Calmodulin
Calcium binding protein involved in intracellular calcium signalling. [1]
Structure
Highly conserved sequence of 152 <a href="Amino acids">amino acids</a> http://www.ncbi.nlm.nih.gov/protein/CAA36839.1
There are four <a href="EFh domains">EFh domains</a> which are responsible to bind 4 <a href="Calcium">Ca2+</a> <a href="Ions">ions</a>http://smart.embl-heidelberg.de/smart/job_status.pl?jobid=939661123169621289746763YwAuDuwGQT
The arrangement of the Ca2+ binding sites are brought about by the N- and C- terminal lobes http://www.cell-signalling.org/csb/004/csb004.pdf.
Calmodulin are dumbbell shaped protein where long and flexible <a href="Alpha-helix">alpha helix</a> connects two <a href="Globular domains">globular domains</a>. Each domain is assembled from two <a href="EF-hand">EF-hand</a> regions attached to antiparalel <a href="Beta-sheet">beta-sheet</a>. Ca2+ binds to <a href="Glutamate">glutamate</a> and <a href="Aspartate">aspartate</a> residues placed in the loop of EF-hand.John T.Hancock (2005).Cell signalling. New York:Oxford University press
It is closely related to <a _fcknotitle="true" href="Troponin C">Troponin C</a>, which is found in the <a _fcknotitle="true" href="Actin filaments">Actin filaments</a> of <a _fcknotitle="true" href="Skeletal muscle">Skeletal muscle</a> cells.W.Boron and E Boulpaep, (2012) Medical Physiology (2nd Ed) p258
Function
Calmodulin functions as a multi-purpose intracellular receptor, governing many Ca2+ regulated processes. [2]
Two or more Ca2+ ions bind to induce a conformational change and activate calmodulin [3]. This results in amplifying the affinity of Ca2+ molecule which later affects other signalling components such as Ca2+/calmodulin-dependent protein kinases (CaMKs), myosin light chain kinase (MLCK), phosphorylase kinase, neuromodulin and so on [4].
In the case of MLCK, four Ca2+ ions bind Calmodulin to form a Ca2+-CaM complex. It is this complex that then activates MLCK, leading to the Phosphorylation of the myosin light chain and increased ATPase activity in smooth muscle cells. [5]