Content Writer | Updated On - Jun 19, 2024
Alpha-helix and Beta-pleated sheets are the common types of secondary protein structures. Both structures are joined together by hydrogen bonds but differ in appearance and arrangement.
- As their name suggests, the alpha helix has a helical or twisted structure, and the beta-pleated sheet resembles with folded sheet.
- The hydrogen bonds in the alpha-helix structure are formed at every fourth amino acid, while the hydrogen bonds in beta-pleated sheets are formed in the adjacent beta strands.
What is a Protein?
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A protein is defined as a complex molecule consisting of long chains of amino acids. About 20 amino acids are required to make up a protein molecule. They are present in every cell of a living organism to perform basic life functions such as:
- Catalyzing metabolic reactions
- Repairing body tissues
- Maintaining pH
- Supporting the immune system
- Replication of DNA
The building blocks of alpha-helix and beta-pleated sheets are amino acids joined together to form complex protein structures. A carboxyl group (-COOH) of an amino acid is joined with the amino group (-NH) of the next amino acid. This bond formation is called the peptide bond.
Figure: Amino acid molecule
Types of Proteins
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The structure of protein depends on the type of folding and the molecular bonding of amino acids. Protein is classified into 4 groups based on their structures: Primary, Secondary, Tertiary, and Quaternary structure of proteins.
Primary Structures
In a primary structure, the amino acids are joined together in a linear sequence through peptide bonds.
- This bond is formed by joining a carboxyl of one amino acid, to an amino group of another amino acid.
- Primary structures function in post-translational modifications. An example of a primary structure is hemoglobin.
Secondary Structures
The folding of a polypeptide chain forms secondary structures of protein. It is a three-dimensional structure. Linus Pauling and Robert Corey proposed two types of secondary structures of protein:
- Alpha helix
- Beta Pleated Sheets
Alpha Helix
In an alpha helix structure, the polypeptide chain is coiled or twisted from the N-terminal to the C-terminal. It is one of the most common types of protein structures.
- The bonds in an alpha helix are formed between the NH and the CO group. Each turn in this structure has about 3.6 residues of amino acids.
- The helix can either be in a clockwise or anticlockwise direction. An example of an alpha helix structure is ferritin (a blood protein).
Figure: Alpha Helix Structure of Ferritin
Beta Pleated Sheet
In this type of protein structure, the beta sheets are joined together by hydrogen bonds. About 3 to 10 amino acids are combined to form a polypeptide of beta-strand.
- The bonds in a beta-pleated sheet are formed between the carbonyl and the amine groups of polypeptide chains.
- The beta sheets are arranged in parallel or anti-parallel directions. Silk contains parallel and anti-parallel arrangements.
Figure: Beta-Pleated Sheets
Difference between Alpha-helix and Beta-pleated Sheet
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The difference between the two secondary structures of protein is shown in the table below:
| Parameters | Alpha-helix | Beta-pleated sheet |
|---|---|---|
| Structure | Right-handed coiled structure. | Sheet-like structure. |
| Bond Formation | Formation of hydrogen bonds with polypeptide chains. | Hydrogen bonds join Beta strands. |
| Bond type | Hydrogen bonds are formed with the NH group of one amino acid to the CO group of another amino acid. | Hydrogen bonds are created between the NH and CO groups of adjacent peptide chains. |
| Types | It has no types. | It has two types: parallel beta-sheet and anti-parallel beta-sheet. |
| Number of Amino acids | 3.6 amino acid residues are present in a helix. | Three to ten amino acids are joined to form a beta-pleated sheet. |
| Type of Amino acids | Alanine, Glycine, Isoleucine, leucine, Methionine. | Tyrosine, tryptophan, and phenylalanine. |
Tertiary Structure
When secondary structures are folded again, tertiary structures are formed. Myoglobin is an example of a tertiary structure.
Quaternary Structure
The protein structures formed from multiple proteins (or protein subunits) are called quaternary structures. Hemoglobin is an example of a quaternary structure.
Things to Remember
- Alpha-helix and Beta-pleated sheets are the types of secondary structures of protein.
