What is the Difference Between Troponin I and Troponin T

The main difference between troponin I and troponin T is that troponin I binds to actin in thin microfilaments to hold the troponin-tropomyosin complex whereas troponin T binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex. Furthermore, troponin I inhibits ATPase activity of actomyosin while troponin T regulates the interaction of troponin complex with thin filaments. 

Troponin I and troponin T are two of the three subunits of troponin, which regulates the contraction of both cardiac and skeletal muscles. However, smooth muscles do not contain troponin. 

Key Areas Covered 

1. What is Troponin I
     – Definition, Structure, Function
2. What is Troponin T
     – Definition, Structure, Function
3. What are the Similarities Between Troponin I and Troponin T
     – Outline of Common Features
4. What is the Difference Between Troponin I and Troponin T
     – Comparison of Key Differences

Key Terms 

Cardiac Muscles, Muscle Contraction, Smooth Muscles, Troponin, Troponin I, Troponin T

What is Troponin I 

Troponin I (TnI) is one of the three regulatory proteins or subunits of the troponin complex. The other two subunits are troponin C and troponin T. Generally, the main function of the troponin complex is to regulate the contraction of both skeletal and cardiac muscles. Furthermore, in the troponin complex, troponin I binds to the actin in thin microfilaments to hold the troponin-tropomyosin complex in place. Due to this binding, myosin cannot bind to the actin in relaxed muscles. 

Figure 1: Troponin-Tropomyosin Complex

Furthermore, troponin I inhibits the ATPase activity of the actomyosin. However, with the binding of calcium to the troponin C, which is the third subunit of the troponin complex, troponin I undergoes conformational changes. Moreover, this leads to the dislocation of troponin I, leaving tropomyosin from the complex. Ultimately, this makes the binding site available for myosin, resulting in the contraction of muscles. 

What is Troponin T 

Troponin T (TnT) is another subunit of the troponin T complex in the skeletal muscles and cardiac muscles. The main function of it in the troponin-tropomyosin complex is to bind to tropomyosin. Both troponin I and troponin T consist of different forms in different types of muscles: cardiac and skeletal muscles. Basically, two isoforms of troponin I and two isoforms of troponin T occur in skeletal muscles. However, only a single isoform of troponin I (cTnI) occurs in the cardiac muscles. Therefore, this isoform is tissue-specific. 

Figure 2: Troponin Function

On the other hand, cardiac troponin T subtype is especially useful in the diagnosis of heart attacks as the damage into the heart muscles releases cardiac troponin T into the bloodstream.  

Similarities Between Troponin I and Troponin T 

• Troponin I and troponin T are two subunits of the troponin complex. 
• They are responsible for the regulation of the contraction of cardiac and skeletal muscles. 
• Moreover, they do not occur in smooth muscles. 
• The levels of cardiac-specific troponin I and T serve as diagnostic and prognostic indicators in the management of myocardial infarction and acute coronary syndrome. 

Difference Between Troponin I and Troponin T 

Definition 

Troponin I refers to a part of the troponin protein complex, which binds to actin in thin microfilaments to hold the actin-tropomyosin complex, while troponin T refers to another part of the troponin complex, especially useful in the laboratory diagnosis of a heart attack. 

Molecular Weight 

The molecular weight of cardiac troponin I is 23.9 kDa while the molecular weight of cardiac troponin T is 37 kDa. 

Function 

Troponin I binds to the actin filaments to hold the troponin-tropomyosin complex in place while troponin T binds to the tropomyosin to form the troponin-tropomyosin complex. 

Other Functions 

Furthermore, troponin I inhibits ATPase activity of actomyosin while troponin T regulates the interaction of troponin complex with thin filaments. 

Conclusion 

Basically, troponin I is one of the three subunits of troponin, binding to the actin filaments of both skeletal and cardiac muscles. Moreover, it is responsible for holding the troponin-tropomyosin complex in place in the actin filament. On the other hand, troponin T is another subunit of the troponin, holding the troponin-tropomyosin complex. Generally, the main function of troponin is to regulate the contraction of cardiac and skeletal muscles. However, the main difference between troponin I and troponin T is their function in the troponin complex.   

References:

1. Stark M, Sharma S. Troponin. [Updated 2019 Feb 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available Here.

Image Courtesy:

1. “Troponino” By Arcadian – Troponino.jpg (Attribution) via Commons Wikimedia   
2. “Troponin-activation” (CC BY-SA 3.0) via Commons Wikimedia