Protein Structures: Alpha-Keratin, Collagen, and Fibroin

Vocabulary-style flashcards covering key concepts from the lecture notes on alpha-keratin, collagen, and fibroin.

Alpha-keratin

Wool, hair, and skin protein composed of two right-handed α-helices wrapped around a left-handed coiled-coil (superhelix); typically very long (≈1000 Å or more) and cross-linked by disulfide bonds.

Coiled-coil

A structural motif where two right-handed α-helices wrap around each other to form a left-handed superhelix; stabilized by hydrophobic interactions and the heptad repeat.

Left-handed superhelix

The overall left-handed twist of the two α-helices in the keratin coiled-coil arrangement.

3.5 aa per turn

Approximate number of amino acids per turn in the keratin α-helix portion of the coiled-coil, describing its helical geometry.

Heptad repeat

A seven-residue repeating pattern (a–g) in coiled-coils; positions a and d are typically hydrophobic and pack in the core.

Disulfide bridge

Covalent S–S bond between cysteine residues; formed by oxidation and cross-links polypeptide chains (e.g., in keratin).

Cysteine

Sulfur-containing amino acid that can form disulfide bonds (cystine) under oxidative conditions.

Tropocollagen

The basic structural unit of collagen: three left-handed helices wrapped into a right-handed triple helix; about 3000 Å long and 15 Å wide.

Glycine (in collagen)

Glycine appears every third residue in collagen’s primary structure, fitting into the small interior space of the triple helix.

Proline

A nonpolar amino acid common in collagen fibers; contributes to the unique geometry of the collagen triple helix.

Hydroxyproline

Hydroxylated form of proline in collagen; its synthesis requires vitamin C and helps stabilize the triple helix.

Vitamin C (ascorbic acid)

Cofactor needed for hydroxylation of proline (and lysine) in collagen; deficiency leads to scurvy.

Lysine/hydroxylysine cross-linking

Post-translational cross-links between collagen molecules (via lysyl oxidase) that stabilize mature collagen.

Fibroin

Silk protein with an antiparallel β-sheet structure; sequence-rich in glycine and alanine; highly strong yet flexible.

Antiparallel β-sheet

β-sheet arrangement in which adjacent strands run in opposite directions; in fibroin, covalent context aligns with fiber axis.

Gly-Ala-Gly-Ala-Gly-Ser-Gly-Ala-Ala-Gly-(Ser-Gly-Ala-Gly-Ala-Gly)8

Repeating fibroin sequence motif rich in glycine and alanine that promotes β-sheet formation and crystalline structure in silk.

Silk fiber mechanics

Mechanical property arising from crystalline β-sheet regions interspersed with amorphous regions, allowing sliding and elasticity in fibroin-filled silk.