Structural Characterization of Procaryotic Metal Reductase Systems

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Recent years have witnessed the emergence of the field of geomicrobiology, which focusses on the influence of microbial processes on shaping our environment through the formation and degradation of mineral deposits. Many geological formations formerly believed to be of purely inorganic origin are today known to be the product of procaryotic metabolism. Herein, becterial reduction of insoluble metal oxides, predominantly of Fe III and Mn IV, is a major respiratory process in anaerobic, aquatic environments and can under certain conditions account for the oxidation of 80 to 90 % of the available organic carbon. This process gains further ecological importance by the fact that many metal-reducing bacteria are able to oxidize aromatic hydrocarbons such as benzene and toluene and that among the described oxidized metal substrates are those of arsenic, chromium and uranium. Metal reductase activity has been found in a range of soluble metalloproteins, but the complete, respiratory machinery consists of a multisubunit multiheme protein system spanning both bacterial membranes. It presents the actual catalytic units on the outside of the bacterial cells, where they can directly interact with their insoluble substrates.