Proteins are the most abundant and functionally diverse molecules in living systems. About 20% of the human body is made up of proteins and almost every life process depends on this class of molecules. Proteins display an incredible diversity of functions, yet all share the common structural feature of being linear polymers of amino acids.

Amino acids are the building blocks of protein. Although more than 300 different amino acids have been described in nature, only 20 are commonly found as constituents of mammalian proteins. These are the only amino acids that are coded for by DNA, the genetic material in the cell.

Each amino acid has the same basic structure , which consists of a carboxyl group (COOH), an amino group (NH2), a side chain group (R group), and a hydrogen atom attached to a central carbon atom, also known as the alpha (α) carbon. As the side chain changes the amino acid also changes and the nature of the side chains (R group) dictates the role an amino acid plays in a protein.

The carboxyl and amino groups are combined through peptide linkage to form long chains of amino acids called polypeptide. The linear sequence of the linked amino acids contains the information necessary to generate a protein molecule with a unique three-dimensional shape. The complexity of protein structure is best described by studying the four organizational levels, namely, primary, secondary, tertiary and quaternary.

The primary structure of protein comprises of number and sequence of amino acids in a polypeptide chain. The primary structure can be studied by various methods including Edman degradation, Overlapping of peptides, and DNA sequencing.

The regular arrangements of amino acids that are located near to each other in the linear sequence are termed as secondary structure of the protein. Examples of secondary structures frequently encountered in proteins are alpha helix (α-helix), beta sheet (β-sheet), and beta bend (β-bend or β-turn).

The tertiary structure of protein is formed when a single polypeptide bends and folds upon itself to form a globular structure. Interactions between the amino acid side chains guide the folding of the polypeptide to form a compact structure. The four types of interactions cooperate in stabilizing the tertiary structures of globular proteins are disulfide bonds, hydrophobic interactions, hydrogen bonds, and ionic interactions.

The quaternary structure of the protein is the arrangement of two or more polypeptide chains that may be structurally identical or totally unrelated. It is the most complicated and highest level organization of protein structure.

Classification of proteins based on structure divide them into globular protein and fibrous protein. Globular proteins are spherical or globular in shape. They have variety of biological functions such as catalysis, transportation, regulation and structure formation. The examples of globular proteins are hemoglobin, myoglobin, hormones, actin, tubulin, and enzymes. on the other hand, fibrous proteins form long protein filaments, which are shaped like rods or wires. They are structural or storage proteins that are typically inert and water-insoluble. The examples include Keratine, elastin, collagen, and fibroin.

If the proteins are not able to achieve the native state, they cannot perform their function properly and may lead to diseases. This might be due to an unwanted mutation in their amino acid sequence or simply because of an error in the folding process of protein. Some examples of diseases associated with protein are Sickle cell anemia, Thalassemia, and Alzheimer's disease.

This course is a valuable resource for students and researchers related to biochemistry, molecular biology, proteomics and biotechnology.

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