Levels of protein structure and how they are affected by denaturation

Protein is a polymer that is made up of amino acids (AA) monomers that are joined together with the aid of peptide bonds. They occur naturally. Proteins are divided into different groupslevels depending on their structural composition. They include primary structure, secondary structure, tertiary and quaternary structure. Their structural composition can be destroyed if they are exposed to different changes in temperature, PH and various chemicals (Barry, p.209).

Different levels of protein structure
Primary structure this is the arrangement of amino acids in reference to different locations of disulphide bonds in the polypeptide chains. The primary structure can be said to be the basis for all other covalent bonds in the polypeptide chain. It is usually denoted by use of a three letter standard abbreviation for (AA) such as gly-gly-ser-ala which is the primary structure for polypeptides containing glycine, serine, and alanine respectively. The N terminal contains glycine while the C-terminal contains alanine (Reginald and Charles, p.79).

Secondary structure this is the arrangement of amino acids in the localized areas of a protein molecule. These folding patterns are stabilized by hydrogen bonds. The main structures in this level are alpha helix structure and anti-parallel beta-pleated sheet. Although there are different periodic conformations, alpha and beta pleated sheets are the most stable. A protein or single polypeptide can be comprised of more than one secondary structure. Alpha-helix is a clockwise spiral with planar peptide bonds and a trans conformation. The amine groups for each of the peptide bond generally run upward at the same time parallel to the helix axis. Carboxyl group generally points in the upward direction. Beta-pleated sheets are made of extensive polypeptide chains. Their neighboring chains extend anti-parallel to one another. Each of their peptide bonds is planar and trans just like the ones for alpha-helix structure. The carboxyl and amine groups of the peptide bonds lie in the same plane pointing at each other making it possible for hydrogen bonds to occur between neighboring polypeptide chains. Hydrogen bonding among carboxyl and amine groups in the same polypeptide stabilizes the helix structure (Kenneth, Enrique and Paul, p.500).

Tertiary structure this is a three dimensional-array of atoms in a polypeptide chain. In polypeptides with single conformational pattern such as alpha helix, tertiary and secondary structures may remain the same. For proteins with one polypeptide molecule, the highest structure level that it can attain is the tertiary structure. This level is mainly kept together by disulphide bonds that form among cysteine side chains through oxidation of 2 thiol groups (Reginald and Charles, p. 97).

Quaternary structure this refers to proteins that contain multiple sub-units. These proteins contain a molecular weight of more than 50,000 with 2 or more non covalently-linked monomers. Arrangements of monomers in a 3 dimensional protein make a quaternary structure. An example of a protein with quaternary structure is hemoglobin protein. The main stabilizer in the quaternary structure is the hydrophobic interaction (Kenneth, Enrique and Paul, p.500).

Protein denaturation
According to Shuryo and Wayne (p.129), this is destruction of the protein structure resulting in loss of biological activity of proteins. It is usually caused by changes in PH, temperature and chemical composition. The primary structure that is mainly held together by covalent bonds is not affected by denaturation. The secondary structure is usually affected with the proteins loosing all their regular repeating patterns and adapting a random coil pattern. In tertiary structure, it involves disruption of interaction of covalent contact among AA side chains, dipole-dipole non-covalent interactions among polar AA side chains and Van der Waals interaction among non-polar AA side chains. In Quaternary denaturation, sub-units of proteins are partially rearranged destroying the three dimensional structure. To maintain the biological activities of proteins, its advisable that optimal conditions are maintained at all times.


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