2.3 - Globular and Fibrous Protein Structure

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Mullins Bich 410 Tamu

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)

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