Bacterial cell envelope and extracellular sulfhydryl binding sites: Their roles in metal binding and bioavailability

Although carboxyl and phosphoryl functional groups within the bacterial cell envelope and on bacterial extracellular polymeric substance (EPS) molecules are the most abundant metal binding sites, recent studies suggest that sulfhydryl sites control the binding of chalcophile and similar elements under environmentally-relevant metal loading conditions. The role of cell surface sulfhydryl sites in metal binding has been demonstrated unambiguously for Zn, Cd, Hg, Cu, Au, and Se. This review article summarizes our current understanding of the nature, concentration, and reactivity of these important metal binding sites, their distribution between the cell envelope and extractable EPS molecules, and their possible role in controlling bacterial bioavailability of some elements. The objective of the review is to summarize the relatively few studies that have focussed on bacterial sulfhydryl sites, and to identify areas in which future research may be most productive. Sulfhydryl sites comprise only approximately 5–10% of the total binding site concentration of bacterial cell envelopes, but exhibit such a high affinity for some metals that under low metal loading conditions, sulfhydryl binding of metals is responsible for nearly 100% of the adsorbed metal budget. Recent experimental results have revealed that the concentration and distribution of sulfhydryl sites between cell envelope macromolecules and cell-produced EPS are dependent on the bacterial species, growth phase, and growth conditions. For example, the cell envelope sulfhydryl site concentrations of Bacillus subtilis increase with increasing glucose concentration in the growth medium. Shewanella oneidensis cells contain high concentrations of sulfhydryl sites within their cell envelopes with much lower concentrations present on EPS molecules, while Pseudomonas putida cells exhibit the opposite. We apply a proteomics approach to explain the observed differences in sulfhydryl distributions for S. oneidensis and P. putida. The proteomics analysis indicates that the outer membrane proteins of S. oneidensis contains a high concentration of cysteine residues, while the cell surface proteins of P. putida are relatively cysteine-poor, with cysteine-rich proteins of P. putida associated predominately with EPS materials. The results of this proteomics analysis demonstrate the potential to identify the range of possible protein hosts for metal binding sulfhydryl sites, and the approach represents a means for predicting the concentration and distribution of sulfhydryl metal binding sites on bacterial cells and EPS molecules.