ABSTRACT
Tungsten (W; atomic number 74) and the chemically analogous and similar metal molybdenum (Mo; atomic number 42) are minor yet equally abundant elements on this planet (Krauskopf, 1972; Evans et al., 1974). The essential role of molybdenum in biology has been known for over 60 years and molybdoenzymes are ubiquitous (Bortels, 1936, for recent reviews, see Stiefel, 1993, and references cited therein). However, it is only recently that a biological role for tungsten has been established in prokaryotes. Yet, W and Mo have similar atomic and ionic radii, they exhibit comparable coordination chemistry (both are group Via elements; Rollinson, 1970), and most W enzymes known have Mo-containing isoenzymes some of which, but not all, are phylogenetically homologous (Kletzin and Adams, 1996). Considering the high atomic number, W is a most unlikely choice for a metal with a biological function, and indeed, there are only two examples of biologically essential elements with atomic numbers above 35, molybdenum and iodine (fifth or sixth row; Frafisto da Silva and Williams, 1991). In contrast, most sixth and many fifth row metals like mercury, lead or cadmium are known for their detrimental effects on higher organisms (Wennig and Kirsch, 1988) and for the elaborate resistance mechanisms bacteria have developed to avoid their effects or deal with them (Silver, 1992, and references cited therein).
