transport and global cycling of metals.