Polypeptides: Best Friends with a Special Bond
Proteins are essential for life - they are needed for growth and repair. Protein molecules are comprised of polypeptides, which are polymers of amino acid chains. They have special properties associated with their structure, which we’ll explore today.
Proteins are made from around 20 amino acid monomer units that can be assembled in any order to produce many different varieties of proteins. These same amino acids are found in almost every living organism and provide evidence for evolution.
Amino acids contain an amine group of - NH2. It is chemically basic because it neutralised acids by accepting H+ ions to become NH3+. Amino acids also have a carboxylic acid group -COOH which dissociates to produce H+ ions when dissolved in water, leaving the group with a COO- charge. Charged groups mean they are attracted to water molecules and are soluble.
Amino acids have an R group attached to the central carbon atom. These are side chains like hydrogen molecules.
There are four main layers to proteins:
When two amino acids join together in a condensation reaction, they form a dipeptide with a peptide bond between them. This can be identified on a diagram as between a carbon (double bonded to an oxygen) and a nitrogen (bonded to a hydrogen).
Long chains of amino acids are called polypeptides. Proteins are made up of one or more polypeptides which have their own unique sequence of amino acids - this order is called the primary structure.
The way these polypeptides coil or pleat is the secondary structure of a protein. Two common structures include alpha helix and beta pleat shapes:
Polypeptide chains coil in a spiral held in place by hydrogen bonds that form between hydrogen atoms of the amine group and oxygen atoms on carboxyl groups.
The tertiary structure is further folding of the polypeptide chains to form 3D shapes. These are held together by hydrogen bonds, ionic bonds and disulfide bridges. These bonds occur between the R groups of amino acids. The 3D shape is determined by where the bonds form which is determined by the primary structure.
Each protein has its own tertiary structure, determining its function.
Proteins can be made up of more than one polypeptide chains, such a haemoglobin molecule. This is their quaternary structure.
So how do these properties relate to their function?
That’s for the next post!
