The word ‘protein’ was originally coined from the Greek word proteios meaning “holding the first place”.
It’s no surprise then that proteins (and hence their subunits amino acids) are one of the most basic biological molecules essential for life. These macromolecules are responsible for tissue growth and repair, along with the formation of hormones and antibodies.
Proteins aren’t your run-of-the-mill macromolecules, they actually have 4 structural levels :
1) Primary Structure:
– found in all proteins; essentially it’s a special sequence of amino acids (joined via peptide bonds) in a peptide chain. This sequence is unique, not unlike every individual on this planet is unique in his/her own way *sheds tear*.
2) Secondary Structure:
– occurs when there is coiling or bending of a polypeptide into sheets (either α- helices or β-pleated sheets). Hydrogen bonds are usually found between the amide groups and are integral to holding these structures together. In an α-helix structure the bending is caused by the alternation of amino acid α-carbons.
Imagine this scenario.. a random guy finds a cork screw (this should represent the α- helix structure) and he also finds a bottle of unopened wine….he opens the wine; drinks all of it; ends up drunk and on top of his neighbour’s roof (galvanized roofing) where the galvanized roofing represents the β-pleated sheet.
3) Tertiary Structure:
-occurs where there is a folding back of a molecule upon its own self, (which is no easy feat) and certain bonds. These bonds include disulfide bridges and ionic,hydrogen and covalent bonds formed by amino acid R-group interactions; they hold everything in place and to add stability to the protein and their folding actually makes them functional (biologically active) proteins(Ferrier, 2014) .
4) Quaternary Structure:
(Woah…things are getting complicated up in here)
These are complex structures formed by the interaction of two or more polypeptide chains . A variety of bonding interactions, namely hydrogen bonding, salt bridges and disulfide bonds, hold the various chains to a particular 3D shape (Ophardt, 2003).
Quaternary proteins are further classified as 1) fibrous & 2)globular.
Unlike their fibrous cousins, globular proteins actually have charged, hydrophilic ends at their surfaces (hence their water solubility) and hydrophobic amino acid residues at their cores. Their peculiar organization works to stabilise the protein structure (nature loves stability). Haemoglobin is a great example; this protein binds to oxygen molecules in red blood cells, which then transport the O2 throughout the body.
Ferrier, D. R. 2014. Biochemistry. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
Ophardt, C. E. 2003. Quaternery Protein. [online] Available at: http://www.elmhurst.edu/~chm/vchembook/567quatprotein.html
Pover, A. 2012. OCR AS Level Biology: Globular and Fibrous proteins. [image online] Available at: http://andrewpover.co.uk/biology/ocr-as-level-biology-globular-and-fibrous-proteins/
School Animated gif. 2013. [image online] Available at: http://giphy.com/gifs/9xrSBojGBTLOg
Contributors: Thalia, Roi (editor)