Amino Acids & the Peptide Bond (Cambridge (CIE) AS Biology): Revision Note

Amino acids & the peptide bond

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 (e.g. carrier)

  • Hormones

  • Immunoproteins (e.g immunoglobulins)

  • Transport proteins (e.g haemoglobin)

  • Structural proteins (e.g keratin, collagen)

  • Contractile proteins (e.g. myosin)

Amino acids

• 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 peptide bond

• In order to form a peptide bond a hydroxyl group (-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

• 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