Fibrous Proteins: Structure, Properties, and Biological Roles

Fibrous Proteins

Overview and Biochemical Properties

Fibrous proteins are a class of structural proteins characterized by their elongated, filamentous shapes and limited sequence variation. They play critical roles in providing mechanical support and strength to cells and tissues in various organisms.

• Composed of long, extended chains: These proteins have repetitive amino acid sequences that form extended secondary structures.

• Hydrophobic amino acids: Their sequences often contain hydrophobic residues, promoting aggregation and insolubility in water.

• Limited sequence variation: The repetitive nature of their sequences contributes to their structural stability.

• Insoluble in water: Due to hydrophobic interactions, fibrous proteins do not dissolve in aqueous environments.

• High tensile strength: Their structure allows them to withstand stretching and mechanical stress.

Examples of fibrous proteins:

• α-Keratin: Found in fingernails, hair, skin, claws, horns, and hooves.

• β-Keratin: Present in feathers and scales.

• Collagen: Major component of bone matrix, tendons, and skin.

• Fibroin: Constituent of silkworm silk and spider webs.

Amino Acid Composition of Fibrous Proteins

The amino acid composition of fibrous proteins is distinct from that of globular proteins, with certain residues being highly prevalent depending on the protein type.

Amino Acid	α-Keratin (wool)	Fibroin (silk)	Collagen (bovine tendon)	All Proteins
Gly	8.1	44.6	32.7	7.9
Ala	5.0	29.4	11.0	8.7
Ser	10.2	12.2	3.0	6.8
Glu + Gln	11.2	0.9	7.7	6.6
Cys	7.0	0.2	0.0	2.0
Pro	7.2	0.5	22.1	5.0
Arg	7.2	0.5	5.0	5.1
Leu	6.6	0.8	2.2	9.2
Thr	6.6	0.9	1.2	5.6
Asp + Asn	5.1	0.2	1.2	5.9
Val	5.1	2.2	1.2	6.6
Tyr	4.2	0.8	0.7	3.2
Ile	4.2	0.2	1.2	5.5
Phe	3.6	0.8	1.2	4.0
Lys	2.0	0.2	3.0	5.9
Trp	0.7	0.2	0.0	1.0
His	0.7	0.2	0.0	2.0
Met	0.5	0.0	0.7	2.0

α-Keratin

Structure and Biochemical Features

α-Keratin is the dominant protein in hair, wool, nails, claws, quills, horns, hooves, and the outer layer of skin. Its structure and amino acid composition confer mechanical strength and resilience.

• Predominant amino acids: Ala, Val, Leu, Ile, Met, Phe (hydrophobic residues).

• Coiled-coil structure: Two α-helices coil around each other, stabilized by hydrophobic interactions at the 'a' and 'd' positions of the heptad repeat.

• Helix properties: Each α-helix has 3.6 residues per turn.

Organization of α-Keratin fibers:

• Single α-helix

• Coiled coil of two α-helices

• Protofilament (pair of coiled coils)

• Filament (four right-handed twisted protofibrils)

Every 3-4 residues in the sequence is hydrophobic, creating a hydrophobic stripe that spirals around the helix, resulting in a left-handed super-helix.

Disulfide Bonds and Mechanical Properties

Disulfide bonds between helices are a key feature of α-keratin, contributing to its hardness and stability.

• Disulfide bond formation: Covalent bonds between cysteine residues in adjacent helices increase crosslinking.

• Mechanical strength: The more disulfide bonds present, the harder the α-keratin. For example, rhinoceros horn contains about 18% of its amino acids cross-linked by disulfide bonds.

Disulfide Bonds in Hair and Chemical Treatments

Human hair contains disulfide bonds that can be chemically altered to change its shape, such as during a 'permanent' at a hair salon.

• Initial state: Hair contains di-sulfide bonds (S-S) between cysteine residues.

• Chemical treatment: Reducing agents break these bonds, allowing hair to be reshaped.

