Tertiary and Quaternary Structure of Proteins

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LEARNING OBJECTIVES

In this article, author has explained the tertiary, quaternary structure of proteins and their characteristics.

Tertiary structure of proteins

The three dimensional structure of proteins is known as tertiary structure of protein. The tertiary structure is formed due to interaction between R groups of amino acid that make up proteins. The protein folds in such a way to attain maximum stability and lowest energy.

Characteristics of tertiary structure

  • The interactions between R groups can be hydrogen bonding, dipole-dipole interaction, London dispersion and ionic bonding.
  • Tertiary structure is a compact structure with hydrophobic side chain extended interior while hydrophilic side is present outwards.
  • The R groups with same charges repel each other, while those opposite charges forms ionic bond.
  • Polar R groups form hydrogen bonds while other form dipole-dipole interactions.
  • Hydrophobic interactions are also important in tertiary structure. The non-polar amino acids with hydrophobic side chains have hydrophobic interactions which are extended inside leaving hydrophilic amino acids outside to interacts water molecules.
  • There is another covalent bond present in tertiary structure which is disulfide bond. Disulfide bonds are present between the sulfur containing cysteines side chains. Disulfide bonds are much stronger than other type of covalent bonds present in tertiary structure. Disulfide bonds act as “safety pins” keeping the parts of polypeptide chain together.
  • There is another basic structure present in tertiary structure known as domain. Domain is related to protein structure (tertiary) and functions. A polypeptide consists of 200-300 amino acids, has two or three domains.
  • Tertiary structure gives rise to two major molecular shapes; globular and fibrous.

Quaternary structure of proteins

Proteins usually are composed of single polypeptide chain. Some proteins have multiple polypeptides also known as subunits. When these subunits come together they give rise to quaternary structure of proteins.

Characteristics of quaternary structure:

  • The quaternary structure containing proteins are known as oligomers,
  • The individual polypeptide chains are known as monomers, protomers and subunits.
  • A dimer consists of two polypeptides while tetramer consists of four polypeptides.
  • The hydrophobic interaction is present between the non-polar side chains at the contact point of subunits.
  • Electrostatic interactions are present between the ionic groups of opposite charges.
  • Hydrogen bonds are present between the polar side chains of polypeptide.
  • Besides these, disulfide bonds are also present.

Example of quaternary structure

Hemoglobin is a example of quaternary structure of proteins. It is a large protein who functions to carry and transport blood throughout the bloodstream. Hemoglobin is compose of four subunits, two alpha subunits and two beta subunits. The subunits come together by specific interactions of side chains.

Bonds responsible for protein structure

Protein structure is stabilized by two types of bonds; covalent and non-covalent.

1. Covalent bonds

Two types of covalent bonds are found in protein structure; peptide and disulfide bonds.

Peptide bonds

This covalent bond is formed between the amino group of one amino acid and carboxyl group of next amino acid. Peptide bonds connect amino acids. A dipeptide has one peptide bond while tripeptide bond has two peptide bonds.

Disulfide bonds

A disulfide bond is formed between the sulfhydryl groups (SH) of two cysteines to produce cystine. A disulfide bond can be formed in a single polypeptide or maybe between different polypetides. The disulfide bonds contribute to structural conformation and stability of proteins.

2. Non-covalent bonds

There are four types of non-covalent bonds:

Hydrogen bonds

The hydrogen bonds are formed between the nitrogen and carbonyl oxygen of different peptide bonds. Individually hydrogen bond is weak but collectively they are strong. A large number of hydrogen bonds contribute to the structure of proteins.

Hydrophobic bonds

The non-polar side chains of neutral amino acids tend to stay close in the protein structure. The hydrophobic interactions are found in aqueous environment where molecules have to stay together.

Electrostatic interactions

Electrostatic interactions are present between the negative charged group (COO–) of acidic amino acid and positively charged group (NH3) of basic amino acid.

Wan der Waal forces

The non-covalent interactions present between electrically neutral molecules are known as Wan der Waal forces. These are electrostatic interactions formed due to permanent or induced dipoles.