Structure and Function of Proteins

Key Concepts

• Proteins
• Amino acids
• Polypeptide chain
• Peptide bond

Introduction: 

Cells are the basic unit of living organisms; from bacteria to humans, all are made up of cells, which contain a nucleus, and the nucleus contains DNA and proteins are the most important substances in our body. Every cell in our body contains proteins.  

Explanation: 

Proteins are found in muscles, bones, skin, hair, and in other organs or tissue in the body. It is the building block of enzymes,  

Amino acids are twenty-plus basic building blocks that makeup protein. Our system produces them in two ways:  

1. From starch  

2. By modifying other amino acids 

Structure of proteins: 

The structure of proteins is a 3-dimensional arrangement of amino acid residues that link up to form a polypeptide chain. There are 20 different types of amino acids that combine together and make up proteins. These amino acids are identical and have different side chains. The amino acid sequence of proteins has a unique 3-dimensional structure. Protein in the human body is required for the structure, regulation, and functions of the tissues and organs of the body. 

Types of Proteins 

Primary Structure:

The primary structure is a linear polypeptide chain made up of a sequence of amino acids. Changing a single amino acid position as there are limited amino acid monomers, for example, 20 present in the human body will result in alteration of the 3-dimensional structure of the protein which leads to different chains and finally a different protein. This simple sequencing of protein by amino acids is called its primary structure. For example, Human insulin has two polypeptide chains, A and B. 

Secondary Structure:

The secondary structure is the folded structure formed by a hydrogen bond between atoms of the polypeptide chain except for atoms of the R-group. The 3-D shape of the protein has two different conformations known as α-helix and β-pleated sheets. This structure results in the hydrogen bonds, which form between the amino H atom of one amino acid and the carbonyl O of another. 

• α-Helix: The carbonyl group (C=O) forms backbone of a hydrogen bond with the amino H (N-H) group. The bond forms a helical structure that resembles curled ribbons within the polypeptide chain. 

• β-Pleated Sheet:  Structure is formed by hydrogen bonds between two or more strands of polypeptide chain which are aligned next to each other, forming a sheet-like structure. N-H groups form hydrogen bonds with the C=O group, the R-group either extends above or below the plane of the sheet. 

The other functional groups that are linked to each protein, like carboxylic acid, alcohols, carboxamides, etc. These are the functional groups responsible for affecting protein folding and its function. 

Tertiary Structure:

The 3-dimensional shape of the polypeptide is called the protein tertiary structure. The tertiary structure is formed by the repulsive and attractive forces of different R-groups of amino acids that make up a protein. 

The secondary interactions are seen in tertiary structures including ionic bonding and hydrogen bonding. R-groups are polar in nature, they form hydrogen bonds and similarly, R-groups with opposite charges form the ionic bonds.  

Non-polar hydrophobic R-groups are formed together within the protein. The disulfide bond also contributes to the tertiary structure by a covalent link between the cysteine chain’s sulfur-containing side by forming polypeptide parts attached firmly to each other, they act as “Molecular safety pins”. 

Quaternary Structure:

Multiple polypeptide chains are linked together to form proteins that are called subunits. The arrangement of subunits forms multi-subunits which results in the quaternary structure of proteins. Proteins and other macromolecules present in the body interact to form complex assemblies. 

Proteins are unable to perform transmission of cell signals and carry out DNA replication.  

For example  

• Hemoglobin carries oxygen in the blood. It is formed by two subunits α and β type, a total of four subunits. 
• DNA polymerase is composed of 10 subunits that an enzyme uses to synthesize new DNA strands.