Proteins

= Structure of proteins =

Proteins are large macromolecules made up of chains of 2-amino acids. Amino acids bond to each other through condensation reactions to form a polypeptide, in which the amino acid residues are joined to each other by an amide link.



A **peptide bond** is formed when via covalent binding of the carbon atom of the carboxyl group of one amino acid to the nitrogen atom of the amino group of another amino acid by dehydration. A **polypeptide chain** is a chain of amino acids linked by peptide bonds.  Proteins are made up of a fixed number of amino acid residues connected to each other, each in a unique linear sequence. This unique sequence resulting in a polypeptide chain is called a **primary structure**. This sequence is held together by peptide bonds. Primary structures are the base structure for any secondary and tertiary structures.   **Secondary structures** refer to the way in which the polypeptide chain folds as a result of intramolecular hydrogen bonding. Secondary structures are repeating sequences of amino acids and therefore create a pattern in which it folds into. The folding of the polypeptide chain results in either an alpha helix or a beta-pleated sheet. In an alpha helix the polypeptide chain coils in a spiraling form, and in the beta-pleated sheet, the polypeptide chain becomes a sheet-like structure as is illustrated in the image below.   The **tertiary structure ** is the overall folding of the chains, or in simpler terms, a 3-D molecule of a single protein. This folding in the single protein molecule is a result of interactions between distant amino acids, the interactions may be hydrogen bonds, van de Waals' attraction between non polar side groups, or ionic attractions between polar groups. Also, disulfide bridges may occur when two cysteine residues form together and their sulfur atoms undergo enzyme catalyzed oxidation.
 * Amino acids** consist of a central carbon atom and an amino group (NH2), a hydrogen atom, and a carboxyl group (COOH) and a side chain (R).
 * Backbone** given by the repeated sequence of three atoms: the amide N, alpha carbon and the carboxyl C.

Finally, a **quaternary structure** is the more complex protein structure that consists of an interaction between primary, secondary and tertiary protein structures; it is a larger assembly of several protein molecules or polypeptide chains. A common example is haemoglobin, which consists of four protein chains: two alpha-chains and two beta-chains grouped together around four haem groups.

Uses of proteins
Proteins have different functions in the body:
 * Act as biological catalysts for specific reactions


 * Give structure (e.g hair, nails) and provide a source of energy
 * Some hormones are proteins (e.g FSH follicle stimulating hormone), responsible for triggering the monthly cycle in females.