The alpha helix is a right-handed and has a backbone consisting of hydrogen bonds binding to each 4th amino acid residue. There are about 3.6 residues for each turn. 

The Alpha Helix

The beta-sheet forms hydrogen bonds with neighboring amino acid chains, not within itself. Appear pleated due to the hydrogen bonding.
-Antiparallel - where the chains are hydrogen bonded in opposite directions
-Parallel – chains are hydrogen bonded in the same direction 


The Beta Sheet

Turns are polypeptide sequences that connect secondary structures (i.e. alpha helices and beta sheets). There are two types of arrangements for turns: 
- Type I
- Type II

Protein Turns

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

Grand Valley State University

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 archive.org.

Fundamental of biochemistry: Life of the molecular level 4th edition: Chapter 6

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 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. 

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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