Calmodulin, calcium binding sites

M.-C. Kilhoffer, J. G. Demaille, and D. Gerard, Terbium as luminescent probe of calmodulin calcium-binding sites. Domains I and II contain the high-affinity sites, FEBS Lett. 116, 269-272 (1980). 

P/Q-type channels undergo a second calcium-dependent process whereby current activity increases following a strong membrane depolarization or a train of action potentials. This increase is only observed in the presence of external calcium, is insensitive to calcium buffers, and requires the high-affinity calcium binding sites on calmodulin (Chaudhuri et al. 2004 DeMaria et al. 2001). This process, termed calcium-dependent facilitation (CDF), is not observed with N-type or R-type channels and is Cav2.1 channel splice isoform dependent (Chaudhuri et al. 2004). The... 

Calcium ions are also transported into the cell by a pump, which is a Ca +-dependent ATPase. This pump is necessary because the calcium ion concentration is four orders of magnitude higher outside than inside living cells. Calmodulin regulates the level of calcium ions and hence the calcimn pump. When the calcium concentration decreases, calcium is dissociated from calmodulin and the calcium pump is inactivated. The structure of such a pump from the sarcoplasmic reticulum is reported at 8 A resolution. This pump couples ATP hydrolysis with cation transport. The protein contains 10 transmembrane helices. A distinct cavity was located that led to the putative calcium-binding site, suggesting a path for a calcium passage. 

The affinities of the two peptides for four calcium binding site mutants of calmodulin also provide important information. The B2K and B2Q calmodulins have Glu67 (in binding site 2) mutated to Lys and Gin respectively, and B4K and B4Q calmodulins have Glul40 (in binding site 4) mutated to Lys and Gin respectively. Each of these mutations effectively eliminates calcium binding to the altered site (3). 

Calmodulin (CaM), a 17-kd protein with four calcium-binding sites, serves as a calcium sensor in nearly all eukaryotic... 

Phosphorylase kinase activity has an absolute requirement for Ca +, which binds to the 5-subunit. The amino acid sequence of this subunit is nearly identical to that of calmodulin, with four calcium binding sites, but unlike calmodulin, the 5-subunit is an integral part of the enzyme and does not dissociate from it in the absence of Ca +. In the presence of Ca +, kinase activity is further increased by phosphorylation of the a- and j6-subunits, catalyzed by cAMP-dependent protein kinase and several other kinases. Phosphorylation may activate the enzyme by disinhibiting... 

Figure 4. The local structures of the classic EF-hand calcium binding site 1 of calmodulin (A) and the relocated Site 1 (B). The local structures of the calcium binding site lof calbindinnst with a pseudo-EF-hand in the natural protein (C) and the relocated Site 1 (D). The calcium binding site 2 of calbindinnsk with a classic-EF-hand in the natural protein (E) and the relocated Site 2 (F).

Figure 5. The deviation of the designed sites fiom the ideal pentagonal bipyramidal geometry (pseudoenergy U(p)) as a function the designed calcium binding sites. The natural calcium binding sites I-IV of calmodulin are shown as black dots.

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