Subunits are the individual polypeptide chains. Each subunit is given a designation Greek letter (alpha, beta, etc.)
A protein with several subunits is an oligomer.

i.e. a protein made with two of the same subunits could be (alpha)2 or (beta)2. while a protein with two subunits that are different could be (alpha)(beta). 

*Note this differs from the alpha/beta 3D formations in secondary protein structure. The Greek alphabet being referred to in quaternary structure involves the subunit being identical to another subunit in the same protein and designating it with a Greek alphabet letter. A subunit that is different from another subunit in the same protein will have a different Greek letter designated to it. 

Protein Subunits

Quaternary structure refers to the proteins that are multisubunit, more than 100 kilo Daltons, and form a specific geometry. These subunits are usually linked non-covalently. 

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Protein structure is the 3-dimensional assembly caused by the string of amino acids translated from RNA . Protein structures varies immensely due to the assembly of the amino acids.
There are four subdivisions of proteins structures:
- Primary structure
- Secondary structure
- Tertiary structure 
- Quaternary structure 

Protein Structure 

Voet, D., Voet, J. G., & Pratt, C. W. (2013). Chapter 6: Proteins: Three-Dimensional Structure. In Fundamentals of biochemistry: Life at the molecular level (4th ed., pp. 127-175). Hoboken, NJ: Wiley. Retrieved January 05, 2021, from https://archive.org.

Fundamentals of biochemistry: Life at the molecular level, 4th edition: Chapter 6

Secondary protein structure refers to only the spatial arrangements of the polypeptide backbone that does not include the sided chain conformations. 
The most common secondary structures include:
-	Alpha helix
-	Beta sheets
- Turns

Secondary Protein Structure

Primary structure refers to the amino acid sequence, or multiple sequences of a protein. Each amino acid in a primary structure is known as a residue. Because the 20 amino acids have individual chemical and physical properties, they influence the 3D structure, the function of the protein and its chemical properties. i.e. hydrophobic and hydrophilic properties. 

Ions can additionally be found in the primary structure. Zinc and calcium ions can form complexes that are present in the protein's primary structure. 

Primary Protein Structure 

Quaternary Protein Structure

Tertiary protein structure combines the folding of the secondary back bone structure and includes the positions of the side chains of each amino acid. 

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