Proteins and Enzymes

ID #2180

From my understanding of the lecture notes, a dimer is a linkage between 2 subunits that make up tertiary or quaternary structure, correct? My question is how do we keep track of how many dimers are in a particular folded protein? For example, in hemoglobin, which has 2 alpha and 2 beta subunits, are there 2 homo dimers (b/w each pair of identical subunits) and one heterodimer (to link these two sets of identical subunits together)? Or how do we keep track of this?

A dimer occurs when two separate polypeptides are bound together (this is quaternary structure). Notice that I am using the word "bound", not "linked" because the word "linkage" implies a covalent bond. A covalent bond is not the only way for two polypeptide chains to come together. 

Because hemoglobin is comprised of four subunits, it is a tetramer. Hypothetically, if the two alpha subunits were bound together alone, this would be a homodimer. But the word dimer only applies to two subunits being associated. In the case of hemoglobin, it is a tetramer. 

There is no common naming system that would give you insight into the quaternary structure of a protein. You will not be expected to simply know by reading the name of a protein if it is made up of multiple subunits. You will, however, be expected to know the make-up of hemoglobin and any other examples from class because they were specifically covered in the lecture notes. 

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