Metal binding by Anabaena flos-aquae cells and cell walls
Cyanobacteria bind metals to a number of different structures. Intracellularly, cyanobacteria may utilize proteins such as metallothionein or polyphosphate bodies to sequester metal ions. Extracellularly, they may bind metal ions to the extracellular mucilaginous material found outside the gram negative-like cell wall. The extracellular polysaccharides (EPS) found on their surfaces are polyanionic, containing neutral sugars and uronic acids. These anionic EPS have been shown to bind ions and thus affect the entry of both essential and toxic metal ions into cells. Microbial cell walls are also known to possess a high ion exchange capacity. This can be attributed to the presence of strongly metal complexing functional groups such as: carboxyl, aldehyde, sulfhydryl, and phosphoryl groups. Together, these functional groups serve to give the cell wall an overall negative charge. The high ion-exchange capacity of isolated cyanobacterial cell wall material and cyanobacterial exopolymers may allow for these biomaterials to be used for bioremediation of water and restoration of copper metal ion contaminated sites. Isolated peptidoglycan of cell walls and whole cells of the cyanobacterium Anabaena flos-aquae (ATCC 22664) were examined for their ability to bind copper ions. Experiments were performed to gain insight into the chemical groups involved in the binding of copper ions. We have taken advantage of physical/chemical tools that allowed us to identify these groups at the molecular level. Extended X-ray absorption fine structure (EXAFS) spectroscopy was utilized to determine the chemical nature of the copper binding sites. The results of these studies indicate that pH-dependent ligands in the purified peptidoglycan of the cell wall consist of amines and carboxylates at pH 5 and pH 2 while a pH-independent ligand in the form of phosphates predominates in the whole cell experimental samples at pH 2. Other ligands such as carboxylates and amines play a larger role at pH 5 in the whole cell group. XANES analysis illustrated that most of the copper in both experimental samples is present as Cu(II) although a significant amount of Cu(I) is found in the whole cell experimental group. We describe the use of a more elaborate model to interpret the EXAFS of Cu bound by lyophilized whole cells and purified peptidoglycan of cell walls, and differentiate and quantify amine, carboxylate, and phosphate ligands. In addition some structural information is obtained describing the ligands.
Kretschmer, Xiomara Carolina, "Metal binding by Anabaena flos-aquae cells and cell walls" (2002). ETD Collection for University of Texas, El Paso. AAI3081130.