Primary structure of proteins: Refers to sequence of amino acids linked together through peptide bonds in a polypeptide chain. It also refers to position of disulphide bonds (-S-S) that are formed between the -SH groups of cysteine residues that are adjacent in space but not in the linear sequence of amino acids.
Secondary structure of proteins: The polypeptide chain undergoes regular folding or coiling to give rise secondary structure of protein. The coiling of polypeptide chain is produced and stabilized by weak, low energy non-covalent hydrogen bonds and covalent disulpide bonds. Each hydrogen bond is weak, but collectively hydrogen bonds are strong enough to stabilize the secondary structure. The two most prominent types of secondary structure of proteins are (i) alpha-Helix & (ii) beta-Pleated sheet.
Tertiary structure of proteins: Refers to overall three-dimensional structure (native conformation) of a protein molecule. The tertiary structure is derived by further folding and super-folding of polypeptide chain(s) exhibiting secondary structure. The tertiary structure is stabilized by covalent disulfide bonds and various weak non-covalent bonds (hydrophobic interactions/van der Waal forces, ionic interactions and hydrogen bonding).
Quaternary structure of proteins: Quaternary structure refers to arrangement of multiple protein subunits (polypeptide chains) in a multi-subunit protein. Proteins having two or more polypeptide chains (protein subunits), are known as multi-subunit proteins or multimeric or oligomeric proteins (like CPK, LDH, glycogen phosphorylase, Hb, etc.) and exhibit quaternary structure. Different multi-subunit proteins have 2-200 subunits. In a multi-subunit protein, all the polypeptide chains (subunits) are arranged in a characteristic manner and held together by non-covalent bonds (hydrogen bonds, ionic bonds & hydrophobic interactions) & covalent disulfide bonds to give rise a quaternary structure.
