Chapter 22, Problem 22.60P

Interpretation Introduction

Interpretation:

Effect of hydrophobic interactions on the tertiary structure of proteins must be explained.

Concept introduction:

Proteins are biological polymers made up of amino acids. Amino acids are connected through peptide bonds to make polypeptide chains.

Amino acids are molecules that contain both amino group and carboxylic acid group attached to the same carbon atom. A peptide bond is formed by condensation of amino group of one amino acid to the carboxylic acid of other amino acid.

Structure of proteins plays a very important role in their function. Proteins are very complex in structure. Structure of protein is studied in four levels: Primary, Secondary, Tertiary and Quaternary structure.

Primary structure:

Primary structure of a protein is the sequence of amino acids in each polypeptide chain that make up the protein. The ultimate structure of protein depends on this sequence.

Secondary Structure:

The peptide backbone of polypeptide chain folds onto itself due to interactions between amino and carboxylic acid residues in the peptide backbone. This folding of polypeptide chains give proteins a unique shape, this makes the secondary structure of proteins.

Two kinds of shapes are formed in the secondary structure of proteins:

1. α-Helix: The backbone folds itself to form a helical structure. Hydrogen bonds are formed with the chain.
2. ß-Pleated sheet: The polypeptide chains are stacked side by side. The outer N-H and C=O form intermolecular hydrogen bonds and give a very rigid structure. These hydrogen bonds are formed between neighboring polypeptide chains unlike α-Helix.

Tertiary Structure:

The overall 3-Dimensional structure of a protein formed when regions in secondary structure fold together, is called the tertiary structure of a protein. The tertiary structure of a protein is primarily due to interactions between the side chains of the polypeptide chains or the side chains in the backbone of the polypeptide.

The interactions between the side chains include: hydrogen bonding, ionic interactions, dipole-dipole interactions and London dispersion forces. Another important interaction that makes up the tertiary structure of proteins are hydrophobic interactions between the hydrophobic r groups of side chain of amino acids.

One special kind of covalent bond is also involved in forming the tertiary structure of proteins that is the disulfide bond formed between the -SH residues of cysteine.

Quaternary Structure:

When proteins contain more than one polypeptide chain, the final arrangement of each polypeptide subunit is known as the quaternary structure. The same kinds of interactions that make the tertiary structure are also involved in forming the quaternary structure.