Proteins

Question 1

Primary structure of proteins

Answer 1

• Type, number and sequence of amino acids joined by peptide bonds
• Determines how polypeptide is folded/3D shape and hence its properties and functions

Question 2

Secondary structure of proteins

Answer 2

• Localised folds and coils within a singular polypeptide chain
• Formation of intra-chain H bonds are regular AAs

Question 3

Alpha-helix

Answer 3

• Localised coiling, helical shape
• H-bonds formed are parallel to the axis
• R-groups projected outwards
• H-bonds between n and n+4 AAs
• 3.6 AAs per turn

Question 4

Beta-pleated sheet

Answer 4

• Localised coiling, sheet
• H-bonds formed between adjacent regions (B-strands)
• R-groups projected above & below beta-sheet
• Beta-sheets are parallel or anti-parallel

Question 5

Tertiary structure

Answer 5

• Extensive folding of a polypeptide chain into a precise, compact and globular 3D shape
• Formation of H-, ionic, disulfide bonds and hydrophobic interactions
• AAs that were far apart are brought together into localised regions to form functional groups

Question 6

Quarternary structure

Answer 6

• Association of two or more polypeptide chains
• Formation of bonds between R-groups of adjacent polypeptides

Question 7

Factors for protein denaturation

Answer 7

1. Temperature
2. pH
3. Reducing agents (disulfide bonds)
4. Organic solvents

Question 8

Haemoglobin structure

Answer 8

1. Globular protein → soluble in aq.
2. Extensively folded and compact → pack more into RBC
3. Specific 3D conformation with clefts lined with hydrophobic R-groups found in the interior → complementary haem group embed into subunit
4. Four haem groups with Fe²⁺ → Fe²⁺ allows O₂ binding, 4 per haemoglobin
5. 4 subunits with haem group → allows cooperativity

Question 9

Haemoglobin cooperativity

Answer 9

When one oxygen molecule binds to a haem group, it causes a slight conformation change in the haemoglobin, increasing the affinity of the other subunits to oxygen

Question 10

Collagen function

Answer 10

• Structural protein with high tensile strength to provide support
• Quarternary protein
• Insoluble

Question 11

Collagen structure

Answer 11

• AA sequence of glycine, proline then hydroxyproline or hydroxyglycine
• Polypeptide chain forms kinked helix
• 3 chains wind tightly to form triple helix tropocollagen with h-bonds
• Tropocollagens interact with others running parallel, forming covalent cross-linkages between lysine R-groups, forming collagen fibril
• Collagen fibrils further aggregate to form collagen fibres

Question 12

Collagen features

Answer 12

1. Fibrous proteins with unshielded hydrophobic R-groups
   →Insoluble in water to allow collagen to provide support
2. Every 3rd AA is glycine
   → Glycine is smallest AA, hence triple helix is more tightly coiled
3. Kinked helix
   → More h-bonds can be formed in triple helix
4. Staggered arrangement of collage fibrils
   → Eliminates any lines of weaknesses in the fibrils

Typically features aim to increase tensile strength of collagen