- Amino acids are the building blocks that make up a protein molecule.
- Hemoglobin, Keratin, Myoglobin, and Ferritin are examples of Alpha-helix structures.
- A protein is not functional if it loses its form at any structural level.
- A beta-pleated sheet is of two types: parallel and anti-parallel sheet.
Sample Questions
Ques. How many amino acids are required to make alpha helix and beta-sheets? (1 mark)
Ans. In the alpha helix structure, one turn has 3.6 amino acid residues, while three to ten amino acids make up the beta sheets. Alpha helix can be a single chain of polypeptide but more than one beta strand is required to make beta sheets.
Ques. Who predicted the Alpha-helix protein structure? Is this a stable protein structure?(2 marks)
Ans. Yes, Alpha-helix is a stable protein structure. The structure of the Alpha-helix protein was predicted by Linus Pauling. His prediction was confirmed when myoglobin (three-dimensional structure) was predicted by X-ray crystallography.
Ques. Explain the types of beta-pleated sheets. (2 marks)
Ans. Beta-pleated sheets are of two types: parallel and antiparallel.
- In a parallel beta-pleated sheet, the strands run in the same direction. These strands are more twisted.
- In an antiparallel beta-pleated sheet, the strands run in the opposite direction. These strands are less twisted as compared to the parallel beta-pleated sheets.
Ques. Mention the four groups of amino acids that forms alpha-helix and beta-pleated sheets. (2 marks)
Ans. There are a total of 20 amino acids that occur in nature. Amino acids have four groups:
- Amino group
- Carboxyl group (COOH)
- Hydrogen atom
- R group
Ques. What is the importance of protein alpha-helix and beta-pleated structure of protein? (2 marks)
Ans. Proteins are divided into different groups based on their structure.
- The structure of protein depends on how amino acids are arranged.
- A protein will not be functional if it loses its structure or shape.
Ques. Is insulin a globular protein? (2 marks)
Ans. Yes, insulin is a globular protein and exhibits a quaternary structure.
- The structure of insulin is composed of two polypeptide chains A and B.
- These chains are joined together by disulfide bonds.
Ques. What technique is used to determine the secondary structure of a protein? (2 marks)
Ans. The most widely used technique for determining the structure of a protein is X-ray crystallography.
- This technique is used to determine the three-dimensional structure of not only proteins but also in organic and inorganic molecules.
Ques. What forms a basis of alpha-helix and beta-pleated structure: amino acid or nucleic acid? Are both same. (2 marks)
Ans. Amino acid forms the basis of alpha-helix and beta-pleated structure. Nucleic acids and amino acids differ from one another.
- Nucleic acids store genetic information and are known as polymers of nucleotides.
- Examples of nucleotides are DNA and RNA.
- Amino acids are monomers that are joined together to form a protein.
- Examples of amino acids are valine, methionine, leucine, isoleucine, etc.
Ques. Which globular protein is found in muscle tissue? (2 marks)
Ans. The globular protein found in muscle tissue is called myoglobin.
This protein gives a red color to the muscles and ensures that a sufficient amount of oxygen reaches the muscles for alpha-helixtioning.
Ques. How many amino acids are present in each chain of an insulin? (2 marks)
Ans. Two polypeptide chains form the structure of insulin.
The first chain contains 21 amino acids, whereas the second chain contains 30 amino acids.
Ques. Mention the main function of fibrous proteins. Give examples. Also, is it a secondary protein? (2 marks)
Ans. Fibrous proteins or structural proteins play a major role in forming connective tissue, tendons, and muscle fibers. For example, collagen, keratin, fibroin, and elastin. Yes, fibrous proteins are secondary protein structures.
Ques. Which amino acids are absent in both alpha-helix and beta-pleated sheets? (3 marks)
Ans. Glycine and proline are the amino acids that are absent in both alpha-helix and beta-pleated sheets.
- These amino acids cause rigidity in the structure of proteins and destabilize the structure by creating breaks.
- In glycine the side chains are absent which makes it difficult to form bonds with the protein structures.
- The nitrogen atom of proline does not have a hydrogen atom and thus cannot form bonds with the amino acids.
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