Tungstoenzymes

Abstract

Tungsten (W, atomic number 74) and molybdenum \n(Mo, 42) are chemically analogous elements, and both \nare relatively scarce in natural environments. The \nnotion that either of them might have a significant \nbiological role is perhaps surprising when one considers \nthat, with the exception of iodine (53), life forms \non this planet typically thrive by utilizing elements \nwith atomic numbers below 35. Yet, from a biological \nperspective, W and Mo provide a fascinating \nstudy in contrasts. The essential role of Mo in \nvarious fundamental biological conversions carried \nout by both microorganisms and higher (larger) life \nforms has been known for many decades. Molybdoenzymes \nare ubiquitous in nature and play intimate \nroles in the global cycles of nitrogen, carbon, \nand sulfur, with nitrogenase, nitrate reductase, formate \ndehydrogenase, and xanthine oxidase being \nprime and extensively studied examples. On the other hand, W has traditionally been regarded as an \nantagonist of the biological functions of Mo. Because \nof the great similarities in the properties of the two \nelements, it was reasoned that insight into the \ncatalytic role of Mo in various enzymes might be \nprovided by replacing Mo with W. Various \norganisms, including plants and rats, were therefore \ngrown with or exposed to W, but they produced either \ninactive metal-free molybdoenzymes or W-substituted \nenzymes with little or no activity. Clearly, \nthe chemical properties of W and Mo are sufficiently \ndifferent that biology can distinguish between them, \neither at the levels of their uptake and/or incorporation \ninto enzymes or in the properties of the enzymes \nthemselves, which function with Mo but not with W.