The ability to move is fundamental to life, and it's made possible in vertebrates by the phenomenon of muscle contraction. Three types of muscles exist in mammals: smooth, cardiac, and skeletal. The latter two are composed of thin and thick filaments, also known as sarcomeres, that enable muscle contraction.
Troponin and tropomyosin are two proteins present on the thin filaments of muscle cells, playing pivotal roles in muscle contraction. Interestingly, they perform opposing roles. Troponin facilitates muscle contraction, whereas tropomyosin inhibits it. These proteins operate based on the
sliding filament theory
. Let's delve deeper into the differences between troponin and tropomyosin.
Understanding Troponin
Troponin is a protein complex found in muscle cells, specifically in skeletal and cardiac cells. It consists of three subunits: Troponin C, Troponin T, and Troponin I, each performing a unique function.
When cellular calcium levels rise, the troponin attached to the actin filaments alters its shape, revealing the sites where myosin heads can bind. This structural change results in the binding of myosin to actin, causing muscle contraction.
Medically, troponin levels are significant. They are tested in conditions like myocardial infarction, stroke, and other acute cardiac conditions.
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Tropomyosin is a double-stranded, coiled, alpha-helical structure found within the cytoskeleton of the cytoplasm. It exists in both muscle and non-muscle cells, hence it's classified into muscle tropomyosins and non-muscle tropomyosins. The latter assists in cellular interactions and functions.
Muscle tropomyosin operates in low calcium conditions. In such circumstances, the tropomyosin proteins bind to the troponin proteins. This binding obscures the actin and myosin interaction site, preventing
muscle contraction
.
Comparing Troponin and Tropomyosin
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Biology
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No, troponin is not always bound to tropomyosin. The binding of troponin to tropomyosin depends on the level of calcium in the cells.
What is the difference between troponin I and troponin T?
The troponin T subunit is responsible for binding with the tropomyosin, forming a troponin-tropomyosin complex. The troponin I subunit, on the other hand, binds to the actin filament.
What does low troponin mean?
Low levels of troponin in the blood is not a major issue, but high levels of troponin are alarming and are indicative of cardiac damage.
What does tropomyosin cover on the actin?
Tropomyosin proteins form chains that run along the length of actin proteins and block up to seven subunits of the actin proteins.
What would happen without tropomyosin?
Without tropomyosin, the sites on the actin filaments will always be accessible to myosin heads, and our muscles will be in a constant state of contraction.