proteins

• structure of an amino acid

  • amino acids are the basic monomer units which combine to make up a polymer called a polypeptide
  • polypeptides can be combined to perform proteins
  • about 100 amino acids have been identified of which 20 occur naturally in proteins
• every amino acid has a central carbon atom to which are attached four different chemical groups:

  • amino group (-NH2)- a basic group from which the amino part of the name amino acid is derived
  • carboxyl group (-COOH)- an acidic group which gives the amino acid the acid part its name
  • hydrogen atom (-H)
  • R (side) group - a variety of different chemical groups. Each amino acid has a different R group. These 20 naturally occurring amino acids differ only in their R (side) group
• the formation of a peptide bond

  • amino acid monomers can combine to form a dipeptide - the process requires a condensation reaction through the removal of a water molecule
  • the water molecule is made by combining an -OH from the carboxyl group of one amino acid with an -H from the amino group of the other amino acid
  • the two amino acids then become linked by a new peptide bond between the carbon atom of one amino acid and the nitrogen atom of another
  • the peptide bond can be broken by a hydrolysis reaction
• the primary structure of proteins - polypeptides 


  • through a series of condensation reactions many amino acid monomers can be joined together in a process called polymerisation. The resulting chain of many hundreds of amino acids is called a polypeptide
  • the sequence of amino acids in a polypeptide chain forms the primary structure of any protein
  • as polypeptides have many of the 20 naturally occurring amino acids joined in different sequences it follows that there is almost a limitless number of possible combinations, and therefore types of primary protein structure
• primary structure of proteins

  • it is the primary structure of a protein that determines its ultimate shape and hence its function
  • a change in a single amino acid in the primary sequence can lead to a change in shape of the protein and may stop it carrying out its function
  • a proteins shape is very specific to its function
  • a simple protein may consist of a single polypeptide chain. More commonly however a protein is made up of a number of polypeptide chains
• secondary structure of protein

  • the linked amino acids that make up a polypeptide possess both NH and C=O groups on either side of every peptide bond
  • the hydrogen of the NH group has an overall positive charge while the O of the C=O group has an overall negative charge
  • these two groups therefore readily form weak bonds called hydrogen bonds
  • this cause the long polypeptide chain to be twisted into a 3D shape such as a coil known as an alpha helix
• Tertiary structure of proteins

  • the alpha helices of the secondary protein structure can be twisted an folded even more to give the complex and often specific 3-D structure of each protein. This is known as tertiary structure
  • this structure is maintained by a number of different bonds. Where the bonds occur depends on the primary structure of the protein
• tertiary structure of proteins - types of bond

  • disulphide bridges - are fairly strong and therefore not easily broken
  • ionic bonds - are formed between any carboxyl and amino groups that are not involved in forming peptide bonds. They are weaker than disulphide bonds and are easily broken by changes in pH
  • hydrogen bonds - which are numerous but easily broken
  • it is the 3D shape of a protein that is important when it comes to how it functions. It makes each protein distinctive and allows it to recognise and be recognised by other molecules. It can then interact with them in a specific way
• quaternary structure of proteins

  • large proteins often form complex molecules containing a number of individual polypeptide chains that are linked in various ways
  • there ma also be non-protein (prosthetic) groups associated with the molecules, such as the iron-containing haem group in haemoglobin
• test for proteins

  • the most reliable protein test is the Biuret test which detects peptide bonds

  1. place a sample of the solution to be tested in a test tube and add an equal volume of sodium hydroxide solution at room temperature
  2. add a few drops of very dilute (0.05%) copper sulphate solution and mix gently
  3. a purple colour indicates the presence of peptide bonds and hence a protein. If no protein is present it will remain blue
• protein shape and function

  • their roles depend on their molecular shape which can have two basic types:
  • fibrous proteins - such as collagen have structural functions
  • globular proteins - such as enzymes and haemoglobin carry out metabolic functions