Proteins

• The different functions of proteins:

  • Structural function e.g. myosin in muscles. 
  • Enzymes are all made from proteins. 
  • Some hormones are proteins e.g. insulin. 
  • Antibodies involved in the immune response are made from protein. 
  • Protein receptors e.g. receptor for insulin 
  • Transport proteins e.g. carrier and channel proteins in plasma membranes 
  • Antigens e.g. recognition of the cell as self and non-self 
• Proteins are used as structural components used in making new cells for growth and repair and are very important as enzymes that which control metabolic reactions. 
• Amino acids are monomer units of proteins.
• Amino acids countain the elements Carbon, Hydrogen, Oxygen and Nitrogen (sometimes Sulphur- S). 
• There are 20 different types of naturally occurring amino acid monomers
• Each amino acid has the same general structure - a central carbon with four groups of atoms attached. Three of the groups are the same on all amino acids – an amine group (-NH2), a carboxyl group (-COOH) and a hydrogen atom (-H), but the forth - the R group differs depending on the amino acid
• Glycine is the simplest amino acid, with its R group being one Hydrogen.
• Some R groups are charged (polar) and are therefore are hydrophilic (i.e. water loving). Whereas others are non-polar and are hydrophobic (i.e. water repelling). 
• An Amino Acid:

  A) R

  B) N

  C) C

  D) C

  E) H

  F) H

  G) H

  H) O

  I) OH

  
• Two Amino acids join together to form a dipeptide via a condensation reaction occurring between the carboxylic acid group of one amino acid and the amine group of another to form a strong covalent bond known as a peptide bond.
• When two amino acids bond together, a dipeptide and a water molecule are produced.
• Primary Structure- The sequence of amino acids held together by peptide bonds. It determines the secondary and tertiary structure and thus the final 3D shape of the protein.
• Secondary structure- folding of the primary structure.
• Secondary structure- the polypeptide chain coils to form an alpha helix or folds to form a beta pleated sheet. Both these secondary structures are held together by many weak hydrogen bonds which overall make the structure.
• Tertiary Structure- This is the further folding of the polypeptide chain to give a more complex 3D shape and is closely related to the function of a particular protein.
• The tertiary structure is stabilised by:

  • Hydrogen bonds
  • Ionic bonds
  • Disulphide Bridges
  • Hydrophobic interactions
• Tertiary structure- Hydrogen bonds – weak bonds between the R groups  - easily broken
• Tertiary structure- Ionic Bonds - between positively and negatively charged R groups of amino acids. 
• Tertiary structure- Disulphide bonds - strong covalent bonds formed between sulphurs in the R- group of the amino acid cysteine. 
• Tertiary structure- Hydrophobic interactions - between non-polar R groups which tend to cluster together towards the centre of the molecule. 
• Quaternary structure- proteins made up of more than one polypeptide chain.
• The 3D shape of molecules can be divided into two main groups:

  • Fibrous proteins
  • Globular proteins
• Fibrous proteins form long fibres. They have regular, repetitive sequences of amino acids and are usually insoluble in water. They tend to have structural roles in living organisms: e.g. myosin in muscles, keratin in hair, collagen in skin. 
• Fibrous Proteins- Collagen is found in skin, bones and ligaments.
• Collagen:

  • It is made from three identical left handed helix polypeptide chains wound around each other to form a triple helix (quaternary structure) 
  • In each polypeptide chain every third amino acid is glycine. Glycine is small so it allows 3 polypeptides to pack closely together.  
  • 3 chains held together by hydrogen-bonds 
  • Collagen molecules cross-link through covalent bonds to form fibres which give collagen its great strength 
• Globular Proteins

  These fold up into a compact ball like shape. Hydrophobic R- groups on amino acids tend to be turned inwards towards the centre of the protein and hydrophilic R -groups tend to be on the outside. This means that they tend to be more water soluble than fibrous proteins.
• Globular proteins tend to have a metabolic role in living organisms e.g. all enzymes, plasma proteins and antibodies have globular structure. Globular proteins have a wide range of amino acid sequence in their structure.
• Globular Proteins- Haemoglobin

  • Has a compact ball shape.
  • Consists of 4 polypeptide chains. There are 2 alpha chains and 2 beta chains
  • Each chain has a haem group which contains a Fe2+ ion. This haem group gives blood its colour.
  • Its function is to carry oxygen from the lungs to the respiring tissues.