And both of these polypeptide chains essentially are composed of these right handed alpha helixes and these right handed alpha helixes together intertwined to form a left handed coil known as the alpha coiled coil. Now, how exactly are these polypeptide chains? How exactly are these two subunits actually held together? So notice because we have two subunits in the alpha carotene, this is an example of a dimer. So a quarterinary protein that contains coordinate structure that is actually a dimer because it consists of two subunits. So how are these two subunits actually held together?