Oxygen atoms are often involved in the bonds of both alkali metal and alkaline earth metal cations, bonds of the latter being relatively stronger. Mg²⁺ is accumulated by cells in exchange for transport of Ca²⁺ in the opposite direction. So Ca⁺⁺ activation enzymes are mostly extracellular ones. Example: Salivary and pancreatic α-amylases.

Intracellular enzymes require Mg²⁺ for activity, and in most cases, this requirement can be replaced in vitro by one for Mn²⁺. Example: Arginase –M²⁺.

For Mg²⁺. Example: Muscle creatine kinase.

 

Creatine + MgATP  MgADP + Phosphocreatine + H⁺

The true substrate is MgATP, and the reaction proceeds via the formation of the complex creatine E ATP Mg. Cohen and colleagues showed that divalent cation binds to α and β phosphate of nucleotide, but not in the terminal γ-phosphate, which is transferred to creative.

In contrast, the reaction catalysed by pyruvate kinase involves a cyclic metal bridge complex.

Enolase is a dimeric enzyme, having two Mg²⁺ ions that are needed to stabilise the active dimer. In addition, two more Mg²⁺ ions are required if each of two sites binds to a substrate.