TROPONIN: CHARACTERISTICS, STRUCTURE, FUNCTIONS AND TEST - BIOLOGY - 2025

Troponin is the name given to a protein present in the skeletal and cardiac muscle of vertebrates, associated with the filaments in muscle fibers and which has functions in the regulation of contractile activity (muscle contraction and relaxation).

Muscle fibers are the cells that make up muscle tissue, whose contraction capacity is based on the interaction between some filaments that are ordered and closely associated inside, occupying most of the cytoplasmic volume.

Graphic representation of the elements of a thin filament in muscle fibers (Source: Raul654, via Wikimedia Commons)

These filaments are known as myofilaments and there are two classes: thick and thin. The thick filaments are composed of myosin II molecules, while the thin filaments are polymers of globular actin or G-actin in association with two other proteins.

Both actin and myosin are also found in other cells of the human body and other organisms, only in a much smaller proportion and participating in different processes such as cell migration, exocytosis, in cytokinesis (during cell division) and even in intracellular vesicular traffic.

Troponin and tropomyosin are the two proteins associated with the thin actin filaments that participate in the regulation of the contraction and relaxation processes of the myofibrils of muscle cells or fibers.

The mechanisms of action through which these two proteins exert their function are related to the intracellular concentration of calcium. The troponin regulatory system is one of the best known systems in the physiology and biochemistry of skeletal muscle contraction.

These proteins are of great importance for the body. At present, it is known for sure that some familial or congenital cardiomyopathies are the product of mutations in the sequence of the genes that code for either of the two (troponin or tropomyosin).

characteristics

Troponin is associated with actin in the thin filaments of muscle fibers in skeletal and cardiac muscle in a stoichiometric ratio of 1 to 7, that is, one molecule of troponin for every 7 molecules of actin.

This protein, as has been highlighted, is found exclusively in the filaments contained within the myofibrils of skeletal and cardiac striated muscle fibers, and not in the smooth muscle fibers that make up the vascular and visceral muscles.

It is conceived by some authors as the regulator protein of tropomyosin. Like this, it has binding sites for the interaction with actin molecules, which gives it the ability to regulate its interaction with the myosin of the thick filaments.

In myofilaments, the ratio between troponin and tropomyosin molecules is 1 to 1, which means that for each troponin complex that exists there is a tropomyosin molecule associated with it.

Structure

Troponin is a protein complex made up of three different globular subunits known as troponin I, troponin C, and troponin T, which together add up to more or less 78 kDa.

In the human body there are tissue-specific variants for each of these subunits, which differ from each other both at a genetic and molecular level (with respect to the genes that encode them), and at a structural level (with respect to their amino acid sequences).

Representation of one of the Troponin subunits (Source: Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute via Wikimedia Commons)

Troponin C or TnC is the smallest of the three subunits and perhaps one of the most important. It has 18 kDa molecular weight and has sites to bind calcium (Ca2 +).

Troponin T or TnT is the one that has the binding sites to anchor the complex of the three subunits to tropomyosin and has a molecular weight of 30 kDa; it is also known as T subunit or tropomyosin binding subunit.

Troponin I or TnI, with little more than 180 amino acid residues, has the same molecular weight as troponin T, but in its structure it has the special sites to bind actin, blocking the interaction between the latter and myosin, which is the phenomenon responsible for the contraction of muscle fibers.

Many textbooks refer to this subunit as the inhibitory subunit and as the molecular "glue" between the three subunits of troponin. Its ability to bind to actin and its inhibitory activity are enhanced by its association with tropomyosin, mediated by the TnT subunit.

It has been shown that, in subunit I, the region of the sequence responsible for the inhibition is defined by a central peptide of 12 amino acid residues between positions 104 and 115; and that the C-terminal region of the subunit also has a role during inhibition.

Features

Troponin's main role in muscle contraction depends on its ability to bind calcium, since this protein is the only component of the thin filaments in skeletal muscle that has this property.

In the absence of troponin, thin filaments are able to bind to thick filaments and contract, regardless of the intracellular calcium concentration, so the function of troponin is to prevent contraction in the absence of calcium through its association with tropomyosin.

