releasing calcium ions to foster muscle relaxation). Calmodulin and calcium
ATPases will be discussed in more detail in Sections 6.3.2 and 6.4.2. Many
other proteins and enzymes require calcium, but these will not be discussed
in any detail here. Extracellular digestive enzymes requiring calcium ions
include staphylococcal nuclease (one calcium site), phospholipase A 2 (two
calcium sites), and trypsin (one calcium site). Structural and storage enzymes
that sequester many calcium ions per protein unit include thrombin, phos-
phodentine (material in teeth), and calsequestrin (calcium storage in the sar-
coplasmic reticulum). Table 6.6 of reference 1 collects some information on
calcium - binding proteins.
Calcium ion movements are sensitive to the concentration of other cations.
In one study, it was found that Ca 2+ infl ux into cells might occur through K +
inward rectifi er channels when extracellular K + ion concentration fell below
1 mM. These channels became Ca 2+ - permeable only when the extracellular K +
concentration decreased to 1 mM or below. The same study found that the
addition of different divalent cations revealed that Ba 2+ , but not Ni 2+ , Cd 2+ ,
Sr2+ , or Mg 2+ , reversibly blocked the Ca 2+ infl ux into cells during low external
K+.^11 Intracellular proteins requiring calcium ion must be very selective for
Ca2+ because the concentration of free Mg 2+ (2.5 mM) and K + ( ∼ 100 mM) are
much higher. Intracellular Ca 2+ concentrations must be kept at very low levels
because calcium ions inhibit the activity of Mg 2+ - dependent enzymes. In addi-
tion, the precipitation of sparingly soluble calcium salts may cause serious
problems within a cell.
5.3 Movement of Molecules and Ions Across Membranes,
ACROSS MEMBRANES
5.3.1 Passive Diffusion,
At equilibrium there is a zero free - energy change, ΔG = 0, that takes place
between compartments separated by a membrane, with the free - energy change
being dependent on the difference in concentration of various ions and the
electrical potential difference that exists across the membrane. The relation-
ships among sodium, potassium, and chloride ions, pH, and electrolytic poten-
tial have become known as Donnan equilibria. The concentrations and
electrolytic potentials are related by the following equation:
ΔGRT
C
C
=+=ln^2 zF
1
ψ 0 (5.3)
where z is the charge on the ion, F is the the Faraday constant, ψ is the elec-
trolytic potential.
If going down the concentration gradient — C 2 is less than C 1 — the fi rst term
in equation 5.3 will be negative (remember ΔG = − R T ln Keq ), and if going up
the concentration gradient — C 1 is less than C 2 — the fi rst term will be positive.
MOVEMENT OF MOLECULES AND IONS ACROSS MEMBRANES 195
