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Overview of the functions (selection):
- Support (endo- and exo-skeletons, teeth): Ca (and Mg)
- Information transfer by migration along a concentration and/or electrochemical
gradient: all ions
- Regulation of the osmotic pressure and of membrane potentials: Na, K
- Activation and regulation of enzymes Ca (and Mg, K)
- Signal transduction, e.g. in neurotransmission Ca, K
- Chlorophyll: Mg
- Phosphate und anaerobic energy metabolism; activation by phosphorylation: Mg
- Stabilisation of cell membranes by the formation of cross-links between membrane
proteins and polysaccharides: Mg (and Ca)

For the physiological functions, the charge density (CD = ionic charge divided by the
ionic radius) is of central importance:

Li + Na + K + Mg 2+ Ca 2+ Mn 2+
r/Å * 0.76 1.02 1.38 0.72 1.00 0.83
LD 1.45 1.10 0.72 2.78 2.0 2.41
*
for the coordination number 6

The larger the charge density, the higher is the ability of the ion to polarise a molecule. The
2+
2+
charge density of Mg is particularly high: contrasting Ca , but in accordance with transition
2+
metal ions, Mg thus forms stable complexes also with N-functional ligands; see e.g.
chlorophyll. Alkaline and alkaline earth metal ions are rather mobile; typically, their complexes
are characterised by small stability constants. In aqueous media, and in the absence of other
ligands, the ions are present in hydrated form. Unlike aqua complexes of transition metal ions,
the number of water molecules in the hydration sphere is hardly defined, and the interactions
are weak. The rate constants for the exchange of water in the hydration sphere and surrounding
water, i.e. for the equilibrium

n+
n+
n+
[M(H 2O) x] (≡ ”M ·aq“) ' M + xH 2O

-7
2+
2+
-5 -1
-7 -1
-10
+
+
are in the order of magnitude of 10 -10 s for Na , K and Ca , and 10 -10 s for Mg .
2+
Mg complexes thus are not only thermodynamically but also kinetically more stable than
those of the other ions.

Magnesium

Magnesium takes over a crucial role in the phosphate (and hence the energy) metabolism in
that it coordinates to diphosphate or phosphate + carboxylate, and thus activates molecules and
triggers activation paths. Examples are kinases, ATPases, phosphatases, isomerases, enolases,
proteinsynthases and –polymerases; see the following examples (ATP hydrolysis;
creatinkinase):

O O O O O O
H O O Ad H O O Ad O O
O P P P O O P P P O HO P O P O Ad O
O - O - O - O - O - O - H O O - O - + H O P OH
2
Mg 2+ O -
Mg 2+ Mg 2+
ATP hydrolysis protected ATP hydrolysis susceptible
2+
2+
(Mg coordinates to Pγ and Pβ) (Mg coordinates to Pα and Pβ) ADP P i