Difference Between Alpha Helix and Beta Pleated Sheet

Key Difference – Alpha Helix vs Beta Pleated Sheet
 

Alpha helices and beta pleated sheets are the two most commonly found secondary structures in a polypeptide chain. These two structural components are the first main steps in the process of folding a polypeptide chain. The key difference between Alpha Helix and Beta Pleated Sheet is in their structure; they have two different shapes to do a specific job.

What is Alpha Helix?

An alpha helix is a right-handed coil of amino acid residues on a polypeptide chain. The range of amino acid residues can vary from 4 to 40 residues. The hydrogen bonds formed between the oxygen of C=O group on the top coil and the hydrogen of the N-H group of the bottom coil help to hold the coil together. A hydrogen bond is formed per every four amino acid residues in the chain in the above manner. This uniform pattern gives it definite features such as the thickness of the coil and it dictates the length of each complete turn along the helix axis. The stability of alpha helix structure depends on several factors.

Difference Between Alpha Helix and Beta Pleated Sheet

O atoms in red, N atoms in blue, and hydrogen bonds as green dotted lines

What is Beta Pleated Sheet?

Beta pleated sheet, also known as beta sheet, is considered as the second form of secondary structure in proteins. It contains beta strands which are connected laterally by a minimum of two or three backbone hydrogen bonds to form a twisted, pleated sheet as shown in the picture. A beta strand is a stretch of polypeptide chain; its length is generally equal to 3 to 10 amino acids, including backbone in an extended confirmation.

Key Difference - Alpha Helix vs Beta Pleated Sheet

4-stranded antiparallel β sheet fragment from a crystal structure of the enzyme catalase.
a) showing the antiparallel hydrogen bonds (dotted) between peptide NH and CO groups on adjacent strands. Arrows indicate chain direction, and electron density contours outline the non-H atoms. O atoms are red balls, N atoms are blue, and H atoms are omitted for simplicity; sidechains are shown only out to the first sidechain C atom (green)
b) Edge-on view of the central two β strands

In beta pleated sheets, the polypeptide chains run alongside each other. It gets the name “pleated sheet” due to the wave-like appearance of the structure. They are linked together by hydrogen bonds. This structure allows forming more hydrogen bonds by stretching out the polypeptide chain.

What is the difference between Alpha Helix and Beta Pleated Sheet?

Structure of Alpha Helix and Beta Pleated Sheet

Alpha Helix:

In this structure, the polypeptide backbone is tightly bound around an imaginary axis as a spiral structure. It is also known as the helicoidal arrangement of the peptide chain.

The formation of alpha helix structure happens when the polypeptide chains are twisted into a spiral. This enables all the amino acids in the chain to form hydrogen bonds (a bonding between an oxygen molecule and a hydrogen molecule) with each other. The hydrogen bonds allow the helix to hold the spiral shape and gives a tight coil. This spiral shape makes the alpha helix very strong.

Difference Between Alpha Helix and Beta Pleated Sheet -2

Hydrogen bonds are indicated by the yellow dots.

Beta Pleated Sheet:

When two or more fragments of polypeptide chain(s) overlap with one another, forming a row of hydrogen bonds with each other, following structures can be found. It can happen in two ways; parallel arrangement and anti-parallel arrangement.

Examples of the structure:

Alpha Helix: Fingernails or toenails can be taken as an example of an alpha helix structure.

Beta Pleated Sheet: The structure of feathers is similar to the structure of beta pleated sheet.

Features of the structure:

Alpha Helix: In alpha helix structure, there are 3.6 amino acids per turn of the helix. All the peptide bonds are trans and planar, and the N-H groups in the peptide bonds point in the same direction, which is approximately parallel to the axis of the helix. The C=O groups of all peptide bonds point in the opposite direction, and they are  parallel to the axis of the helix. The C=O group of each peptide bond is bonded to the N-H group of the peptide bond forming a hydrogen bond. All R- groups are pointed outward from the helix.

Beta Pleated Sheet: Each peptide bond in the beta pleated sheet is planar and has the trans-conformation. The C=O and N-H groups of peptide bonds from adjacent chains are in the same plane and  point toward each other forming hydrogen bonding between them. All the R- groups in any chain can alternatively occur above and below the plane of the sheet.

Definitions:

Secondary structure: It is the shape of a folding protein due to hydrogen bonding between its backbone amide and carbonyl groups.

References:
“Protein Structure”. ChemWiki: The Dynamic Chemistry Hypertext
“Protein Secondary Structure: α-Helices and β-Sheets” . Proteinstructures.com by Salam Al Karadaghi
“Organic Chemistry”. The Virtual Cell Textbook
“Beta Sheet”.Wikipedia
 
Image Courtesy:
“Helix electron density myoglobin 2nrl 17-32″ By Dcrjsr – Own work (CC BY 3.0) via Commons Wikimedia
“Protein secondary structure” by en:User:Bikadi (CC BY-SA 3.0) via Commons Wikimedia 
“1gwe antipar beta Sheet both” by Dcrjsr – Own work (CC BY 3.0) via Commons Wikimedia