The protein primary sequence and secondary structure are essentially linear, the primary sequence describes which amino acids are present in which order in the protein, and the secondary structure describes which local structure each of those amino acids adopts. However proteins are three dimensional molecules, where these protein chains are bundled together into a 3 dimensional shape (Figure 1). The protein tertiary structure is what describes the three dimensional shape of the protein.

Figure 1: The tertiary structure of Mouse Dihydrofolate Reductase (DHFR) (PDB ID: 3D80). The linear chain with secondary structures of α-Helix (cyan), β-Sheet (red) and Loops (purple) folds into the final 3 dimensional structure, which is the protein tertiary structure.

The helices, sheets, turns and loops that make up the secondary structure fold around one another to form the tertiary structure. This gives the gross three dimensional shape of the protein, and is an important factor in determining the protein function.

Rather than fitting into a few descrete amino acid or structural compositions as with the lower orders of protein structure, the protein tertiary structures are more variable. However there are some broad categories of protein tertiary structure that are described. There are globular (such as enzymes) and fibrous (such as muscle proteins) proteins as two large groups of structure.

Owing to universal common descent, protein structures adopt similar shapes as they have evolved from other protein structures. Two proteins may have been duplicated in the genome at some point in the past and evolved separately to have different functions, but due to their common ancestry retain much of their gross structure. Furthermore physics limits the scope of possible structural conformations, as protein folding is fundamentally driven by the physical properties of the amino acids and their interactions with one another.

For many proteins, the finished protein structure is the tertiary structure. However some proteins also exhibit a quaternary structure, which is the highest level of protein structure.