Proteins

Question 1

Structure and properties of amino acids

Answer 1

• consists of a hydrogen atom, amino group, carboxyl group and variable R group.

Properties of amino acids:
- exist as zwitterions
- act as buffers and amphoteric

Question 2

Formation and breakage of peptide bonds

Answer 2

• when two amino acids are joined by a peptide bond via condensation reaction which involves the removal of one molecule

Question 3

Primary structure of proteins

Answer 3

a single polypeptide chain that has a specific number and sequence of amino acids, linked together by peptide bonds between amino acids

Question 4

Secondary structure

Answer 4

each polypeptide chain is either folded into B pleated sheets or coiled into a helices, held together by hydrogen bonds between C=O and N-H groups of amino acids

a helices:
- each turn has about 3.6 amino acid residues
- hydrogen bonds formed between C=O and N-H groups of amino acid residues that are 4 amino acid residues apart along backbone of the polypeptide chain
- made up of a single polypeptide chain in a coiled structure

B-pleated sheets:
- single polypeptide chain is folded into two or more regions lying side by side linked by hydrogen bonds
- a hydrogen bonds is formed between C=O and N-H groups of amino acids that are on an adjacent segment along the backbone of a single polypeptides
- chains may run parallel or anti-parallel
- forms flat sheets which become folded

Question 5

tertiary structure

Answer 5

single polypeptide is then further folded and coiled into a globular protein, held together by 4 interactions between R groups of amino acids
- hydrogen bonds
- hydrophobic interactions
- disulfide bonds
- ionic bonds

Question 6

quaternary structure

Answer 6

more than 1 polypeptide is joined by 4 interactions to form a functional protein molecule

Question 7

structure and protein of haemoglobin

Answer 7

• each Hb molecule has a quaternary structure with 4 subunits, 2 a-globin subunits and 2 B-globin subunits, each subunit consists of a haem group, which consists of a porphyrin ring and Fe2+ ion

–> Fe2+ allows oxygen to bind to it reversibly to form oxyhaemoglobin, each Hb molecule can bind to 4O2

• each subunit is arranged. so that the hydrophilic amino acid side chain are on the external surface and the hydrophilic amino acid side chain is facing interior

–> this is so that HB is soluble in water and can take part in chemical reactions so that it can be easily transported to transport O2 to all parts of the body

• the 4 subunits held together by hydrophobic interactions, hydrogen bonds and ionic bonds between R groups of amino acids

–> allows for cooperative binding where one subunit binds to one O2 molecule this triggers a conformational change to the rest of the subunits to increase affinity for O2

Question 8

structure and properties of collagen

Answer 8

• 3 helical polypeptide chains wounded around tightly together to form a tropocollagen molecule
• each chain consists of a repeating sequence of glycine -X-Y, where X is usually proline and Y is usually hydroxyproline, glycine is the smallest amino acid so it can fit into the centre of the tight triple helix
  –> this is to increase tensile strength
  –> bulky and inflexible proline and hydroxyproline confer rigidity to the molecule
• extensive hydrogen bonds formed between amino acid residues of adjacent polypeptides hence insoluble in water
  –> increase tensile strength and insoluble in water hence does not affect wpt
• adjacent tropocollagen molecules have a staggered arrangement
  –> minimise points of weaknesses across fibrils
• covalent cross links between lysine residues of C and N ends of adjacent tropocollagen molecules hence formation of fibrils
  –> increase tensile strength
• bundles of fibrils unit to form long collagen fibres
  –> increase tensile strength