Structure of proteins

• How many amino acids can polypeptide or protein molecules have?


  Anywhere from 3 (glutathione) to more than 34,000(Titan).
• Describe the primary structure of proteins.


  It is the sequence of amino acids bonded by covalent peptide bonds.
• What determines the primary structure of a protein?


  The DNA of a cell- by instructing the cell to add certain amino acids in specific quantities in a certain sequence.
• What does the structure of a protein determine?


  Function- one alteration in the sequence of amino acids can affect the function of the protein.
• What are always free on the ends of a polypeptide chain?


  A free amino group(NH2)-> called the N-terminus

  A free carboxyl group(COOH)-> called the C-terminus
• What is the next structure called?

  Secondary structure.
• Why does the secondary structure occur?


  When the weak negatively charged nitrogen and oxygen atoms interact with weak positively charged hydrogen atoms to form hydrogen bonds.
• What are the two shapes that can form within proteins due to the hydrogen bonds?


  α-helix or β-pleated sheet
• When does the alpha helix occur?


  When the hydrogen bonds form between every fourth peptide bonds- between the oxygen of the carboxyl group and the hydrogen of the amino group.
• When does the beta pleated sheet occur?


  When the protein folds so that two parts of the polypeptide chain are parallel to each other enabling hydrogen bonds to form between parallel peptide bonds.
• What proteins usually have a secondary structure?


  most fibrous proteins- e.g. collagen and keratin.
• What does the secondary structure of proteins relate to?

  Hydrogen bonds in the protein backbone
• How do hydrogen bonds contribute to the secondary structure of proteins?

  They form between the amino and carboxyl groups
• How can the hydrogen bonds be broken?


  By high temperatures and pH changes.
• What is the 3rd structure that occurs


  tertiary structure.
• Why does the tertiary structure occur?


  Due to further conformational change of the secondary structure leading to additional bonds forming between the R groups.
• What are these additional bonds?

  hydrogen (between the R groups)

  disulphide (between cysteine amino acids)

  ionic (between charged R groups)

  weak hydrophobic interactions (between non-polar R groups)
• When is the tertiary structure common?


  In globular proteins.
• When does the quaternary structure occur?


  It occurs in proteins that have more than one polypeptide chain working together as a functional macromolecule, e.g. haemoglobin.
• What is each polypeptide chain in the quaternary structure called?


  A subunit of the protein.