Views: 4
Structural composition
Proteins are made of amino acids. Each amino acid consists of an amino group (-NH2), a carboxyl group (-COOH), and a unique side chain (R-group). The sequence of amino acids in a protein determines its structure and function.
There are 20 common amino acids from which all proteins in living organisms are made. Nine of them are considered essential amino acids, as they cannot by synthesised in the body from other compounds, and must be obtained from the diet. Amino acids are bonded together by peptide bonds to form peptides. A long peptide chain forms a protein, which folds into a very specific three-dimensional shape. This three-dimensional shape is completely determined by the identity and order of the amino acids in the peptide chain.
There are four different levels of protein structure as shown below :
The primary, secondary, tertiary and quaternary levels of protein structure
- Primary structure: This refers to the sequence of amino acids joined together by peptide bonds to form a polypeptide chain. Some proteins have fewer than a hundred amino acids, while others have several thousand.
- Secondary structure: This is the first level of three dimensional folding. It is driven completely by hydrogen bonding. Hydrogen bonding usually results in regions of the chain coiling and other regions forming sheets.
- Tertiary structure: This is the second level of three dimensional folding and is the overall final shape of the protein molecule. The secondary structures and unstructured regions of the chain further fold into a globular shape, driven by hydrophobic interactions (non-polar regions trying to escape the water in the cell environment) and electrostatic interactions (polar and charged regions wanting to interact with the water environment and each other).
- Quaternary structure: Some proteins are complex: two or more peptide chains fold into their tertiary structures, then these complete structures associate together by hydrophobic and electrostatic interactions to form the final protein.