Final answer:
Alpha keratin has a secondary structure characterized by an alpha-helix stabilized by hydrogen bonds. Multiple helices form higher-order structures like protofibrils, microfibrils, and macrofibrils. Disulfide bonds provide elasticity and strength.
Step-by-step explanation:
The structure of alpha keratin, a type of structural protein found in skin, hair, and nails, is characterized by the alpha-helix structure (a-helix). This secondary structure is stabilized by hydrogen bonds, forming a right-handed spiral shape. The alpha-helices are further arranged into higher-order structures. Three alpha-helices are twisted together forming a protofibril, and then eleven protofibrils are bonded together to compose a microfibril. Numerous microfibrils then come together to form a macrofibril. In hair, these macrofibrils are cemented into bundles, contributing to the hair's strength and flexibility.
Furthermore, the alpha-helices of keratin are extensively cross-linked with disulfide bonds from cysteine amino acids, which impart elasticity to the protein. When these helices are stretched, the disulfide cross-links help in returning them to their original position upon release of tension.
Keratin's resilience and durability come from this elaborate structure, along with its water-resistant properties, making it an essential protein in the integumentary system.