The Four Levels of Protein Structure

This is the structure of human hemoglobin (2hhb.pdb), a protein that transports oxygen in red blood cells. Like many proteins, there are four levels of structure in the hemoglobin protein. 

A protein is a chain of amino acids. The primary structure is the sequence of amino acids in the chain.

As you examine more of the protein, it helps to visualize the backbone of the protein as a ribbon.

As you follow the backbone, it usually folds to form a regular repeating pattern. This is secondary structure. There are two major kinds of secondary structure - an elongated beta sheet and a coiled alpha helix (as seen here in hemoglobin). 

The protein folds upon itself when regions of secondary structure are interrupted by irregularly loops and turns. It helps to visualize the helices as pink and the turns as white.

The irregular folding of the entire protein is tertiary structure.

Some proteins are actually a collection of smaller proteins called subunits. Hemoglobin is made of four subunits. The arrangement of subunits in a protein is its quaternary structure. It help to visualize the subunits as different colors.

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