The four levels of protein structure, in order of increasing complexity and structural organization, are:

1. Primary structure
   • This is the simplest level, consisting of the linear sequence of amino acids in a polypeptide chain linked by peptide bonds. It determines all higher levels of structure. For example, the sequence of amino acids in insulin chains

2. Secondary structure
   • This level involves local spatial arrangements of the polypeptide backbone, stabilized mainly by hydrogen bonds. Common secondary structures include alpha helices and beta pleated sheets

. These structures form patterns like coils and folds within segments of the chain.

3. Tertiary structure
   • The tertiary structure is the overall three-dimensional folding of a single polypeptide chain, formed by interactions among side chains (R groups) such as hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges. It creates a compact, functional 3D shape and may include domains-independently folded regions within the protein

4. Quaternary structure
   • This highest level occurs when multiple polypeptide chains (subunits) assemble into a larger, biologically active complex. The spatial arrangement and interactions between these subunits define the quaternary structure. Examples include hemoglobin with its four subunits

Sorting images of proteins by level of structure shown

• Primary structure images show only the linear amino acid sequence without folding.
• Secondary structure images depict alpha helices and beta sheets, often shown as ribbons or coils.
• Tertiary structure images illustrate the full 3D folded shape of a single polypeptide, including side chain interactions.
• Quaternary structure images display multiple polypeptide subunits assembled into a multi-subunit complex.

This hierarchy reflects increasing complexity from a simple sequence to complex multi-subunit assemblies