2.3.4 Amino Acids & Peptide Bonds

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 acids

• Amino acids are the monomers of polypeptides
• 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 (also called amino) 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 general structure of an amino acid

• Peptide bonds form between amino acids
• Peptide bonds are covalent bonds and so involve the sharing of electrons
• In order to form a peptide bond :
  • A hydroxyl (-OH) is lost from the carboxylic group of one amino acid
  • 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

Peptide bonds are formed by condensation reactions (releasing a molecule of water) and broken by hydrolysis reactions (adding a molecule of water)