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Enniatin B
Antamanid

+
+
Figure 33. Ionophores for K (nononactin and enniatine B) and Na (antamanide).

Calcium

Sparingly soluble calcium compounds (such as carbonates and phosphates) can act as bearings
by being incorporated into exo- and endo-skeletons. Examples are bone (calcium phosphate:
hydroxyapatite) and sea shells (calcium carbonate: aragonite, calcite). The bones of the
vertebrates are composite materials, containing about 50% collagen (a fibrous protein) and
50% hydroxyapatatite Ca 5(PO 4) 3(OH) xF 1-x (x ≤ 0.01). A person of average weight (70 kg)
contains ca. 1.1 kg calcium, mainly as a constituent of the bone tissue. Only about 10 g are not
confined to bony materials. These 10 g are used for a variety of functions in the organism,
including the regulation of cell function, muscle contraction, blood clotting, and enzyme
2+
regulation, the latter with the help of specific Ca binding proteins (calmodulins; see below).

2+
On a general basis, Ca acts as a second messenger by activating, regulating and
2+
reinforcing signals. Additionally, Ca can take over the role of a co-factor in hydrolases (e.g.
in nucleases responsible for the hydrolysis of the phosphodiester bonds), and exert structure
2+
function in proteins (e.g. in thermolysin and in proteinase-K). In this respect, it resembles Zn .
As a rule, only very low cytosolic calcium concentrations are necessary (about 0.1 to 1 µM).
2+
The extracellular Ca concentrations are about 1 mM, and they can go up to 5 mM in special
cellular compartments (SR; see below). The exchange between the extra- and intracellular
space is achieved by Ca-ATPases (formation of ATP in case of a transport with the
concentration gradient; see for a Na,Ca-ATPase below left). Malfunctions of the calcium
metabolism can result in the deposition of sparingly soluble calcium compounds (oxalate,
phosphate, steroids) in the blood vessels (where they cause calcification and cardiovascular
diseases) and secretion organs (where these deposits are responsible for stones in gall, bladder,
kidney).

extra intra O NH
Ca 2+ O O
ADP + P i → ATP H O Ca
2
2Na + O O O