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)