Tungstoenzymes

Abstract

Tungsten (W, atomic number 74) and molybdenum (Mo, 42) are chemically analogous elements, and both are relatively scarce in natural environments. The notion that either of them might have a significant biological role is perhaps surprising when one considers that, with the exception of iodine (53), life forms on this planet typically thrive by utilizing elements with atomic numbers below 35. Yet, from a biological perspective, W and Mo provide a fascinating study in contrasts. The essential role of Mo in various fundamental biological conversions carried out by both microorganisms and higher (larger) life forms has been known for many decades. Molybdoenzymes are ubiquitous in nature and play intimate roles in the global cycles of nitrogen, carbon, and sulfur, with nitrogenase, nitrate reductase, formate dehydrogenase, and xanthine oxidase being prime and extensively studied examples. On the other hand, W has traditionally been regarded as an antagonist of the biological functions of Mo. Because of the great similarities in the properties of the two elements, it was reasoned that insight into the catalytic role of Mo in various enzymes might be provided by replacing Mo with W. Various organisms, including plants and rats, were therefore grown with or exposed to W, but they produced either inactive metal-free molybdoenzymes or W-substituted enzymes with little or no activity. Clearly, the chemical properties of W and Mo are sufficiently different that biology can distinguish between them, either at the levels of their uptake and/or incorporation into enzymes or in the properties of the enzymes themselves, which function with Mo but not with W.