Main Difference – Alpha Helix and Beta Pleated Sheet
Alpha helix and beta plates are two different secondary structures of protein. Alpha helix is a right handed-coiled or spiral conformation of polypeptide chains. In alpha helix, every backbone N-H group donates a hydrogen bond to the backbone C=O group, which is placed in four residues prior. Here, hydrogen bonds appear within a polypeptide chain in order to create a helical structure. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds; they form a generally twisted, pleated sheet. In contrast to the alpha helix, hydrogen bonds in beta sheets form in between N-H groups in the backbone of one strand and C=O groups in the backbone of the adjacent strands. This is the main difference between Alpha Helix and Beta Pleated Sheet.
What is an Alpha Helix
Proteins are made up of polypeptide chains, and they are divided into several categories such as primary, secondary, tertiary, and quaternary, depending on the shape of a folding of the polypeptide chain. α-helices and beta-pleated sheets are the two most commonly encountered secondary structures of a polypeptide chain.
Alpha-helical structure of proteins forms due to hydrogen bonding between its backbone amide and carbonyl groups. This is a right handed coil, which typically contain 4 to 40 amino acid residues in the polypeptide chain. The following figure illustrates the structure of alpha helix.
Hydrogen bonds form between an N-H group of one amino residue with a C=O group of another amino acid, which is placed in 4 residues earlier. These hydrogen bonds are essential to creating the alpha helical structure, and every complete turn of the helix has 3.6 amino residues.
Amino acids whose R-groups are too large (i.e. tryptophan, tyrosine) or too small (i.e. glycine) destabilize α-helices. Proline also destabilizes α-helices due to its irregular geometry; its R-group bonds back to the nitrogen of the amide group, and causes steric hindrance. In addition, the lack of a hydrogen on Proline’s nitrogen prevents it from participating in hydrogen bonding. Apart from that, the stability of alpha helix depends on the dipole moment of the entire helix, which is caused due to individual dipoles of C=O groups involved in hydrogen bonding. Stable α-helices typically end with a charged amino acid to neutralize the dipole moment.
A Hemoglobin molecule, which has four heme-binding subunits, each made largely of α-helices.
What is Beta Pleated Sheet
Beta pleated sheets are another type of protein secondary structure. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet. Generally, a beta strand contains 3 to 10 amino acid residues, and these strands are arranged adjacent to other beta strands while forming extensive hydrogen bond network. Here, N-H groups in the backbone of one strand create hydrogen bonds with the C=O groups in the backbone of the adjacent strands. Two ends of the peptide chain can be assigned to the N- terminus and C- terminus in order to illustrate the direction. N- terminus indicates one end of the peptide chain, where free amine group is available. Similarly, C- terminus represent the other terminal of peptide chain, where free carboxylic group is available.
Adjacent β strands can form hydrogen bonds in antiparallel, parallel, or mixed arrangements. In the anti-parallel arrangement, N-terminus of one stand is adjacent to the C- terminus of next stand. In a parallel arrangement, N- terminus of adjacent strands are oriented in the same direction. The following figure illustrates the structure and hydrogen bonding pattern of parallel and anti-parallel beta strands.
a) anti parallel
b) parallel
Difference Between Alpha Helix and Beta Pleated Sheet
Shape
Alpha Helix: Alpha Helix is a right-handed coiled rod-like structure.
Beta Pleated Sheet: Beta sheet is a sheet-like structure.
Formation
Alpha Helix: Hydrogen bonds form within the polypeptide chain in order to create a helical structure.
Beta Pleated Sheet: Beta sheets are formed by linking two or more beta strands by H bonds.
Bonds
Alpha Helix: Alpha helix has n + 4 H-bonding scheme. i.e. Hydrogen bonds form between N-H group of one amino residue with C=O group of another amino acid, which is placed in 4 residues earlier.
Beta Pleated Sheet: Hydrogen bonds are formed in between the neighboring N-H and C=O groups of adjacent peptide chains.
-R group
Alpha Helix: -R groups of the amino acids are oriented outside of the helix.
Beta Pleated Sheet: -R groups are directed to both inside and outside of the sheet.
Number
Alpha Helix: This can be a single chain.
Beta Pleated Sheet: This cannot exist as a single beta strand; there are must be two or more.
Type
Alpha Helix: This has only one type.
Beta Pleated Sheet: This can be parallel, anti-parallel or mixed.
Qualities
Alpha Helix: 100o rotation, 3.6 residues per turn and 1.5 Ao rise from one alpha carbon to the second
Beta Pleated Sheet: 3.5 Ao rise between residues
Amino Acid
Alpha Helix: Alpha helix prefers the amino acid side chains, which can cover and protect the backbone H-bonds in the core of the helix.
Beta Pleated Sheet: The extended structure leaves the maximum space free for the amino acid side chains. Therefore, amino acids with large bulky side chains prefer beta sheet structure.
Preference
Alpha Helix: Alpha helix prefers Ala, Leu, Met, Phe, Glu, Gln, His, Lys, Arg amino acids.
Beta Pleated Sheet: Beta sheet prefers Tyr, Trp, (Phe, Met), Ile, Val, Thr, Cys.
Image Courtesy:
“Alpha Helix Protein Structure” [Public Domain] via Commons Wikimedia
“Hempglobin molecule” By Zephyris at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
“Parellel and Antiparellel” by Fvasconcellos – Own work.Created by uploader upon request by Opabinia regalis., (Public Domain) via Commons Wikimedia
