Chemistry Reference
In-Depth Information
Chapter 17
Molybdenum, Tungsten, Vanadium,
and Chromium
Introduction
323
Mo and W Chemistry and Biochemistry
323
Molybdenum Enzyme Families
324
The Xanthine Oxidase Family
325
The Sulfite Oxidases and DMSO Reductases
328
INTRODUCTION
In this last chapter to consider particular metal ions, we have regrouped four metals, which, together with
manganese and nickel, are used as alloys to produce specialist steels.
1
However, this is not on account of their
capacity to confer particular properties on steel, but rather for their biological chemistry. For further details see
Brondino et al., 2006; Crans et al., 2004; Enemark et al., 2004; Mendel and Bittner, 2006; Vincent, 2000a
.
Molybdenum is the only second row transition element that is essential for most living organisms, and the few
species that do not need molybdenum use tungsten, which is molybdenum's third row homologue. In the case of
vanadium, the close similarity between its chemical properties and those of molybdenum has led to its replace-
ment in the FeMo-cofactor in some bacterial nitrogenases. However, it is also involved in the activity of hal-
operoxidases, and four-coordinate vanadate can mimic cellular metabolites via its analogy with phosphate. All
three of these metal ions play important roles in enzymes. In contrast, chromium (Cr), a little like Pirandello's 'Six
Characters in Search of an Author', is an element which may be biologically necessary, but for which we have yet
to find a function or a protein to which it binds.
MO AND W CHEMISTRY AND BIOCHEMISTRY
While it is relatively rare in the earth's crust, Mo is the most abundant transition metal in seawater. When we
consider that the oceans are the closest we get today to the primordial soup in which life first arose, it is not
surprising that Mo has been widely incorporated into biological systems, and the only organisms which do not
require Mo use W instead. The biological versatility of Mo and W result not only from their redox-activity, ranging
through oxidation states VI to IV, but because the intermediate V valence state is also accessible, they can act as
interfaces between one- and two-electron redox systems, which allows them to catalyse hydroxylation of carbon
atoms using water as the ultimate source of oxygen (
Figure 17.1
)
. This contrasts with systems in which O
2
is the
ultimate source of the hydroxyl oxygen incorporated into the product. These monooxygenase systems range from
1. Steel is an alloy of iron and carbon, containing typically up to 2% carbon. The addition of other metals in alloys can give special properties
such as superior strength, hardness, durability, or corrosion resistance.
