Difference between Troponin and Tropomyosin

Muscle contraction is a phenomenon in vertebrates that gives the organism an ability to move. There are three types of muscles in a mammal; smooth, cardiac and skeletal. Out of the three, cardiac and skeletal muscles are composed of thin and thick filaments called sarcomere that enable muscle contraction.

Troponin and tropomyosin are two proteins that are present on the thin filaments of the muscle cells, and help in the contraction of muscles. However, they have opposite functions. While troponin promotes muscle contraction, tropomyosin blocks muscle contraction. Both the proteins function on the basis of the sliding filament theory. Let us look at more such differences between troponin and tropomyosin in the table below.

What is Troponin?

Troponin is a protein found in muscle cells. However it is only found in skeletal and cardiac cells and not smooth cells. It is a protein complex that is made up of three subunits, Troponin C, Troponin T and Troponin I. Each subunit has its own unique function.

When the calcium levels in our cells increase, the troponin bound to the actin filaments changes its shape and exposes the sites of actin where the myosin heads can bind. This structural change leads to binding of the myosin to actin, and as a result there is a contraction of muscle.

Troponin levels are important medically. Levels of troponin in blood are checked for conditions like myocardial infarction, stroke, and other acute cardiac conditions.

What is Tropomyosin?

Tropomyosin is a double-stranded, coiled and an alpha helical structure found in the cytoskeleton of cytoplasm. They are present in both muscle and nonmuscle cells, and are hence categorised as muscle tropomyosins and nonmuscle tropomyosins. The nonmuscle tropomyosins help in cellular interactions and functions.

The muscle tropomyosin works in low calcium levels. When the concentration of calcium is low in cells, the tropomyosin proteins remain bound to the troponin proteins. This binding covers the site of actin and myosin interaction, and hence muscle contraction does not occur.

Troponin vs Tropomyosin

Troponin

Tropomyosin

Definition

Troponin is a protein found in muscle cells that is important for muscle contraction.

Tropomyosin is a protein found in the cytoskeleton in cytoplasm.

Present in

They are present in cardiac and skeletal muscle cells.

They are present in both muscle and nonmuscle cells.

Structure

It is a complex that is composed of three subunits: Troponin C, Troponin T and Troponin I.

It is a double-stranded, coil protein that is alpha-helical in nature.

Function

It binds with calcium that leads to a change in its shape. This modified shape renders the myosin sites active, thus promoting muscle contraction.

Tropomyosin functions to block the contact between myosin heads and actin filaments, thereby preventing muscle contraction.

Calcium Levels

It works in high intracellular calcium levels.

It works in low intracellular calcium levels.

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Frequently Asked Questions

Is troponin always bound to tropomyosin?

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 blood is not a major issue, but high levels of troponin is alarming and is 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 upto 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.

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