Fibrous Proteins (Module 4 continued)

Question 1

Keratin’s primary and secondary structure:

Answer 1

Primary:
- contains pseudo-seven-repeat (repeating pattern of 7 amino acids)
- positions a and d are hydrophobic residues (non-polar)

Secondary:
- forms alpha helix
- positions a and a end up on same side creating a hydrophobic strip along one side of helix

Question 1

What proteins are included in the fibrous proteins?

Answer 1

• Keratin
• Collagen
• Silk

Question 2

Keratin coiled-coils (secondary):

Answer 2

• two amphipathic helices interact so their hydrophobic faces together (coiled-coil)
• two right handed helices wrapping in left handed manner

Question 3

Keratin post-translational stabilization (secondary):

Answer 3

• stagger stacked chains for max overlap
• covalent linkages
• individual units are linked together through disulfide bonds
• extent of disulfide bonds will determine strength of structure (more disulfide bonds = more rigid)

Question 4

Collagen primary and secondary structure:

Answer 4

Primary:
- Triplet repeat: repeats of Gly-X-Y (X is often proline)
- Sharp turns with proline being rigid and glycine being flexible

Secondary:
- forms left-handed helix of 3 residues per turn (not an alpha helix)

Question 5

Collagen coiled-coils (secondary)

Answer 5

• three left handed helices wrap around in right handed fashion
• bulky side chains of proline are on the outside
• small side chains of glycerine are the core tightly packed

Question 6

Collagen post-translational modifications (secondary):

Answer 6

• covalent linkages
• linkages occur from residues undergo post-translational modification (hydroxyproline, hydroxylysine)
• modifications require vitamin C (collagen cannot form stabilizing cross links without)
• Vitamin C deficiency leads to scurvy

Question 7

Silk primary and secondary structure:

Answer 7

produced for webs and cocoons which need to be both strong and flexible

Primary:
- six residue repeat (GSGAGA)

Secondary:
- composed primarily from beta sheets
- side chains tend to be in trans (one side glycerine and the other alanine)
- fully extended polypeptides offer strength while still being flexible

Question 8

Strength and flexibility of silk:

Answer 8

Strength: fully extended polypeptide chains
Flexible: association of strands by hydrogen bonding and association of sheets by van der waals and hydrophobic interactions