CALMODULIN: STRUCTURE, FUNCTIONS AND MECHANISM OF ACTION - BIOLOGY - 2025

Calmodulin is a term that comes to mean "calcium modulated protein" and refers to a small intracellular protein that has the property of binding to the calcium ion (Ca ++) and mediating many of its intracellular actions. The origin of the word comes from the combination of the English words 'calcium', 'modulated' and 'protein' which, when put together, derive in CAL cium MODUL ated prote IN.

Among the mineral elements that become part of the constitution of animal organisms, calcium, followed by phosphorus, is by far the most abundant, since bone is formed by the deposition in its matrix of large amounts of mineral salts formed from this ion.

Schematic of calmodulin and its calcium-binding sites (Source: PDB via Wikimedia Commons)

Of course, these mineral salts of calcium are essential for the constitution and conformation of the skeletal system of vertebrates, but it is the ionized form of calcium (Ca ++) in solution in body fluids that acquires a relevant physiological importance for the life of animals. organisms.

This cation, with two excess positive electrical charges in its structure, can act as a current transporter by moving through the cell membrane and modifying its level of electrical potential in many of the excitable cells of the body, mainly in the heart muscle.

But of greater physiological relevance is the fact that many cellular regulatory reactions triggered by external stimuli, such as neurotransmitters, hormones, or other physical or biochemical factors, are species of metabolic cascades in which several proteins sequentially participate, some of which are enzymes that require calcium for their activation or inactivation.

It is then said in these cases that calcium acts as a second messenger in a metabolic cascade destined to have a final result that would be like the cellular response necessary to satisfy a need detected at a different level than the cell itself, and that it requires hers that particular answer.

Calcium can act directly on its biochemical target to influence its activity, but it often requires the participation of a protein with which it must bind in order to exert its effect on the protein (s) to be modified. Calmodulin is one of those mediator proteins.

Structure

Calmodulin, highly ubiquitous, since it is expressed in almost all cell types of eukaryotic organisms, is a small acidic protein with a molecular weight of about 17 kDa, the structure of which is highly conserved among species.

It is a monomeric protein, that is, it is made up of a single polypeptide chain, which at its terminal ends takes the form of globular domains joined by an alpha helix. Each globular domain has two motifs known as EF hand (EF hand) that are typical for calcium-binding proteins.

Calmodulin bound to four calcium ions (Source: Webridge via Wikimedia Commons)

These "EF hand" topological motifs represent a kind of supersecondary structures; They are linked to each other, in each globular domain, by a region of great flexibility and in each of them there is a binding site for Ca ++, which yields 4 sites in total for each calmodulin molecule.

The binding of positively charged calcium ions is made possible by the presence of amino acid residues with negatively charged side chains at the calcium-binding sites of calmodulin. These residues are three aspartates and one glutamate.

Functions of calmodulin

All the functions known so far for calmodulin are framed within a group of actions promoted by the increases in cytosolic calcium produced by its entry from the extracellular space or its exit from the intracellular deposits: mitochondria and endoplasmic reticulum.

Many of the actions of calcium are carried out by this ion acting directly on its target proteins, which can be of different types and functions. Some of these proteins cannot be directly influenced, but require calcium to bind to calmodulin and it is this complex that acts on the protein influenced by the ion.

These target proteins are said to be calcium-calmodulin dependent and include dozens of enzymes, such as protein kinases, protein phosphatases, nucleotide cyclases, and phosphodiesterases; all of them involved in a myriad of physiological functions that include:

- The metabolism

- The transport of particles

- Visceral mobility

- The secretion of substances

- Fertilization of the ovules

- Genetic expression

- Cell proliferation

- The structural integrity of cells

- Intercellular communication, etc.

Among the calmodulin-dependent protein kinases, the following are mentioned: myosin light chain kinase (MLCK), phosphorylase kinase, and Ca ++ / calmodulin kinases I, II and III.

