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? Well, they're held together by Covalent and non Covalent interactions. So we have Vanderwal's forces that are basically the London dispersion forces between the non polar side chains of the amino acids found on these two opposing subunits. We also have ionic bonds which are basically the bonds between the negatively charged side chains and the positively charged side chains.