Titin is the largest known naturally occurring protein and is composed of 34,350 amino acids. It is also known as connectin and is abundant in striated muscle. Though it is 189,819 letters long, its status as the longest word in the English language is debatable, given that it is a technical word not found in the dictionary.
A man on YouTube read the name for nearly 3.5 hours straight. See it here: http://bit.ly/176k5db