Structure of Protein

Proteins are composed of polymers of amino acid residues. The amino acids are joined together by peptide bonds. Each protein is a linear polymer built from different amino acids. The type and the sequence of amino acids in a protein are specified by the DNA in the cell that produces them. The genetic code typically specifies 20 standard amino acids. However, some organisms, such as archaea, have a genetic code that specifies more. This sequence of amino acids is essential since it determines the overall structure and function of a protein. Some proteins can form a complex together with another protein. Others form a complex with other biomolecules other than the peptide. Some of these non-peptide groups in a protein are referred to as cofactors or prosthetic groups. (Ref. 5)

There are four distinct types of protein structure: (1) primary structure, (2) secondary structure, (3) tertiary structure, and (4) quaternary structure. (Ref. 6)

• The primary structure pertains to the sequence of amino acids in a polypeptide chain.
• The secondary structure of a protein refers to the regularly repeating local structures that are stabilized by hydrogen bonds. Examples of a secondary structure are α-helix, β-sheet, and Turns and Loops.
• The tertiary structure (sometimes called fold) pertains to the spatial relationship of the secondary structures from one to another. It is often stabilized by nonlocal interactions, e.g. by disulfide bonds, salt bridges, and the formation of a hydrophobic core. This structure of the protein is what determines the fundamental function of the protein.
• The quaternary structure refers to the protein complex, i.e. when several polypeptide chains (sometimes referred to as protein subunits) constitute it. The tertiary and quaternary structures are often called conformations. The protein may transition from one structure to another and the transition between tertiary and quaternary by a conformational change. Conformational changes may be induced for example when a substrate binds to a protein (particularly, an enzyme) at its active site.