Thus, troponin plays an important role in maintaining muscle relaxation when there is not enough intracellular calcium, and in muscle contraction when the electrical nerve stimulus allows calcium to enter the muscle fiber.

How does this happen?

In skeletal and cardiac striated muscles, muscle contraction occurs due to the interaction between thin and thick filaments that slide over each other.

In the cells of these muscles, calcium is essential for the actin-myosin interaction (thin and thick filaments) to occur, since the actin binding sites for myosin are "hidden" by the joint action of tropomyosin and troponin, which is the one that responds to calcium.

Calcium ions from the sarcoplasmic reticulum (the endoplasmic reticulum of muscle fibers) bind to the C subunit of troponin, neutralizing troponin-mediated inhibition and triggering muscle contraction.

The "neutralization" of the inhibition caused by subunit I occurs after the binding of calcium to subunit C, which generates a conformational change that spreads between the three subunits and allows their dissociation from both actin and tropomyosin molecules.

This dissociation between troponin, tropomyosin, and actin reveals the binding sites for myosin on actin. It is then that the globular heads of the latter can interact with actin fibers and initiate ATP-dependent contraction by displacement of one filament over the other.

Troponin test

Troponin is the preferred biomarker for the detection of cardiac lesions. For this reason, the troponin test is widely used in the biochemical, early and / or preventive diagnosis of some cardiac pathological conditions such as acute myocardial infarction.

This test is found by many treating physicians to help make decisions about what to do and what treatment to administer to patients with chest pain.

It is generally associated with detection of troponin T and I subunits, since the troponin C isoform is also found in slow-twitch skeletal muscles; that is, it is not specific to the heart.

What is the troponin test based on?

The troponin test is usually an immunological test that detects the cardiac isoforms of the T and I subunits of troponin. So, it is based on the differences that exist between both isoforms.

Isoform of the troponin I subunit (cTnI)

In myocardial muscle tissue there is only one isoform of troponin I subunit, characterized by the presence of a 32 amino acid post-translational "tail" at its N-terminal end.

This isoform is detected thanks to the development of specific monoclonal antibodies that do not recognize other non-cardiac isoforms, since the tail of amino acids is more or less 50% different from the ends of other isoforms.

CTnI is not expressed in damaged tissues, but is unique to adult cardiac tissue.

Isoform of the troponin T subunit (cTnT)

The cardiac isoform of the troponin T subunit is encoded in three different genes, the mRNAs of which can undergo alternative splicing resulting in the production of isoforms with variable sequences at the N- and C-termini.

Although the human heart muscle contains 4 isoforms of TnT, only one is characteristic of adult heart tissue. This is detected with specific antibodies designed against the N-terminal end of its amino acid sequence.

"Next generation" tests for the T subunit of the cardiac isoform pay close attention to the fact that some injured skeletal muscle tissue can re-express this isoform, thus cross-reaction with antibodies can be obtained.

References

1. Babuin, L., & Jaffe, AS (2005). Troponin: the biomarker of choice for the detection of cardiac injury. CMAJ, 173 (10), 1191-1202.
2. Collinson, P., Stubbs, P., & Kessler, A.-C. (2003). Multicentre evaluation of the diagnostic value of cardiac troponin T, CK-MB mass, and myoglobin for assessing patients with suspected acute coronary syndromes in routine clinical practice. Heart, 89, 280–286.
3. Farah, C., & Reinach, F. (1995). The troponin complex and regulation of muscle contraction. FASEB, 9, 755–767.
4. Keller, T., Peetz, D., Tzikas, S., Roth, A., Czyz, E., Bickel, C.,… Blankenberg, S. (2009). Sensitive Troponin I Assay in Early Diagnosis of Acute Myocardial Infarction. The New England Journal of Medicine, 361 (9), 868–877.
5. Ross, M., & Pawlina, W. (2006). Histology. A Text and Atlas with correlated cell and molecular biology (5th ed.). Lippincott Williams & Wilkins.
6. Wakabayashi, T. (2015). Mechanism of the calcium-regulation of muscle contraction. In pursuit of its structural basis. Proc. Jpn. Acad. Ser. B, 91, 321-350.