Fibrous Proteins

globular proteins

a type of protein that includes most proteins (hemoglobin, enzymes, regulatory proteins…)

fibrous proteins

structural proteins that provide support, shape, and external protection; often single type of secondary structure and simple/no tertiary structure

Integral membrane proteins

third type of proteins

structure

a protein’s function is determined by its _____

a-keratin

protein that gives hair its strength (also wool, nails, claws, quills, horns, hooves, outer layer of skin); structure is build of a-helices in layers (2 helices interact to form coiled coil-quaternary)

coiled-coil

when 2 helices wrap around each other to give a supertwisted shape; helices are parallel (N-termini same end); held by hydrophobic interactions *(3.6 residues per turn)

protofilaments

2 chained coiled coiled combine into higher structures; stabilization comes from disulfide bonds

disulfide bonds

covalent bonds that are important in folding and stability between cysteine bonds; usually extracellular proteins (S-S)

reducing agent

used to break a disulfide bond (ex. mercaptoethanol)

silk fibroin

protein produces by spiders and insects (silk worm uses to make cocoons); 87% is composed of 3 amino acids (45% gly, 30% ala, 12% ser)

antiparallel B-pleated sheet

secondary structure of silk fibroin; sheets stack with glycines facing each other, stabilized by noncovalent interactions (close packing of B-sheets makes sidechains strong)

collagen

most abundant protein in body - 25% of all; insoluble & strong; major structural protein in bones teeth, cartilage, tendons, cornea

collagen primary sequence

Gly, x, y repeats (x & y - any amino acid); 1/3 gly, ¼ pro+hydroxyproline, no cys trp & little tyr

hydroxyproline

example of post-translational modification that makes collagen stable; carried out by enzyme that needs vitamin C

secondary structure of collagen

collagen structure stabilized by steric repulsion of rings on Pro and Hy-Pro (left-handed polypro helix, 3 residues/turn)

Polypro helix

left handed helix with 3 residues/turn; stabilized by steric repulsion of pyrrolidone rings of proline and hypro

collagen quaternary structure

structure of collagen with right-handed triple helix & glycine residues in centre of triple helix; heterotrimer

polypro helix

all R groups face out, not an a-helix, collagen type of this helix is a sided helix (all glycyl residues on same side)