User:Anordquist/sandbox

Calmodulin Page

Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the binding of Ca2+ is required for the activation of Calmodulin. Once bound to Ca2+, Calmodulin acts as part of a calcium signal transduction pathway by modifying its interactions with various target proteins such as kinases or phosphotases.^[1][2]

Structure

Calmodulin is a small, highly conserved protein 148 amino acids long (16706 Daltons). The protein has two approximately symmetrical globular domains each containing a pair of EF-hand motifs (the N- and C-domain) separated by a flexible linker region. Each globular domain contains a pair of EF-hand motifs, which allows calmodulin to sense intracellular calcium levels by binding up to four Ca²⁺ ions. Calcium participates in an intracellular signaling system by acting as a diffusible second messenger to the initial stimuli.

Similarity to Troponin C

Calmodulin's structure is very similar to the structure of Troponin C (which is another calcium binding protein). They are both members of the EFh super family. Troponin C, like Calmodulin, has two globular domains that are connected by a linker region. However, Troponin C and Calmodulin differ in the length of the linker region; the linker region of Calmodulin is smaller than that of Troponin C. These remarkably similar structures are an example of how the EF hand motif is highly conserved in calcium binding proteins. Though they have similar structures, their functions are very different. Troponin C has a very specific function (to illicit a conformational change in Troponin I) ultimatley causing a contraction in skeletal muscles. Calmodulin, evolved to bind a wider variety of target proteins, allowing it to play a role in many physiological events.

Importance of Flexibility in Calmodulin

Since Calmodulin binds such a wide variety of target proteins, it is especially important for it to have flexibility. Though Calmodulin's flexibility is more evident when it is bound to a target protein, NMR studies have shown that the linker region of Calmodulin is flexible, even when it is not bound to a target protein. (INSERT PICTURE FROM PDB???)

Mechanism

Overall

Up to four calcium ions are bound by calmodulin via its four EF hand motifs. EF hands supply an electronegative environment for ioncoordination. After calcium binding, hydrophobic methyl groups from methionine residues become exposed on the protein via conformational change. Using both X-Ray and NMR studies, scientists were able to determine that the conformational changes occured in the alpha-helices of the EF motif, which changes the binding affinity for target proteins. When the alpha helices are perpendicular to one another, the Calmodulin is in an open confirmation giving it a higher affinity for target proteins.^[3] More specifically, this conformational change presents hydrophobic surfaces, which can in turn bind to Basic Amphiphilic Helices (BAA helices) on the target protein. These helices contain complementary hydrophobic regions. The flexibility of calmodulin's hinged region allows the molecule to wrap around its target. This property allows it to tightly bind to a wide range of different target proteins. The C-domain of calmodulin has a higher affinity for calcium than does the N-domain.

Dynamic features

Compared to the X-ray crystal structure, the C-terminal domain solution structure is similar while the EF hands of the N-terminal domain are considerably less open. The backbone flexibility within calmodulin is key to its ability to bind a wide range of targets.^[4] Protein domains, connected by intrinsically disordered flexible linker domains, induce long-range allostery via protein domain dynamics.

Function

General role in the body:

CaM mediates many crucial processes such as inflammation, metabolism, apoptosis, smooth muscle contraction, intracellular movement, short-term and long-term memory, and the immune response. CITE. CaM is expressed in many cell types and can have different subcellular locations, including the cytoplasm, within organelles, or associated with the plasma or organelle membranes, but it is always found intracellularly. Many of the proteins that CaM binds are unable to bind calcium themselves, and use CaM as a calcium sensor and signal transducer. CaM can also make use of the calcium stores in the endoplasmic reticulum, and the sarcoplasmic reticulum. CITE CaM can undergo post-translational modifications, such as phosphorylation, acetylation, methylation and proteolytic cleavage, each of which has potential to modulate its actions. CITE

Specific examples:

Role in metabolism: (ex breakdown of glycogen)

Activation of phosphorylase kinase by calmodulin--which then activates glycogen phosphorylase which cleaves glucose from glycogen--allowing for thing such as glycolysis...

Lipid metabolism and calcitonin...

Role in smooth muscle contraction:

Calmodulin plays an important role in excitation contraction coupling and the initiation of the cross-bridge cycling in smooth muscle, ultimately causing smooth muscle contraction. In order to activate contraction of smooth muscle, the head of the myosin light chain must be phosphorylated. This phosphorylation is done by Myosin Light Chain (MLC) Kinase. This MLC Kinase is activated by a Calmodulin when it is bound by Calcium, thus making smooth muscle contraction dependent on the presence of Calcium, through the binding of Calmodulin and activation of MLC kinase.

Another way that Calmodulin affects muscle contraction is by controlling the movement of Ca2+ across both the cell and sarcoplasmic reticulum membranes. The Ca2+ channels, such as the ryanodine receptor of the sarcoplasmic reticulum, can be regulated by Calmodulin bound to calcium, thus affecting the overall levels of Calcium in the cell.

This is a very important function of Calmodulin because it indirectly plays a role in every physiological process that is affected by smooth muscle contraction such as digestion and contraction of arteries (which helps distribute blood and regulate blood pressure).

Role in short-term and long-term memory:

Long-term potentiation (LTP) requires a depolarization of GABAergic neurons in the hippocampus. Calmodulin Kinase II (CaM-K II) plays a crucial role in achieving LTP. It contributes to the phosphorylation of an AMPA receptor which increases the sensitivity of AMPA receptors. Furthermore, research shows that inhibiting CaM-K II interferes with LTP.