• Curling: The position of the bonds is changed as the hair is curled.

• Neutralizer: Oxidizing agents reform the disulfide bonds, locking the new shape in place.

Example: The process of a hair permanent involves breaking and reforming disulfide bonds to achieve a lasting curl.

β-Keratin

Role and Features

β-Keratin is found in feathers and scales, providing rigidity and protection. It is composed of β-sheet structures stabilized by hydrogen bonding.

• Intermediate filaments: Contribute to the structure of nuclei and cytoplasm in cells.

• Mechanical properties: High strength and resistance to deformation.

Collagen

Structure and Biochemical Features

Collagen is the most abundant protein in animals, forming the main component of skin, bone, teeth, tendons, and cartilage. It is synthesized by fibroblast cells and is essential for tissue strength and integrity.

• Predominant sequence: Repeats of Gly-X-Y, where X is often Proline and Y is often Hydroxyproline.

• Triple helix: Three left-handed helices wind together to form a right-handed triple helix, which is long and rigid (approximately 3,000 Å x 15 Å).

• Hydroxylation: Proline and lysine residues are hydroxylated to form hydroxyproline and hydroxylysine, a process catalyzed by enzymes requiring ascorbic acid (vitamin C) as a cofactor.

Clinical relevance: Vitamin C deficiency (scurvy) impairs collagen synthesis, leading to weakened connective tissues.

Stabilization and Cross-Linking

• Glycine residues: Glycine is the only amino acid small enough to fit in the crowded center of the triple helix.

• Hydrogen bonds: Amide hydrogen of glycine forms hydrogen bonds with carbonyl oxygen of adjacent chains, and hydroxyproline side chains also participate in hydrogen bonding.

• Covalent cross-links: Allysine (oxidized lysine) forms covalent bonds with lysine or other allysine residues, stabilizing the collagen fibrils.

Example: The strength of tendons and bone matrix is due to extensive cross-linking between collagen triple helices.

Fibroin (Silk Protein)

Structure and Biochemical Features

Fibroin is the main protein in silk produced by silkworms and spiders. It is notable for its unique sequence and mechanical properties.

• Sequence: Predominantly repeats of six amino acids: Gly-Ser-Gly-Ala-Gly-Ala.

• Secondary structure: Composed of antiparallel β-strands stabilized by hydrogen bonds between backbone NH and CO groups of different strands.

• Stacking: Alternating small side chains (Gly, Ala, Ser) allow β-sheets to stack closely, forming crystalline regions.

• Mechanical properties: Silk is both strong and elastic due to the combination of crystalline sheets and amorphous regions.

Example: Spider silk and silkworm silk are used in nature for web construction and cocoons, respectively, due to their high tensile strength and elasticity.

Summary Table: Key Features of Major Fibrous Proteins

Protein	Main Location	Structure	Key Amino Acids	Mechanical Properties
α-Keratin	Hair, nails, skin, horns	Coiled-coil α-helices	Ala, Val, Leu, Ile, Met, Phe, Cys	Hardness, flexibility (disulfide bonds)
β-Keratin	Feathers, scales	β-sheets	Varied	Rigidity, protection
Collagen	Skin, bone, tendons	Triple helix	Gly, Pro, Hydroxyproline	Tensile strength, rigidity
Fibroin	Silkworm silk, spider webs	Antiparallel β-sheets	Gly, Ala, Ser	Strength, elasticity

Key Equations and Biochemical Reactions

• Disulfide bond formation:

  • $\ce{2 R-SH -> R-S-S-R + 2H^+ + 2e^-}$

• Hydroxylation of proline (collagen synthesis):

  • $\ce{Proline + O_2 + \text{ascorbic acid} -> Hydroxyproline + H_2O}$

Additional info: The study notes have expanded on the brief points in the original slides, providing definitions, structural details, and biochemical context for each major fibrous protein. Tables have been recreated and equations added for clarity.