Protein surfaces, metal accumulation

Direct Observation of Metal Accumulation on Protein Surfaces. 34... 

Some confusion has prevailed in the literature regarding the use of terms "biosorption" and "bioaccumulation". Biosorption (or bioadsorption) is a passive immobilization of metals by the biomass. Mechanisms of cell surface sorption in contrast to bioaccumulation are independent of cell metabolism they are based upon physicochemical interactions between metal and functional groups of the cell wall. The microorganism s cell wall mainly consists of polysaccharides, lipids and proteins, which have many binding sites for metals. This process is independent of the metabolism and metal binding is fast. Bioaccumulation, in contrast, is an intracellular metal accumulation process which involves metal binding on intracellular compounds, intracellular precipitation, methylation and other mechanisms (Mohan and Pittman, 2007). 

The reduction of U(VI) by sulfate-reducing bacteria appears to occur at the cell surface of the anaerobes, since the reduced products of these elements accumulates in the environment outside of the cell. The proteins of cyt Cs and cyt c have been demonstrated to function as nonspecific metal dehydrogenases however, these cytochromes are found in the periplasm and not in the outer membrane. Thus, if it were analogous to Fe(III) reduction, uranyl ions would most appropriately be reduced by a cytochrome in the outer membrane of the sulfate reducers. As demonstrated by Laishley and Bryant (see Chapter 18) cytochromes are located in the outer membrane of certain sulfate reducers however, their role in reduction of U(VI) remains to be demonstrated. 

Figure la shows a quick fall of the mercury concentration in the mercury solution to the end of one hour of treatment this could be explained by mercury adsorption on different immobilization supports and also to mercury adsorption on the bacteria cellular envelope due to the peripheral charges and functional groupings present in the envelope and also to absorption phenomena by which these the bacteria accumulates the metals inside the cell by active or passive transport (Jairo-Alberto, 1990). Then we are witnessing a progressive decrease of the load polluting and this for the different effectuated experiences due to mercury detoxification. In fact mercury can bind with cell surface proteins, highly specific transport of Hg2+ into the cell in the protein-bound form. 

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