Amino Acids & the Peptide Bond (AQA 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 the carboxylic group of one amino acid and a hydrogen atom is lost from the 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
• Dipeptides are formed by the condensation of two amino acids
• Polypeptides are formed by the condensation of many (3 or more) amino acids
• A protein may have only one polypeptide chain or it may have multiple chains interacting with each other
• During hydrolysis reactions, the addition of water breaks the peptide bonds resulting in polypeptides being broken down to amino acids

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