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Structure of an amino acid

  • the basic monomer units which combine to make up polymers called polypeptides.
  • polypeptides can be combined to form proteins.
  • every amino acid has a central carbon atom and four different chemical groups

1. amino group: a basic group (-NH2)

2. carboxyl group: acidic group (-COOH)

3. hydrogen atom (-H)

4. R group: different chemical groups (R) 

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The formation of a peptide bond

  • amino acid monomers can combine to form a dipeptide
  • this happens in a condensation reaction, the bond combines the -OH group of one amino acid and the -H group of another 
  • the two amino acids are joined by a peptide bond 
  • the bond can be broken by adding water (hydrolysis
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Primary Structure

  • many amino acid monomers joined together is called polymerisation 
  • the resulting chain is called a polypeptide 
  • the sequence of amino acid forms the primary structure of any protein
  • there is a limitless number of combinations of amino acids and so a limitless number of primary protein structures 
  • the primary structure determines the utimate shape and function of the protein 
  • a change in amino acids can lead to a change in shape and may stop it carrying out its function 
  • a proteins shape is specific to its function 
  • a protein is commonly made up of a number of polypeptide chains   
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Secondary Structure

  • the linked amino acids that make a polypeptide posses both -NH and -C=O groups on either side of the peptide bond 
  • the -NH group is positive and the -C=O group is negative so they form a weak hydrogen bond 
  • this causes the structure to twist into a 3-D shape called an alpha helix 
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Tertiary Structure

  • the secondary structures can be twisted and folded to give a more complex and unique 3-D structure 
  • the structure is maintained by a number of different bonds 

1. disulfide bonds: fairly strong 

2. ionic bonds: weak and easily broken 

3. hydrogen bonds: numerous but easily broken 

  • the 3-D shape is important when it comes to functions 
  • it makes each protein specific and allows it to be identified by other molecules 
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Quaternary Structure

  • large proteins form complex molecules containg multiple polypeptide chains that are linked in various ways 
  • they also contain non-protein groups (prosthetic) which contribute to the shape and function  
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Biuret test

1. place a sample of the solution in a test tube with the same amount of sodium hydroxide

2. add dilute copper sulfate and mix 

3. if the solution turns purple then there are proteins present  

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