Use of specific metal binding of self-assembling S-layer proteins for metal bioremediation and recycling

Most bacteria and all archaea possess as outermost cell envelope so called surface-layers (S-layers). These S-layers are formed by self-assembling proteins having a number of interesting intrinsic properties, e.g. they mediate selective exchange of molecules and therefore function as molecular sieves. Furthermore, S-layers from bacterial isolates recovered from heavy metal contaminated environments have outstanding metal binding properties and are highly stable. Thus they selectively bind several metals with different affinity. For using S-layer proteins for metal bioremediation and recycling three aspects of the metal-interactions with S-layer proteins must be taken into account. First, S-layers possess different functionalities, e.g. carboxyl, phosphoryl groups, binding toxic metals and metalloids, like U and As, unspecifically depending on pH-value. Second, precious metals like Au and Pd are likewise unspecifically bound to functional groups, but presumably covalently, making the binding irreversible unless the S-layer protein is destroyed completely. Third, some metals are needed for native protein folding of the S-layer protein monomer, self-assembly, and the formation of highly-ordered lattices. These particular metals are specifically bound bivalent cations, e.g. Ca2+. Important is that these binding sites allow selective binding not only of Ca2+, but also of chemical-equal elements including the trivalent lanthanides (Eu3+, Tb3+), possessing comparable ionic radii. Thus the metal dependent binding behavior of the proteins not only contributes to the development of biohybrid materials for the separation, removal or recovery of strategic relevant metals regulated by pH and allowing a conceivably release, but also allows a biochemical and mechanistic understanding of the interaction of different metal ions with S-layer proteins .