What is the Difference Between Troponin and Tropomyosin

The main difference between troponin and tropomyosin is that troponin is a complex of three regulatory proteins: troponin T, troponin C, and troponin I, whereas tropomyosin is a double-stranded coiled protein, which lies within the groove between actin filaments in muscle tissue. Furthermore, troponin is attached to tropomyosin, while tropomyosin is responsible for the contraction of cardiac and skeletal muscles.  

Troponin and tropomyosin are two types of proteins that occur in the cardiac and skeletal muscles, regulating muscular contraction via calcium-binding. 

Key Areas Covered 

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

Key Terms 

Cardiac Muscles, Muscular Contraction, Skeletal Muscles, Tropomyosin, Troponin

What is Troponin 

Troponin is a complex of three regulatory proteins, which play a key role in the contraction of cardiac and skeletal muscles. These three regulatory proteins are troponin C, troponin T, and troponin I. Generally, the sarcomere is the structural unit of a myofibril in striated muscles. Moreover, it is a repeating unit between two Z lines. Moreover, the two main types of proteins in these muscles are actin and myosin. Actin forms thin filaments while myosin forms thick filaments. They also have a long fibrous tail and a globular head. The globular head binds to actin as well as ATP. 

Figure 1: Troponin and Tropomyosin Structure

However, myosin can only bind to actin when the myosin-binding sites on actin are exposed. At rest, tropomyosin covers the myosin-binding sites on actin. Since troponin remains attached to tropomyosin, the main function of troponin is to move tropomyosin away, exposing myosin-binding sites.  

What is Tropomyosin 

Tropomyosin is a coiled protein consisting of double-stranded alpha-helical structure. It forms smooth and thin filaments, which lie along the α-helical groove of most actin filaments, covering the myosin-binding sites on the actin filaments at rest. Thus, this prevents muscular contraction. However, upon the stimulation by an action potential, muscle cells open calcium channels on the sarcoplasmic membrane, releasing calcium into the sarcoplasm. Routinely, this calcium binds to troponin, changing its conformation. Basically, the calcium-bound troponin moves tropomyosin filaments away from the myosin-binding sites on the actin filaments. Ultimately, the binding of actin to myosin filaments causes the cross-bridge formation, contraction the muscle fibers.

Figure 2: Muscle Contraction

Moreover, there are two groups of tropomyosin in the body as muscle tropomyosin isoforms and nonmuscle tropomyosin isoforms. While muscle tropomyosin isoforms regulate muscular contractions,  nonmuscle tropomyosin isoforms regulate the cytoskeleton and other cellular functions. 

Similarities Between Troponin and Tropomyosin 

• Troponin and tropomyosin are two types of proteins that form a complex in cardiac and skeletal muscles. 
• Moreover, they are responsible for the regulation of sarcomere contraction via calcium-binding. 
• However, their dysfunctions may cause a number of disease conditions. 

Difference Between Troponin and Tropomyosin 

Definition 

Troponin refers to a globular protein complex involved in muscle contraction, occurring with tropomyosin in the thin filaments of muscle tissue, while tropomyosin refers to a protein related to myosin, involving in muscle contraction. 

Structure 

While troponin is a complex of three regulatory proteins: troponin T, troponin C, and troponin I, tropomyosin is a double-stranded, alpha-helical, coiled protein, which lies within the groove between actin filaments in muscle tissue.  

Function 

Moreover, troponin binds to calcium ions, moving tropomyosin away from the myosin-binding sites on actin filaments while tropomyosin covers the myosin-bind9ing sites on the actin filament in a relaxed muscle, preventing contraction. 

Conclusion 

Basically, troponin is a complex protein made up of three subunits: troponin C, troponin T, and troponin I. Moreover, it attaches to the tropomyosin, which is a smooth protein filament, covering the myosin-binding sites on the actin filament. Thus, they prevent the contraction of cardiac and skeletal muscles at rest. However, upon binding to calcium ions, troponin removes tropomyosin from the actin filament, exposing myosin-binding sites on the actin filament, allowing contraction. Therefore, the main difference between troponin and tropomyosin is their structure and function.  

References:

1. Marques, Mayra de A, and Guilherme A P de Oliveira. “Cardiac Troponin and Tropomyosin: Structural and Cellular Perspectives to Unveil the Hypertrophic Cardiomyopathy Phenotype.” Frontiers in physiology vol. 7 429. 23 Sep. 2016, doi:10.3389/fphys.2016.00429.

Image Courtesy:

1. “Actin-myosin” By Jeff16 – Own work (CC BY-SA 4.0) via Commons Wikimedia   
2. “1008 Skeletal Muscle Contraction” By OpenStax (CC BY 4.0) via Commons Wikimedia