Proteins

Proteins

  • Proteins are a diverse class of biological molecules
  • They are polypeptides built of long chains of monomers called amino acids
  • All amino acids contain; carbon, nitrogen, oxygen and hydrogen
  • Many also contain sulphur or phosphorous

Structure of amino acids

  • All amino acids have the same general structure
  • The only difference between each amino acid is the nature of the R group. There are 20 different R groups giving 20 different amino acids
  • The R group represents a side chain from the central carbon. It can be anything from a simple hydrogen atom to a more complex structure
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Dipeptides and Polypeptides

Dipeptide

  • Two amino acids bond to form a dipeptide
  • A peptide bond forms between the carboxylic group and the amino group of the other amino acid
  • This is a condensation reaction so a molecule of water is released 

Polypeptides

  • Three or more amino acids joined together by peptide bonds form a polypeptide
  • There are up to four levels of protein structure; primary, secondary, tertiary and quarternary. Each of these play an important role in defining the overall structure and function of the protein

Primary

  • Determined by the type, number and sequence of amino acids in the chain
  • If the sequence changes (due to a mutation in the DNA) then the protein will be non functional even though the type and number of amino acids stays the same
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Polypeptides

Secondary 

  • The polypeptide chain can then twist to form either an alpha helix or a beta pleated sheet
  • Hydrogen bonds between the peptide bonds maintain the shape of the structure

Tertiary

  • A tertiary structure forms when the secondary structured polypeptide folds into a specific 3 dimensional shape
  • This structure is held in place by hydrogen bonds, ionic bonds and disulphide bonds

Quaternary

  • Formed when two or more polypeptides with a tertiary structure combine
  • The polypeptides are held together by hydrogen bonds, ionic bonds and disulphide bonds
  • An example of a quaternary structured protein is haemoglobin. It is made up of 4 tertiary structured polypeptides bonded together (2 alpha helix and 2 beta pleated sheet)
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Protein Classification

Proteins can be divided into two types;

  • Fibrous 
  • Globular

Fibrous

  • Have a secondary structure 
  • Long parallel polypeptide chains of amino acids joined by peptide bonds twisted into an alpha helix or beta pleated sheet and held in position by hydrogen bonds
  • Several long polypeptide chains twist together to form long, rope like fibres
  • Fibrous proteins are insoluble, so cannot dissolve

They have a structural function. Examples include; 

  • Collagen - The main component of connective tissue found in ligaments, tendons and cartilage 
  • Keratin - The main component of hard structures such as hair, nails, hooves and claws. Also a resistant protein in skin cells which helps prevent entry of pathogens
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Protein Classification

Globular Proteins 

  • Have a tertiary or quaternary structure 
  • Long polypeptide chains tightly fold into a specific 3D shape held in position by hydrogen bonds, ionic bonds and disulphide bonds between R groups of amino acids
  • Globular protiens are soluble in water so they can dissolve and diffuse into water

They have a biochemical function relying on the molecule's 3D shape. Examples include;

  • Enzymes - Tertiary structured globular proteins that catalyse chemical reactions 
  • Transport proteins - Such as haemoglobin, myoglobin and those embedded in membranes
  • Hormones - Such as oestrogen and insulin
  • Antibodies

Denaturing Proteins

  • If the bonds that maintain a proteins shape are broken, the protein will stop working properly and is denatured
  • Changes is pH, temperature and salt concentration can all denature a protein
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Proteins

  • Fibrous proteins lose their structural strength when denatured wheras globular proteins become insoluble

Testing for Proteins

  • Add Biuret agent (Copper sulphate solution and sodium hydroxide)
  • A purple/lilac colour shows protein is present
  • If it stays blue it is a negative result
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