Calmodulin

• the EF hand

  • protein motif
  • resembles a hand
  • characterised by helix-loop-helix
• Kd:

  • if Kd is low, then the affinity of the ligand for the receptor is high
  • calculated using ratio
  • unit expressed in concentration (M)
• EF hand can bind to calcium loosely
• EF hand is formed by 29 amino acids
• calmodulin is highly conserved among different species
• calmodulin stands for calcium modulated protein
• calmodulin contains EF hand motifs
• calmodulin changes shape when bound to calcium
• calmodulin participates in numerous biological pathways
• EF hand was named due to Ca2+ binding site being between the E and F alpha helices in parvalbumin
• what is a motif?

  conserved amino acid sequence alignment
• EF hand

  label

  A) EF hand

  
• Kd is a measure of the affinity between 2 molecules
• Kd is calculated by

  k2/k1

  • where k1 is the association rate constant and k2 is the dissociation rate constant
• if Kd is low then the affinity of the ligand for the receptor is ver y high
• if Kd is high then the affinity of the ligand for the receptor is very low
• Kd is the concentration at which 50% of the ligand is free and 50% is bound to the receptor
• members of the EF hand family are found solely in the cytosol
• EF hands bind to calcium with an affinity of Kd~10^6
• calcium ion concentrations

  • extracellular= 10^-3
  • intracellular (unstimulated)= 10^-7
  • intracellular (stimulated)= 10^-5/10^-6
• there are free calcium ions at a low concentration in an unstimulated cell because


  calcium ion concentration is lower than EF hand Kd
• the first alpha helix of the EF hand has a Glu at position 1
• first alpha helix has hydrophobic residues facing the core of the molecule at position 2, 5, 6 and 9
• second alpha helix has a Glu at position 21
• second alpha helix has hydrophobic residues at positions 22, 25, 26 and 29
• alpha helices are amphipathic
• residues that bind calcium ions

  • acidic amino acids (carboxylate oxygen atoms that can ligate calcium)
• glycine at position 15 permits a sharp bend in the structure of the EF hand
• calmodulin

  • 17kDa protein
  • highly conserved amino acid sequence
  • 4 EF hands
  • binds to calcium and changes shape to be able to activate kinases
• calcium is an important secondary messenger involved in muscle contraction
• calcium is important in release of hormones and neurotransmitters
• calmodulin is one of the main effectors of calcium signalling
• primary structure of calmodulin

  labels

  A) Ca2+

  
• structure of calmodulin

  • dumbbell shape
  • both terminals has 2 EF hands each
  • terminals EF hands are separated by 6-turn single alpha helix
• apo calmodulin is a structure that is missing its ligand or binding partner
• binding of calcium causes a conformational change, which exposes a hydrophobic patch in each globular domain
• calmodulin folds at a central region (Ser81) to expose the hydrophobic patch
• when binding to target peptides

  • long helix of the dumbbell unwinds
  • 2 lobes of calmodulin swing to enfold the alpha helical target peptide
  • hydrophobic patch on each lobe contacts the hydrophobic side of the helical target peptide
  • target peptide sits in the hydrophobic channel
• target enzymes of calmodulin

  • phosphorylase kinase
  • myosin light chain kinase
  • adenylate cyclase
  • Ca2+ ATP-ase