Amino Acids & Peptide Bonds (Edexcel A (SNAB) A Level Biology)

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Cara Head

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Cara Head

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Amino Acid: Structure

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)

Amino acid groups

The general structure of an amino acid

The Peptide Bond

  • 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

Dipeptide formation

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

Examiner Tip

When asked to identify the location of the peptide bond, look for where nitrogen is bonded to a carbon which has a double bond with an oxygen atom, note the R group is not involved in the formation of a peptide bond.

Structures of specific amino acids are not required.

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding