Formation of Proteins (HL IB 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 (e.g. carrier)
  • Hormones
  • Immunoproteins (e.g. immunoglobulins)
  • Transport proteins (e.g. haemoglobin)
  • Structural proteins (e.g. keratin, collagen)
  • Contractile proteins (e.g. myosin)
• Because all genes code for proteins, all of the reactions necessary for life are dependent on the function of proteins

Amino acids

• Amino acids are the monomers of polypeptides
• There are 20 amino acids found in polypeptides common to all living organisms
• The general structure of all amino acids is a central carbon atom, the alpha carbon, bonded to:
  • An amine/amino group -NH₂
  • A carboxylic acid/carboxyl 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 R group can be as simple as another hydrogen atom (glycine), right through to complex aromatic ring structures  (e.g. phenylalanine)

Structure of an amino acid diagram

The generalised structure of an amino acid

Peptide bond

• In order to form a peptide bond, a hydroxyl group (-OH) is lost from the carboxylic group (-COOH) of one amino acid and a hydrogen atom is lost from the amine group (-NH2) 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
  • The word equation for this reaction is amino acid + amino acid → dipeptide
• 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 into amino acids
• Molecular modelling kits can be used to build physical models that demonstrate peptide bond formation between different types of amino acids

Peptide bond diagram

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

Drawing a peptide bond diagram

These steps can be followed to draw a peptide bond and a generalised dipeptide

• There are 20 naturally occurring amino acids
• Our cells can synthesise 11 of these from other amino acids
  • These are termed non-essential amino acids
• The remaining nine we need to consume via our diets 
  • These are called essential amino acids
• A healthy, varied, well balanced diet will contain all the nine essential amino acids required
• Diets that restrict certain foods may require supplementation 
  • Meat contains all nine essential amino acids so a vegetarian or vegan diet needs to be well balanced and varied to ensure all essential amino acids are consumed regularly 

Essential amino acid sources diagram

Plant based (vegan) sources of the nine essential amino acids