Download Peptide Bonds - Newcastle University

Amino acids join together via condensation reactions to form peptide bonds. Chains of amino acids are
called polypeptides. The order of amino acids in these chains is specific and this determines the primary
structure of a protein.
Peptide Bonds
Peptide bond formation is a nucleophilic
addition-elimination reaction. The nitrogen lone
pair of one amino acid attacks the carbonyl
carbon of another, leading to the formation of a
peptide bond. This is the addition step; the
elimination stage of the reaction is shown
below. Follow the curly arrows and the
movement of hydrogen atoms until you reach
the dipeptide and eliminated water molecule.
Remember that the peptide bond is rigid and planar because it is stabilised by delocalisation of
the nitrogen lone pair with the carbonyl oxygen.
Primary structure
Two amino acids joined together are called a dipeptide. The condensation reaction you have
just seen can repeat so any more amino acids can add together in the same way, forming a long
chain called a polypeptide. For each protein, the order of amino acid residues is specific and
different – it is this order that gives the protein its primary structure.
Remember that the order of amino acid
residues determines how the protein
will eventually fold – the shape it will
take. This is important to the protein’s
function – how it works. One single
change to the amino acid sequence can
completely change the proteins ability
to function – that’s how important it is!
Imagine these amino acids join together like you
have seen on the previous page:
Leucine – cysteine – methionine – valine –
glycine
The primary structure of this protein could be
represented as:
LEU – CYS – MET – VAL – GLY
At the left hand side of the protein chain, there
will be an amino acid with an unchanged –NH2
group. In this example it is leucine, and this is
called the protein’s N-terminal.
At the other end of the protein chain is and
unchanged –COOH group, in this example it
belongs to glycine. This is known as the Cterminal.
Or like this:
LCMVG
To save time, each amino acid has a three-letter
and one-letter code.
C=O
C-terminal
Nitrogen
N-terminal
The 3D representation of LCMVG – notice the repeating amino acid units and the R-group side
chains extending from either side of the polypeptide chain.
Produced by Lucy Jakubecz at Newcastle University as part of an MChem project.