Lecture 7 - Fibrous Proteins

Question 1

Collagen

Answer 1

Most abundant proteins in the human body
Found in connective tissue
- tendons, cartilage, bone matrix etc.
Different forms of collagen in different tissues
- forms depends on type of polypeptide chain
- collagen contain 3-stranded helical segments, unique based on regions that interrupt triple helix and fold into other 3D structures

Question 2

Collagen - Structure

Answer 2

Consists of three aplha-chains super-twisted around each other to form long thin protein
Collagen polypeptides are left handed helixes
Many H bonds between polypeptides

Question 3

Collagen - Amino Acid Composition

Answer 3

Triple helical structure due to abundance of 3 aa’s
- Glycine (35%)
- Proline (21%)
- Hydroxyproline
Prolines and lycines can also be hydroxylated (when OH group added)
Only Gly residues accomodated in tight junctions between collagen chains
Pro and Hyp allow tight twisting of helix

Question 4

Collagen Fibril Structure

Answer 4

Collagen polypeptides interacting with other triple helices forms a collagen fibril
Short segments at either end of collagen chains important
Segments contain hydroxylysine
Covalent links form between 2 lysine or hydroxylysine residues at C-terminus of one collagen molecule with two similar residues at N-terminus
These cross links stabilise side by side packaging of collagen

Question 5

Collagen Synthesis

Answer 5

Signal sequences cleaved off Prepro-a-cahin to make Pro-a-chain
Post translational modification of some lysine and proline to 4-hydroxy derivatives, enzymes require ascorbic acid
Procollagen secreted from cell
Formation of triple helix by H bonds
Forms fibrils by covalent cross links

Question 6

Scurvy

Answer 6

Lack of ascorbic acid resulting in less hydroxyproline and hydroxylysine
Makes skin, blood vessels, tendons fragile

Question 7

a-Keratin

Answer 7

Found in hair, outer layer of skin, horns, wool, nails etc.
Only in mammals
Right handed helix
Rich in hydrophobic amino acids
- Met, Phe, Val, Leu etc.

Question 8

a-Keratin - Structure

Answer 8

Two a-keratin proteins combine into left handed super twisted coil, with N terminals at same end
These combine to form protofilaments and protofibrils
Strength of keratin enhanced by disulphide bonds between coiled coils

Question 9

a-Keratin and the ‘Perm’

Answer 9

Hair wrapped around roller and reducing agent added plus heat
Reducing agent breaks disulphide bonds and heat breaks H bonds - leads to uncoiling
Reducing agent removed and oxidising agent added to help form disulphide bonds
Hair reverts back to a-helical structure but hair now culrs becuase disulphide bonds exert torsion or twists

Question 10

Elastin

Answer 10

Found in connective tissue
- Extracellular matrix
- Lungs
- Walls of arteries 
- Ligaments
Secreted as a single polypeptide
Elastic property
High proportion of small and non-polar amino acids

Question 11

Elastin - Structure

Answer 11

Cross link by conversion of amine groups of lysine to reactive aldehydes by lysyl oxidase
Results in spontaneous formation of desmosine cross links
Desmosine composed of four lysine residues allowing for bonding of multiple chains

Question 12

Formation of Elastin

Answer 12

Start with tropoelastin
Association with glycoproteins
Oxidative deamination of lysine residues
Leads to formation of desmosine cross links
Elastin made with extensive network of individual polypetides connected by desmosine cross links

Question 13

Fibroin

Answer 13

Found in silk produced by spiders and shit
Polypeptide chain mostly B-conformation
Rich in small amino acids
- Ala and Gly
Silk doesn’t stretch as B sheet structure highly extended
Flexible as sheets held together by weak bonds