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Learn About the 4 Types of ProteinStructure

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Updated May 04, 2019

Proteins are biological polymers composed of amino acids. Amino acids, linked together bypeptide bonds, form a polypeptide chain. One or more polypeptide chains twisted into a 3-Dshape form a protein. Proteins have complex shapes that include various folds, loops, andcurves. Folding in proteins happens spontaneously. Chemical bonding between portions ofthe polypeptide chain aid in holding the protein together and giving it its shape. There aretwo general classes of protein molecules: globular proteins and fibrous proteins. Globularproteins are generally compact, soluble, and spherical in shape. Fibrous proteins are

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typically elongated and insoluble. Globular and fibrous proteins may exhibit one or more offour types of protein structure.

Four Protein Structure Types

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

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1. Primary Structure

Primary Structure describes the unique order in which amino acids are linked together toform a protein. Proteins are constructed from a set of 20 amino acids. Generally, amino acidshave 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, carboxyl group, and anamino group. The "R" group varies among amino acids and determines the differencesbetween these protein monomers. The amino acid sequence of a protein is determined by theinformation found in the cellular genetic code. The order of amino acids in a polypeptidechain is unique and specific to a particular protein. Altering a single amino acid causesa gene mutation, which most often results in a non-functioning protein.

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2. Secondary Structure

Secondary Structure refers to the coiling or folding of a polypeptide chain that gives theprotein its 3-D shape. There are two types of secondary structures observed in proteins. Onetype is the alpha (α) helix structure. This structure resembles a coiled spring and issecured by hydrogen bonding in the polypeptide chain. The second type of secondarystructure in proteins is the beta (β) pleated sheet. This structure appears to be folded orpleated and is held together by hydrogen bonding between polypeptide units of the foldedchain that lie adjacent to one another.

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3. Tertiary Structure

Tertiary Structure refers to the comprehensive 3-D structure of the polypeptide chain ofa protein. There are several types of bonds and forces that hold a protein in its tertiarystructure.

Hydrophobic interactions greatly contribute to the folding and shaping of aprotein. The "R" group of the amino acid is either hydrophobic or hydrophilic. Theamino acids with hydrophilic "R" groups will seek contact with their aqueousenvironment, while amino acids with hydrophobic "R" groups will seek to avoid waterand position themselves towards the center of the protein.

Hydrogen bonding in the polypeptide chain and between amino acid "R" groupshelps to stabilize protein structure by holding the protein in the shape established bythe hydrophobic interactions.

Due to protein folding, ionic bonding can occur between the positively and negativelycharged "R" groups that come in close contact with one another.

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Folding can also result in covalent bonding between the "R" groups of cysteine aminoacids. This type of bonding forms what is called a disulfide bridge. Interactionscalled van der Waals forces also assist in the stabilization of protein structure. Theseinteractions pertain to the attractive and repulsive forces that occur between moleculesthat become polarized. These forces contribute to the bonding that occurs betweenmolecules.

4. Quaternary Structure

Quaternary Structure refers to the structure of a protein macromolecule formed byinteractions between multiple polypeptide chains. Each polypeptide chain is referred to as asubunit. Proteins with quaternary structure may consist of more than one of the same type ofprotein subunit. They may also be composed of different subunits. Hemoglobin is anexample of a protein with quaternary structure. Hemoglobin, found in the blood, is an iron-containing protein that binds oxygen molecules. It contains four subunits: two alphasubunits and two beta subunits.

How to Determine Protein Structure Type

The three-dimensional shape of a protein is determined by its primary structure. The orderof amino acids establishes a protein's structure and specific function. The distinctinstructions for the order of amino acids are designated by the genes in a cell. When a cellperceives a need for protein synthesis, the DNA unravels and is transcribed intoan RNA copy of the genetic code. This process is called DNA transcription. The RNA copy isthen translated to produce a protein. The genetic information in the DNA determines thespecific sequence of amino acids and the specific protein that is produced. Proteins areexamples of one type of biological polymer. Along with proteins, carbohydrates, lipids,and nucleic acids constitute the four major classes of organic compounds in living cells.

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