Titin is a protein that in humans is encoded by the TTN gene. It is a giant protein that functions as a molecular spring which is responsible for the passive elasticity of muscle. It is composed of 244 individually folded protein domains connected by unstructured peptide sequences. These domains unfold when the protein is stretched and refold when the tension is removed. Titin is important in the contraction of striated muscle tissues. With its length of ~27,000 to ~33,000 amino acids—depending on the splice isoform— titin is the largest known protein. (Wikipedia)
Interestingly enough, due to it’s size, it’s actual name is 189,819 letters long. A man was recorded while spending about three hours trying to pronounce the word. If you want to see the length of the word itself, view this link:
The word is also sometimes considered the lengthiest in the English language.