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Fibrous proteins form extended, long and narrow structures, unlike compact globular proteins. They are typically located in the extracellular matrix and provide structural support to tissues. Fibrous proteins are composed of repetitive amino acid sequences and usually consist of either alpha-helices or beta-sheets, rarely both, with hydrophobic amino acids on outer surfaces that facilitate monomer interactions.

Collagen is a fibrous protein composed of extended alpha-helices that wind around each other to form a coiled-coil structure. Three helices twist together to create collagen fibrils, which provide both structure and flexibility in connective tissues. These fibers have significant tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength.

Fibrous proteins serve primarily structural and supportive functions throughout the body. Many are located in the extracellular matrix and connective tissues, imparting strength and enabling joint mobility. They hold tissues together during body movement and are found in skin, ligaments, organs, and other structures requiring mechanical support and resilience.

Fibroin is a fibrous protein composed of beta-sheets, which gives silk its unique mechanical properties. The beta-sheet structure allows silk strands to be both flexible and strong, enabling the material to stretch without breaking while maintaining structural integrity. This combination of properties makes silk valuable for both biological and commercial applications.

Elastin is a fibrous protein found in elastic fibers that returns to its original shape after being stretched or compressed. Unlike collagen, which resists stretching, elastin provides elasticity and recoil. Elastic fibers containing elastin are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column, enabling tissues to withstand repeated deformation.

Reticular fibers are formed from type III collagen proteins and remain narrow, arranged in a branching network throughout the body. They are most abundant in reticular tissue of soft organs such as the liver and spleen, where they anchor and provide structural support to functional cells, blood vessels, and nerves.

Fibrous proteins often have hydrophobic amino acids on their outer surfaces that interact with other monomers to form larger structures. These hydrophobic interactions drive the assembly of individual protein subunits into extended fibers and complex networks. The repetitive amino acid sequences in fibrous proteins further support their ability to polymerize into organized, functional structures.