Definition
The protein troponin is an important component of the contractile apparatus of the heart and skeletal muscles. Together with tropomyosin, its main task is the regulation of muscle contraction at the microscopic level. Troponin is a complex of the building blocks troponin T, I and C, each of which has its own partial function and can only function together.
Both the skeletal muscle and the heart muscle each have their own group of these different troponins, which differ in their structure and function. Therefore, the so-called troponin value is of great importance in the diagnosis of various heart diseases. In the so-called smooth muscles, such as in the intestinal wall, troponin is completely absent.
Components of the troponin complex
Troponin T is the largest subunit of the troponin complex. Together with troponin I and C, it regulates the conversion of the electrical nerve signal into a muscle contraction. In doing so, it acts as a brake on the contractile muscle proteins via tropomyosin.
When a nerve signal reaches the muscle, this brake is released when the muscle proteins are released. These can now contract unhindered. There are three types of Troponin T in the body, so-called isoforms.
One is typical for the heart muscle, two others are found in the skeletal muscles. The troponin T of the skeletal muscle is further divided into one form, which is mainly found in slow but persistent muscles, and one, which is mainly found in muscles of the faster type. The form of troponin T typical of the heart muscle is also found in high concentrations only in the heart.
It is therefore of great importance for clinical diagnostics. The so-called hs – troponin T is determined in particular. If we talk about elevated troponin values, we usually mean an increase in the troponin T concentration in the blood.
As part of the troponin complex, troponin I is also involved in the regulation of muscle strength. On the one hand, it serves to fix the entire troponin complex to its position in the muscle cell. According to new findings, however, it also has a regulatory effect together with troponin T and tropomyosin.
This is done by preventing muscle contraction until the signal for contraction is received via a nerve. Just like troponin T, troponin I has three isoforms. While one is found only in the heart muscle, the other two are distributed between fast and slow muscle fibers in the skeletal muscle.
Only about 4% of troponin I is freely available in the muscle cell, i.e. it is not bound to proteins of the contractile apparatus or involved in the troponin complex. When the muscle cell is damaged, this free portion is first released into the blood where it can be detected chemically in the laboratory. Although troponin C is the smallest protein in the troponin complex, it plays a key role in the regulation of muscle contraction.
When a nerve activates a muscle, the concentration of free calcium ions increases inside the muscle due to electrical activation. A single troponin C in turn binds four of these calcium ions and can then induce a change in the shape of troponin I and T. Only now can the muscle contract. This is known as electromechanical coupling, because the electrical signal from the nerve is converted into mechanical muscle movement. Unlike troponin T and I, there is no myocardial-specific form of troponin C. Only so-called fast-twitching skeletal muscles have their own isoform of troponin C, whereas slow-twitching skeletal muscles and the heart muscle share a second isoform. Since an increase in the concentration of troponin C is therefore not specific for one of the two muscle groups, it is only determined in exceptional cases in the laboratory.