Thus, the information encoded by calcium signals (increase or decrease in its intracellular concentration) is "decoded" by this and other calcium-binding proteins, which convert the signals into biochemical changes; in other words, calmodulin is an intermediate protein in calcium-dependent signaling processes.

Mechanism of action

Calmodulin is a very versatile protein, as its "target" proteins are considerably diverse in shape, sequence, size, and function. Since it is a protein that functions as a “sensor” for calcium ions, its mechanism of action depends on the changes induced in its structure and / or conformation once it binds to four of these ions.

Its mechanisms of action can be exemplified by briefly reviewing its participation in a couple of physiological processes such as contraction of visceral smooth muscle and adaptation to odors suffered by the hair cells of the olfactory mucosa in the nose.

Calmodulin and smooth muscle contraction

Structure of the myosin 1A and calmodulin cross-bridges in the actin bundles of microvilli. Source: Jeffrey W. Brown, C. James McKnight

Skeletal and cardiac muscle contraction is triggered when the increase in cytosolic Ca ++ reaches levels above 10-6 mol / l and this ion binds to troponin C, which undergoes allosteric changes that affect tropomyosin. In turn, tropomyosin moves, exposing its myosin-binding sites in actin, causing the contractile process to fire.

Troponin C does not exist in smooth muscle, and the increase in Ca ++ above the indicated level promotes its binding with calmodulin. The Ca-calmodulin complex activates myosin light chain kinase (MLCK), which in turn phosphorylates this light chain, activates myosin, and triggers the contractile process.

The increase in Ca ++ occurs through its entry from the outside or its exit from the sarcoplasmic reticulum by the action of inositol triphosphate (IP3) released by phospholipase C in the cascade activated by receptors coupled to Gq protein. Relaxation occurs when Ca ++, by the action of transporters, is removed from the cytosol and returns to its sites of origin.

An important difference between both types of contraction is that in striated muscles (cardiac and skeletal) Ca ++ induces allosteric changes by binding with its protein, troponin, while in smooth muscles the changes produced by Ca-calmodulin are covalent and involve phosphorylation of myosin.

Therefore, once the action of Ca ++ has ended, the participation of another enzyme is required to remove the phosphate added by the kinase. This new enzyme is myosin light chain phosphatase (MLCP) whose activity does not depend on calmodulin, but is regulated by other pathways.

In reality, the contractile process of smooth muscle does not cease completely, but the degree of contraction remains at an intermediate level as a result of the balance of the actions of both enzymes, the MLCK controlled by Ca ++ and calmodulin, and the MLCP subjected to other regulatory controls.

Adaptation in olfactory sensors

The odorous sensation is triggered when olfactory receptors located in the cilia of cells located on the surface of the olfactory mucosa are activated.

These receptors are coupled to a heterotrimeric G protein known as "Golf" (olfactory G protein), which has three subunits: "αolf", "ß" and "γ".

When olfactory receptors are activated in response to an odor, the subunits of this protein dissociate and the "αolf" subunit activates the enzyme adenyl cyclase, producing cyclic adenosine monophosphate (cAMP).

The cAMP activates CNG-like channels (activated by cyclic nucleotides) for calcium and sodium. These ions enter the cell, depolarize it and cause the initiation of action potentials whose frequency will determine the intensity of the odor.

The calcium that enters, which tends to depolarize the cell, has an antagonistic effect of negative feedback, somewhat later, by binding to calmodulin and between them closing the channel and eliminating the depolarizing stimulus, even though the odorous stimulus persists. This is called sensor matching.

Calmodulin in plants

Plants also respond to differences in the intracellular concentration of calcium ions by means of the calmodulin protein. In these organisms, calmodulins share many structural and functional characteristics with their animal and yeast counterparts, although they differ in some functional aspects.

For example, calmodulin in plants binds to short peptide sequences within their target proteins, inducing structural changes that alter their activities in response to internal variations in calcium.

To what extent calmodulin controls processes analogous to those that occur in animals in plants is still a matter of discussion today.

References

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