Some of the enzymes require certain inorganic metallic cations like Mg²⁺, Mn²⁺, Zn²⁺, Ca²⁺, Co²⁺, Cu²⁺, Na⁺, K⁺, and so on, for their optimum activity. Rarely anions are also needed for enzyme activity (e.g. chloride ion Cl⁻ for amylase).

Two categories of enzymes requiring metals for their activity are distinguished:

1. Metal-activated enzymes: The metal is not tightly held by the enzyme and can be exchanged easily with other ions.Example: ATPase – (Mg²⁺ and Ca²⁺)
          Glucokinase – (Mg²⁺)
2. Metalloenzymes: The enzymes hold the metals rather tightly which are not readily exchanged.Example: Cytochrome oxidase (Fe²⁺, Cu2⁺) and Pyruvate oxidase (Mn²⁺)

METAL-ACTIVATED ENZYMES AND METALLOENZYMES

Almost ¼ of enzymes require metal atoms for activity. Metal atoms usually exist as cations and often have more than one oxidation state. Examples: Ferrous (Fe²⁺) and Ferric (Fe³⁺).

Metal ion can bind a particular number of groups (ligands) by accepting free electron pair to form coordinate bonds in specific orientation.