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The Structure of Proteins – Overview of the Primary, Secondary, Tertiary, and Quaternary Structure

Learn about the structure of proteins. Discover what amino acids are and find out what the primary, secondary, tertiary, and quaternary structure of a protein is.

A protein is a large molecule that consists of individual units called amino acids. A small group of amino acids linked together is called a peptide. As more and more amino acids are added, the peptide becomes larger and is known as a polypeptide. Eventually, the entire sequence of a specific protein is formed. The function of the protein is largely dependent on the sequence of amino acids, the shape of the protein, and its orientation. The overall structure of a protein can be described in four parts: primary, secondary, tertiary, and quaternary structure.

Primary Structure

The primary structure of a protein is the sequence of amino acids. There are 20 amino acids, each consists of a carboxyl group and an amino group bonded to the same carbon atom. Each amino acid also has an R-group, which distinguishes it from the rest. The order in which the amino acids are joined together is different for each type of protein.

Secondary Structure

The secondary structure of a protein is the shape of the polypeptide chain at different regions. The individual amino acids aren’t linked together in a straight line. Instead, there are many twists and turns in the polypeptide. One type of secondary structure is alpha helix, which is a helical structure, similar to the twisted ladder orientation of DNA. The second type of secondary structure is beta sheets, which is a folded orientation whereby the polypeptide chain runs back and forth over a certain distance.

Tertiary Structure

The tertiary structure of a protein describes the entire polypeptide chain. It is a three dimensional view that encompasses all atoms in the protein.

Quaternary Structure

The quaternary structure of a protein describes the relationship between individual polypeptide subunits in the overall three dimensional structure of the protein molecule. This only applies to proteins that have two or more separate polypeptide chains, like hemoglobin.

Reference:

Nelson, David L. Lehninger Principles of Biochemistry 4th ed. W.H. Freeman and Company. 2005.

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