Titin Structure
A protein called Titin which is also known as Connectin is about 3.6 megawalt and it is composed of 30,000 amino acids and 320 protein domains.(1) This makes this the largest human protein known so far. Apart from that it’s chemical name is known to be the longest word in English with 189,819 letters and apparently it would take one three and half hours to pronounce it correctly. This protein is part of the smallest functional unit in the strained muscle called the sarcomere. It spans half the sarcomere starting from the N-terminus in the Z-line and the C-terminus in the M-line. (2) Titin has also been found to be a multi-domain structure similar to immunoglobulin and fibronectin. Present in titin are about 300 immunoglobulins and fibronectin and closer to the C-terminus are kinase domains. Overall, titin is about half the size of a sarcomere which includes the I-band, A-band, M-line and the Z-line. The I-band of titin consists of only Ig domains and other unique domains. The largest part of the protein is the A-band and it consists Ig and fibronectin domains. In this band the Ig and the fibronectin are arranged in long patterns called super repeats. Toward the end of the A-band there are small repeats. The M-line has the overlapped C-terminus region and the Z-line has the overlapped A-terminus region of Titin.
Titin’s soft elasticity is caused by its generic behavior of random coils due to PEVK or other similar domains. It has been revealed that the individual domains that are stretched due to a strong force reveal their secondary structure also known as “secondary structure elasticity.” Straightening of the multi-domain segments is also why titin has soft elasticity.
Function
The function of titin doesn’t not include muscle contraction it basically ensures elasticity and even the stability of the muscle. To do this it stabilizes the thick filaments by centering between the thin filaments. It also prevents the sarcomere from over-stretching by recoiling the sarcomere like a spring. Titin increases its length under some applied force and then returns to its original length once the applied force is removed.
Cardiac Muscle
Titin plays a crucial role in the determinant of diastolic function. It influences the rigidity of cardiomyocyte and even the passive properties of the ventricle. (CITE LAST). In the heart titin does more than determine ventricular rigidity; while the muscle is stretched it generates passive tension and while the sarcomere shortens it generates a restoring force. This allows a fast recovery rate of the resting length of the sarcomere. The restoring force due to the titin increases ventricular filling especially in the initial diastolic phase. This would be important during physical exertion or even tachycardia situations (CITE LAST). Titin also helps signaling the end of muscle contraction due to sarcomere’s shortened length lower than resting length, which in turn signals the titin to deactivate the cross-bridges.
Mutations
Titin plays a important role in the elasticity of muscles, but a mutation in a gene called TTN plays a huge role in heart failure. The mutation causes the muscles in the heart to become large and weak which makes pumping blood difficult in the body. The lack of oxygen and nutrients in the body causes shortness of breath and retains fluid. The only option in this case is to have a heart transplant. This condition is called dilated cardiomyopathy and about one percent of the population has this. Research from The National Heart Center Singapore and others has shown that even if the mutation has not been expressed one could be at the risk of heart failure. (3)
The conclusion that Titin was involved in this heart diseases took a long time was because Titin is a huge protein and back then it was hard and expensive to sequence something this big so they overlooked this protein. Over the last decade technology has drastically changed making it easier, cheaper and faster to sequence a large sequence. This is when they acknowledge the fact that a mutation in the TTN gene plays a big role in dilated cardiomyopathy. (4)
References
1. ↑ Castro-Ferreira, Ricardo, and Ricardo Fontes-Carvalho. "SciFinder." The role of titin in the modulation of cardiac function and its pathophysiological implications. Sociedade Brasileira De Cardiologia, n.d. Web. 13 Feb. 2017.
2. ↑ Benichou, Tamar, and Sefi Givli. The hidden ingenuity in titin structure. American Institute of Physics, n.d. Web. 13 Feb. 2017.
