C.2.4: Describe and Explain the Primary, Secondary, Tertiary and Quaternary structure of proteins

Structural features of proteins are usually described at four levels of complexity:

Primary structure
The primary structure of a protein describes the linear sequence of amino acids in the protein chain. Each amino acid is joined to another by a peptide bond. A peptide bond forms between the carboxyl group of one amino acid and the amine group of the next. The primary structure is usually written in three-letter code, for example: ala for alanine, leu for leucine. The sequence of amino acids in a protein is determined experimentally by reacting amino acids on one end of the molecule with a substance that creates colored or fluorescent derivatives. The protein is then hydrolyzed by a highly specific enzyme that removes only the terminal amino acid.
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Figure 1.1: Primary Structure of Protein

Secondary structure
The secondary structure describes the folding or coiling within a protein/polypeptide chain into a α-helix or a β-sheet. Hydrogen bonds between lone pairs of electrons on the oxygen atom in the carboxyl group of one amino acid and the hydrogen atom in the amino group of another amino acid that causes the protein to form this structure. If the hydrogen bonds occur within a single protein chain, they are intramolecular hydrogen bonds and they cause the protein chain to form a spiral structure called an α-helix or alpha helix.
If the Hydrogen bonds occur between different protein chains, the protein forms a sheet β-sheet or beta pleated sheet.

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Figure 1.2: α-helix structure - the yellow dots symbolizes hydrogen bonds between the amino acids in the protein chain.

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Figure 1.3: β-sheet structure - the yellow dots symbolizes H-bonds between different chains of amino acids.

These different structures gives the protein special physiological characteristics; the α-helix structure gives the protein elasticity and is found in fibrous proteins like hair. The β-sheet structure gives the protein flexibility and strength as well as a resistance to stretching.


Tertiary structure
The overall folding of the polypeptide chain into a 3D shape is called its tertiary structure. Proteins in their tertiary structures are either fibrous or globular in shape. The tertiary structure is maintained by many different inter and intramolecular bonds which include: hydrogen bonds, dipole-dipole interactions, ionic bonds, covalent bonds, Van der Waal's forces and disulphide bridges. Disulphide bridges are covalent bonds that form between sulphur atoms on the oxidation of two cysteine amino acids.
Fibrous proteins consists of parallel chains cross-linked at intervals to form long strands, while globular proteins are made out of folded polypeptide chains so that their hydrophobic groups are on the inside of the molecule and the hydrophilic ends are facing outwards. The specific 3D shape of the protein gives a protein its function. Consequently, by denaturing the protein with heat, radiation or chemical reactions, the bonds that holds the shape together are disrupted and the protein unfolds, losing its function.

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Figure 1.4: Examples of Fibrous and Globular Proteins

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Figure 1.5: Tertiary Structure with Disulfide Bridges

Quaternary structure
Quaternary structures refers to a protein composed of proteins bound by non-covalent interactions; not all proteins have this structure. One of the most known quaternary structure protein is haemoglobin, which has 4 polypeptide chains (2 alpha chain and 2 beta chain) and 4 non-protein haem groups.

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Figure 1.6: Haemoglobin Quaternary Structure

A Figurative Review of Protein Structures:
primary structure to quaternary structure