Difference Between Alpha and Beta Helix

The key difference between alpha and beta helix relies on the type of Hydrogen bonding they form in developing these structures. Alpha helices form intra-molecular hydrogen bonds while the beta helices form inter-molecular hydrogen bonds.

Complex proteins have four structural organizational levels – primary, secondary, tertiary and quaternary. The secondary structures of proteins form the peptide chains in different orientations. The peptide chains consist of amino acid sequences bound by peptide bonds. Therefore, there are two main secondary structures in proteins as alpha helix and beta helix. In addition, there are other secondary structures called the beta turn and hairpin structures. Mainly, this article focuses on the difference between alpha and beta helix.

CONTENTS

1. Overview and Key Difference
2. What is Alpha Helix
3. What is Beta Helix
4. Similarities Between Alpha and Beta Helix
5. Side by Side Comparison – Alpha vs Beta Helix in Tabular Form
6. Summary

What is Alpha Helix?

Proteins have four structural levels of organization. Of these, the alpha helix is the commonest secondary structure of proteins. And, this structure appears as a rod that is wound around a central axis. Furthermore, the alpha helix is a right-handed helix. However, left-handed helices could also be present. Here, the peptide bonds form from the amino-terminal to the carboxy-terminal. Amino acids link with each other via these peptide bonds. Intra-molecular hydrogen bonds are the main cause for forming the alpha helix.

Arrangement of the alpha helix depends on the hydrophilic and hydrophobic nature of the protein. If the amino acid sequence consists of a high number of hydrophilic R (variable) groups, the R groups orient to the aqueous phase. If the variable groups are hydrophobic, they will jut out to the hydrophobic phase of the environment. In either scenario, the R groups appear to extend out of the helical structure. Due to these structural characteristics, alpha helix is more resistant to mutations. Thus, the presence of Hydrogen bonds stabilizes the structure of the alpha helix. There is an average of 3.6 residues per turn in an alpha helix as it takes 3.6 residues for the hydrogen bonds to develop. Some structural proteins such as collagen and keratin are rich in alpha helices.

What is Beta Helix?

A beta helix is the second most common secondary structure of a protein. Although it is not as common as the alpha helix, the presence of beta helices also plays a major role in protein structure. Formation of the beta helix takes place via two beta sheets arranged either in a parallel fashion or an anti-parallel fashion. These sheets then form into a helical structure. Inter-molecular Hydrogen bonds between two sheet strands aid in the formation of a beta helix.

Beta helices can be both right handed or left handed depending on their binding patterns. When forming a beta helix, the variable groups of the two beta sheets will arrange within the core of the helix. Therefore, the majority of the groups forming beta sheets have hydrophobic functions.

In contrast to the alpha helix, 17 residues form one turn in Beta helices. Metal ions have the ability to activate the formation of Beta helix. Similar to the alpha helix, the Hydrogen bonds support to maintain the structure of the Beta helix. Carbonic anhydrase enzyme and pectate lyase are two proteins rich in beta helices.

What are the Similarities Between Alpha and Beta Helix?

• Alpha and Beta Helix are two secondary structures of proteins.
• Amino acids are the monomers of both secondary structures.
• Furthermore, the chemical constituents of the alpha and beta helices are carbon, hydrogen, oxygen, nitrogen, and sulfur.
• Also, both secondary structures develop into a higher level organization.
• Moreover, both are stabilized by hydrogen bonds.
• In both structures, the hydrophobicity is determined by the presence of the R groups of the amino acids.

What is the Difference Between Alpha and Beta Helix?

The key difference between the alpha and beta helix is the type of hydrogen bonding they show. Alpha helix shows intra-molecular hydrogen bonding while beta helix shows inter-molecular hydrogen bonding. In addition, the alpha helix forms a right-handed helix, while beta helix can form both right and left-handed helices. So, this is also a significant difference between alpha and beta helix.

Moreover, a further difference between alpha and beta helix is that the alpha helix formation takes place by the twisting of the amino acid sequence, whereas in beta helix formation the two beta sheets either parallel or anti-parallel are bound to form the helical structure.

The below info-graphic presents more information regarding the difference between alpha and beta helix.

Summary – Alpha vs Beta Helix

Both alpha helices and beta helices are important in identifying and deducing complex protein structures. Both types are secondary structures of proteins. However, alpha helix is a helical twist of amino acid sequences. In contrast, beta helix formation happens via the Hydrogen bonding of parallel or anti-parallel beta sheets. Furthermore, the Hydrogen bonding is intra-molecular in alpha helix form while the hydrogen bonding is inter-molecular in beta helix form. Besides, both these structures have an R group, which determines the hydrophobicity of the protein. Thus, this summarizes the difference between alpha and beta helix.

Reference:

1. “Orders of Protein Structure.” Khan Academy, Khan Academy, Available here.
2. “Protein Secondary Structure: α-Helices and β-Sheets.” Structural Bioinformatics: Practical Guide, Available here.

Image Courtesy:

1. “Alpha beta structure (2)” By Thomas Shafee – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “1m8n Choristoneura fumiferana” By WillowW at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia