Amino Acids & the Peptide Bond (CIE A Level Biology)

Proteins

• Proteins are polymers (and macromolecules) made of monomers called amino acids
• The sequence, type and number of the amino acids within a protein determines its shape and therefore its function
• Proteins are extremely important in cells because they form all of the following:
  • Enzymes
  • Cell membrane proteins (eg. carrier)
  • Hormones
  • Immunoproteins (eg. immunoglobulins)
  • Transport proteins (eg. haemoglobin)
  • Structural proteins (eg. keratin, collagen)
  • Contractile proteins (eg. myosin)

Amino acid

• Amino acids are the monomers of proteins
• There are 20 amino acids found in proteins common to all living organisms
• The general structure of all amino acids is a central carbon atom bonded to:
  • An amine group -NH₂
  • A carboxylic acid group -COOH
  • A hydrogen atom
  • An R group (which is how each amino acid differs and why amino acid properties differ e.g. whether they are acidic or basic or whether they are polar or non-polar)

The generalised structure of an amino acid

Peptide bond

• In order to form a peptide bond a hydroxyl (-OH) is lost from a carboxylic group of one amino acid and a hydrogen atom is lost from an amine group of another amino acid
• The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid
• This is a condensation reaction so water is released. The resulting molecule is a dipeptide
• When many amino acids are bonded together by peptide bonds the molecule formed is called a polypeptide. A protein may have only one polypeptide chain or it may have multiple chains interacting with each other
• During hydrolysis reactions polypeptides are broken down to amino acids when the addition of water breaks the peptide bonds

Amino acids are bonded together by covalent peptide bonds to form a dipeptide in a condensation reaction