Tertiary structure describes the association of units within domains, but tertiary structure also includes the way in which domains fit together. This should not be confused with quaternary structure, which concerns how separate polypeptide chains associate with each other. Again these terms might have slightly different meanings amongst the literature. For the purposes of this course, we will define them as follows. Architecture will describe the orientations of secondary structures and the way they pack together.
Some proteins consist of a single polypeptide chain, such as myoglobin.
The biologically functioning form of others is an aggregation of several copies of the same chain. For example, the quaternary structure of haemoglobin consists of four chains, each of which is similar to a myoglobin molecule. Other proteins exist of an aggregation of one or more copies of two different chains. Note that some chains are formed by the proteolytic cleavage of an intact precursor chain. Such cases should not be considered as a quaternary association of different chains.