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2+
2+
Contrasting Mg , which prefers octahedral coordination, Ca has a tendency to form
complexes of coordination number 7 or 8. Preferential ligands are H 2O, carboxylate (Asp,
Glu), the carbonyl groups of the peptide bonds, and alcoholate (Ser). An example is
2+
parvalbumin (see the picture above, right), a Ca protein in the smooth muscles, which takes
part in muscle relaxation.

2+
Ca also plays an essential role in muscle contraction. Muscle cells contain protein
filaments (so-called myofibrils), which are embedded within the sarcoplasmatic reticulum
2+
2+
(SR). The SR contains vesicles (ves) which store Ca at concentrations of 1-5 mM. Ca
storage is provided by calsequesterin, an acidic protein of 50 kDa molecular weight, which can
2+
2+
bind up to 50 Ca to Asp and Glu. Muscle contraction is achieved by release of Ca into the
cytoplasm (cyt) of the SR via the SR membrane, a process where there is again an ATPase (E)
involved, which switches between two conformations E 1 and E 2:

- Transport out of the vesicles into the cytoplasm (following the concentration gradient),
coupled with the synthesis of ATP, triggers the contraction of the muscle fibrils:
2+
2+
2Ca (ves) + E 2-phosphate + ADP → 2Ca (cyt) + E 1 + ATP

2+
- Return transport of the Ca ions from the cytoplasm into the vesicles results in muscle
relaxation and consumption of ATP:
2+
2+
2Ca (cyt) + E 1 + ATP → 2Ca (ves) + E 2-phosphate + ADP

2+
The activation of Ca -dependent enzymes is
initiated by proteins of the calmodulin family.
Calmodulin = calcium modulating protein. These + 4Ca
2+
are small proteins of 17 kDa molecular weight,
2+
2+
which can bind four Ca ions. Ca binding leads to
a conformational change (Fig. 34 top), allowing for
2+
coupling of Ca ⋅calmodulin to the enzyme (grey in
Fig. 34), the substrate of which (blue) becomes
activated (red). Examples are Ca-ATPases, NO-
synthases [see. ch. 8], NAD-kinases, adenylate-
cyclase.

Figure 34. Model for the activation of enzymes
2+
(grey; e.g. NO-synthase) by Ca -calmodulin. Blue:
substrate (e.g. arginine); red: activated substrate.