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2+
2+
The complex formation constant for the complex formed between Mg and ATP (Mg +
-1
-
3-
4
ATP ' [MgATP] ) is about 10 M (the dissociation constant correspondingly 0.1 mM). The
2+
-
coordination sphere of Mg is supplemented by water (and, depending on the pH, OH ). The
free enthalpy of reaction for the ATP hydrolysis amounts to ∆G ≈ -35 kJ/mol. The phosphate
group (P i = inorganic phosphate) generated by ATP hydrolysis can also be transferred to
suitable substrates. An example is the phosphorylation of sugars. In cells with a high turnover
rate for ATP, phosphate can be transferred to creatine. Creatinephosphate on its part serves as a
source for rapid regeneration of ATP. The daily turnover of ATP at rest corresponds to about
half of the body weight.
H C CO 2 - (Creatinkinase) H C CO 2 - O O
2
2
H C N NH 2 + MgATP - H C N N P OH + MgADP
3
3
H
NH 2 NH 2
Creatin
Phosphocreatin

2+
Mg also mediates the hydrolysis of phosphoester bonds by phosphatases by stabilising a
trigonal-bipyramidal transition state for phosphorus:
R R R + R
O O O O H OH
O P O O P O
(H 2 O) n Mg 2+ P O HO O
O (H 2 O) n Mg 2+ OH
OH R' O (H 2 O) n Mg 2+ R'
Transition state R'
Übergangszustand
(trigonal-bipyramidal)

Interlude: The Gibbs-Helmholtz Equation

This equation connects the reaction enthalpy (∆H) with the free reaction enthalpy (∆G, Gibbs
free energy), the reaction entropy (∆S) and the temperature T:
∆G = ∆H – T∆S
According to this relation, part of the reaction enthalpy is converted to entropy changes in the
reaction system. In the global system, the entropy always increases (+∆S); in a subsystem, the
entropy can also decrease (-∆S). A reaction will only take place voluntarily in case of a
negative ∆G.
Example: H 2 + ½ O 2 → H 2O: ∆H = -286 kJ/mol, T∆S = -49 kJ/mol (at 298 K); ∆G = -239
kJ/mol
Even in case of a negative ∆G (thermodynamically allowed reaction), a reaction system may
be metastable (i.e. the reaction does not take place) because it is kinetically hindered due to a
high activation barrier (example: the formation of molecular oxygen from ozone and oxygen
atoms in the stratosphere). Catalysts reduce this activation barrier.

Sodium and potassium

+
+
In order to adjust to the intra- and extra-cellular concentrations of Na und K it is
essential that these ions can cross the membrane of the cells. Such a trans-membrane transport
can be passive (via diffusion) or active by use of an ion pump. Since the lipophilic cell