Titin

Titin, also known as connectin (UniProt name: Q10466_HUMAN; accession number: Q10466), is a protein that is important in the contraction of striated muscle tissues. Titin connects the Z line to the M line in the sarcomere. The protein limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle. Variations in the sequence of titin between different types of muscle (e.g. cardiac or skeletal) has been correlated with differences in the mechanical properties of the muscles.

Titin is the largest known protein, consisting of 26,926 amino acids. The molecular weight of the mature protein is approximately 2,993,451.39 Da, and it has a theoretical pI of 6.01. The protein's chemical formula is C132983H211861N36149O40883S693. It has a theoretical instability index (II) of 39.69, indicating that it would be stable in a test tube. The protein's in vivo half-life, the time it takes for half of the amount of protein in a cell to disappear after its synthesis in the cell, is predicted to be approximately 30 hours (in mammalian reticulocytes).

As the largest known protein, titin has the longest full chemical name. The full chemical name, containing 189,819 letters, is sometimes stated to be the longest word in the English language.