Protein Structure 101: From Primary to Quaternary Levels

Protein Structure 101: From Primary to Quaternary Levels

Question 1. Functions of proteins.
Answer:

• Proteins have a variety of specialized and essential functions.
• Its functions are grouped into static and dynamic functions.

Question 2. Classification of Amino acids.
Answer:

Question 3. Limiting amino acids
Answer:

• It refers to essential amino acids in the protein which fall short of meeting the amino acids required by humans.
• They are determined by comparing the number of amino acids in a gram of protein to the number of amino acids per gram of protein required.
• These are limiting amino acids because if a person’s diet is deficient in any one of the amino acids, it will limit the usefulness of the others even if they are present in large quantities.
• It includes.

1. Sulfur-containing amino acids.
2. Methionine.
3. Lysine.

Question 12. Structure of proteins.
Answer:

• Proteins are the polymers of L -α – amino acids.
• Its structure is divided into four levels.

1. Primary structure:

• It is the linear sequence of amino acids forming polypeptides.
• Amino acids are held together by peptide bond which is formed between the amino group of one amino acid and the carboxyl group of another amino acid,

2. Secondary structure:

• The spatial arrangement of protein formed by twisting of the polypeptide chain is referred as secondary structure.

Types:

1. α-Helix.
   • Proposed by Pauling and Corey.
   • It is a tightly packed coiled structure with amino acid side chains extending outwards from the central axis.
   • It is stabilized by extensive hydrogen bonding formed between the H atom attached to peptide N and
     O atom attached to peptide C.
   • It is the common spatial arrangement.
   • It has a rigid arrangement of the polypeptide chain
   • A right-handed helix is more stable than a left-handed one.
2. β – pleated sheet.
   • Proposed by Pauling and Corey.
   • It is composed of two or more segments of fully extended peptide chains.
   • Hydrogen bonds are formed in between the neighboring segments of polypeptide chains.

3. Tertiary structure:

• The three-dimensional arrangement of protein is referred to as a tertiary structure.
• It consists of hydrophobic side chains inside the molecules while hydrophilic on the surface of the molecule.

4. Quaternary structure:

• Some of the proteins contain two or more polypeptide chains called subunits.
• The spatial arrangement of it is referred to as a quaternary structure.
• Subunits are held together by non-covalent bonds like hydrogen bonds, hydrophobic bonds, and ion bonds.