Secondary Structure of Proteins

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LEARNING OBJECTIVES

In this article, author has explained the secondary structure; alpha helix and beta pleated sheet structure and their properties.

The conformation of the polypeptide chain formed by twisting and folding is known as the secondary structure of proteins. The amino acids are close to each other in the secondary structure. There are two types of secondary structures found, α-helix and β-pleated sheet. The hydrogen bonding in a protein determines whether an alpha or beta structure will form. Polypeptides can have alpha helix or beta sheet or both.

α-helix

α-helix is the most common spiral structure found in proteins. It has a rigid arrangement of a polypeptide chain. α-helix was proposed by Pauling and Corey.

Characteristics of α-helix

  • The α-helix is a tightly packed coiled structure with the amino acid side chains extending outward.
  • The helix is stabilized by hydrogen bonding. A hydrogen bond is formed between the NH and CO group.
  • The hydrogen bonds are weak but collectively they are strong and stabilize the helix.
  • All the peptide bonds participate in hydrogen bonding except the first and last peptide bond of the polypeptide chain.
  • Every turn of the helix has 3.6 amino acids. The space between the amino acids is 0.15nm.
  • The alkyl group of the alpha helix is not involved in hydrogen bonding but maintains the structure of the alpha helix.
  • Alpha helix is the most stable structure formed by the lowest energy.
  • The right-handed alpha helix is more stable than the left-handed. The direction of the right-hand alpha helix is like the fingers of the right-hand curling when the thumb points upward.
  • Some amino acids like proline disrupt the structure of the alpha helix. The acidic (glutamic acid, aspartic acid) and basic (arginine, lysine, and histidine) amino acids also disrupt the alpha helix.
  • Keratin, myoglobin, and hemoglobin are examples of an alpha helix.

β-pleated sheet

β-pleated is the second secondary structure of proteins proposed by Pauling and Corey. Two or more segments of the polypeptide chain line up next to each other and are called beta structures. This forms a sheet-like structure that is held together by a hydrogen bond. The hydrogen bonds are formed between the neighboring segments of polypeptide chains.

Characteristics of beta-pleated sheet

  • The hydrogen bonding may be interchain (forms between the two separate chains) or intrachain (forms between a single polypeptide chain by the folding back of the chain upon itself).
  • The hydrogen bond is formed between the carbonyl and amino group of the backbone while the R group is present above and below the plane of the sheet.
  • When three to ten amino acids are combined, a beta strand is formed between the polypeptides.
  • The beta structure may be formed between two polypeptides or by folding the back of a single polypeptide.
  • The alkyl groups are oriented outside and inside the sheet.
  • Skin fibers or fibroin is an example of a beta pleated sheet.

Parallel and Anti-parallel beta sheets

Parallel beta-sheets: When beta sheets are in the same direction means their N and C terminal are matchup, they are parallel beta sheets.

Anti-parallel beta sheets: When beta sheets are in the opposite direction, the N terminal of one strand is positioned next to the C terminal of the other strand.

The parallel arrangement is less stable as compared to the anti-parallel beta-sheet.

Other types of secondary structures

Besides the alpha helix and beta sheet, there are many other secondary structures found in proteins. Beta turns, loops and random coils are types of secondary structures.

When the chain changes direction, beta turns are formed to connect two other elements of the secondary structure. The beta-turn which is formed between two anti-parallel chains is termed beta-hairpin.