Proteins

Proteins consist of strands of amino acids folded into a specific shape. The different protein structures can be classified by four levels of folding, each successive one being constructed from the preceding one.

Primary Structure - The very basic strand of amino acids is the called the primary structure.
Secondary Structure - The hydrogen-bond interaction among strands of amino acids gives rise to the first level of folding, alpha-helices and beta-sheets.
Tertiary - Interaction between alpha-helices and beta-sheets comprise the second level of folding, protein domains. These protein domains are then strung together through third level folding to form small globular proteins. The combination of second and third level folding yields tertiary structure.
Quaternary Structure - In order to achieve enhanced function, small globular proteins often come together to form protein aggregates. This fourth level of protein structure is called the quaternary structure. A famous example of quaternary structure is hemoglobin.