Eduqas Biology for A Level Book 1: 2nd Edition

Link The relationship between DNA and protein structure is explained on p105. 1.7 Knowledge check For A–D, state whether a primary, secondary, tertiary or quaternary structure is described. A. Folding of the polypeptide into a 3D shape. B. α -helix held together with hydrogen bonds. C. The sequence of amino acids in the polypeptide chain. D. The combination of two or more polypeptide chains in tertiary form, associated with a non-protein group. Protein structure The structure of a protein can be thought of at different levels of organisation: Primary structure : This is the order of the amino acids in a polypeptide chain. Polypeptides have up to 20 different types of amino acid. They can be joined in any number, order and combination, so there is a huge number of possible polypeptides. The primary structure is determined by the base sequence on one strand of the DNA molecule. Secondary structure : This is the shape that the polypeptide chain forms as a result of hydrogen bonding between the =O on –CO groups and the –H on –NH groups in the peptide bonds along the chain. This causes the long polypeptide chain to be twisted into a 3D shape. The spiral shape is the α -helix. Another, less common, arrangement is the β -pleated sheet. The protein keratin has a high proportion of α -helix and the protein fibroin, in silk, has a high proportion of β -pleated sheet. Structure of α -helix Structure of β -pleated sheet Chemical bonds in a polypeptide Fig 12 hydrogen bond Fig 11 polypeptide chain hydrogen bond hydrogen bond one amino acid hydrogen bond Fig 11 polypeptide chain hydrogen bond hydrogen bond one amino acid hydrogen bond hydrogen bond one amino acid Fig 13 disulphide bridge (covalent bond – strong link) hydrogen bond (very weak link) ionic bond (weaker than covalent bond) COO – C=O ------ H–N C=O ------ H–N NH + S S hydrophobic interaction R R R ionic bond (weaker than Tertiary structure Eduqas Year 1/AS Biology 26