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36 terms

2.3 - Globular and Fibrous Protein Structure

Mullins Bich 410 Tamu
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Globular protein
Tightly folded into a more or less spherical structure
More functional
Globular proteins are (soluble/insoluble) in water.
Soluble
Examples of globular proteins
Enzymes!!!
Hemoglobin
Ribonuclease
Globular proteins can contain?
A significant number of only alpha helices or only beta sheets
Globular proteins usually have ___ residues facing the interior (interact with one another) and ___ residues facing the exterior (reacting with the solvent).
(hydrophilic/hydrophobic)
Hydrophobic
Polar/Hydrophilic
Why is it important that the exterior of a globular protein has polar residues facing the exterior?
The interact with the solvent, making the protein soluble.
What is different about the interior of a protein?
It is more analogous to an organic solvent, thus, the pKas of side chains are perterbed
Supersecondary Structures
(Motifs)
Specific combination of alpha helices and B sheets
Tell structural information
Proteins with the same motif tend to?
Fold via similar processes
What are examples of a motif?
BAB - two antiparallel B, connected by and A
AA - two antiparallel As
B meander - Antiparallel sheets formed by series of tight reverse turns
Greek Key - repetitive structure (check out my ring!)
Helix-loop-Helix
Often found in calcium binding proteins
Calcium binding proteins are often enriched in? Why?
Asp, Glu, Ser, and Thr.
Are (-) so they attract the Ca2+ cation
Beta Helices
Beta strands in a helical pattern with two to three faces
Examples of a Beta Helix?
Pathogen (bacteria, virus, or toxin) proteins that bind the protein to the host cell.

Such as: Bordetella, ulcers, malaria, and anthrax
Domain
Segments of very large proteins that can be structurally and functionally independent
Domains with similar conformation tend to?
Have similar functions
Ex. Myoglobin and Hemoglobin
(Sequences/3D structures) change faster due to evolutionary pressure.
Sequences
Lipocalins
extracellular transport proteins for small hydrophobic compounds
Up to 10 % of enzymes have a ___ structural motif?
Alpha Beta Barrel
Fibrous Protein
Organized more or less parallel to a single axis
More structural
Fibrous proteins are (soluble/insoluble) in water.
Insoluble
Examples of fibrous proteins (3)
Keratin
Silk
Collagen
Fibrous proteins contain:
Tend to only have one type of secondary structure
Alpha OR Beta, but not both
Where is Alpha Keratin found?
Hair, fingernail, claws, horns and beaks.
What is the basic unit of alpha keratin?
Dimer of A-helices
Has high Cys content

PSEUDO 7-REPEAT WITH A AND D HYDROPHOBIC
When you see alpha keratin, you think:
PSEUDO 7-REPEAT WITH A AND D HYDROPHOBIC
What causes the alpha keratin in hair to be curly? What is the difference between a "permanent" and a "set" in curly hair?
Disulfide Bonds (remember, high Cys content)

Permanent - redox reaction rearranges disulfide bonds
Set - water rearranges H-bonds
What does Silk Fibroins consist of?
Antiparallel B-sheets aligned paralled to axis of fiber
Forms a microcrystalline array
What is the sequencing of silk?
Gly-X-Gly-X where X = Ala or Ser
Why is the sequencing of silk important?
All the Gly on one side and all the Ala/Ser on other side because B-sheet alternates residues!!
Where is collagen found?
Connective tissues such as tendon, cartilage, bone, and teeth
What is the composition of collagen?
3 L-handed helical chains interwoven to produce a R-handed superhelical
What is the unique sequence of the amino acid composition?
About 33% Glycine
About 30% Pro and HyPro together
Has a high level of non-standard amino acids
Why are Gly and Pro so important to collagen?
Only Gly is small enough to fit into the center
Pro permits the sharp twists
Collagen has unusual crosslinking between ___ and ___ which aids in strengthening the fibrils.
Lys and His
Name two collagen diseases.
Ehlers-Danlos Syndrome and Osteogenesis Imperfecta (Brittle Bone)