General Properties

Mineral cofactors comprise a large group of inorganic substances the bulk of which are the metal ions. The domain of metal ions include macro metals, such as Na+, K+, Ca2+ and Mg2+, trace metal ions, including Fe2+, Zn2+, Cu2+ and Mn2+, and metalloids, such as Se, Si and B (Table 1). In seeking a reason for their necessity, one must realize that metal ions are suited to the task of executing dangerous chemical reactions on enzyme surfaces, reactions that would otherwise harm the more sensitive organic side chains of amino acids in an enzyme. For example, redox metals such as iron, manganese, and copper can accept electrons into their structure, hold them temporarily, and then donate them to oxygen, forming water as a way to dispose of the electrons safely. In essence, one should consider a metal cofactor as extending the

Table 1 Inorganic cofactors

Metal Common biological form or valence

Zinc Zn2+

Manganese Mn2+, Mn4+

Vanadium VO2+

Molybdenum MoO2+, MoOij-

Nickel Ni2+

Selenium Selenocysteine, selenomethionine

Silicon Si(OH)4, SiO2

Potassium K+

Sodium Na+

Calcium Ca2+

Magnesium Mg2+

Boron B(OH)3

repertoire of catalytic functions available to and performed by enzymes.

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