Learn About the 4 Different Types of Protein Structure (2024)

Updated on July 26, 2024

Four Protein Structure Types

The four levels of protein structure are distinguished from one another by the degree of complexity in the polypeptide chain. A single protein molecule may contain one or more of the protein structure types:primary, secondary, tertiary, and quaternary structure.

1. Primary Structure

Primary Structuredescribes the unique order in which amino acids are linked together to form a protein. Proteins are constructed from a set of 20 amino acids. Generally, amino acids have the following structural properties:

• A carbon (the alpha carbon) bonded to the four groups below:
• A hydrogen atom (H)
• A carboxyl group (-COOH)
• An amino group (-NH2)
• A "variable" group or "R" group

All amino acids have the alpha carbon bonded to a hydrogen atom, a carboxyl group,andan amino group. The"R" groupvaries amongamino acidsand determines the differences between these protein monomers. The amino acid sequence of a protein is determined by the information found in the cellulargenetic code. The order of amino acids in a polypeptide chain is unique and specific to a particular protein. Altering a single amino acid causes agene mutation, which most often results in a non-functioning protein.

2. Secondary Structure

Secondary Structure refers to the coiling or folding of a polypeptide chain that gives the protein its 3-D shape. There are two types of secondary structures observed in proteins. One type is thealpha (α) helixstructure. This structure resembles a coiled spring and is secured by hydrogen bonding in the polypeptide chain. The second type of secondary structure in proteins is thebeta (β) pleated sheet. This structure appears to be folded or pleated and is held together by hydrogen bonding between polypeptide units of the folded chain that lie adjacent to one another.

3. Tertiary Structure

Tertiary Structurerefers to the comprehensive 3-D structure of the polypeptide chain of aprotein. There are several types of bonds and forces that hold a protein in its tertiary structure.

• Hydrophobic interactionsgreatly contribute to the folding and shaping of a protein. The "R" group of the amino acid is either hydrophobic or hydrophilic. The amino acids with hydrophilic "R" groups will seek contact with their aqueous environment, while amino acids with hydrophobic "R" groups will seek to avoid water and position themselves towards the center of the protein.​
• Hydrogen bondingin the polypeptide chain and between amino acid "R" groups helps to stabilize protein structure by holding the protein in the shape established by the hydrophobic interactions.
• Due to protein folding,ionic bondingcan occur between the positively and negatively charged "R" groups that come in close contact with one another.
• Folding can also result in covalent bonding between the "R" groups of cysteine amino acids. This type of bonding forms what is called adisulfide bridge. Interactions calledvan der Waals forcesalso assist in the stabilization of protein structure. These interactions pertain to the attractive and repulsive forces that occur between molecules that become polarized. These forces contribute to the bonding that occurs between molecules.

4. Quaternary Structure

Quaternary Structurerefers to the structure of a protein macromolecule formed by interactions between multiple polypeptide chains. Each polypeptide chain is referred to as a subunit. Proteins with quaternary structure may consist of more than one of the same type of protein subunit. They may also be composed of different subunits. Hemoglobin is an example of a protein with a quaternary structure. Hemoglobin, found in theblood, is an iron-containing protein that binds oxygen molecules. It contains four subunits: two alpha subunits and two beta subunits.

How to Determine Protein Structure Type

The three-dimensional shape of a protein is determined by its primary structure. The order of amino acids establishes a protein's structure and specific function. The distinct instructions for the order of amino acids are designated by thegenesin a cell. When a cell perceives a need for protein synthesis, theDNAunravels and is transcribed into anRNAcopy of the genetic code. This process is calledDNA transcription. The RNA copy is thentranslatedto produce a protein. The genetic information in the DNA determines the specific sequence of amino acids and the specific protein that is produced. Proteins are examples of one type of biological polymer. Along with proteins,carbohydrates,lipids, andnucleic acidsconstitute the four major classes of organic compounds in livingcells.