Which type of protein is characterized by being insoluble in water?

Fibrous proteins are characterized by their insolubility in water due to their elongated, filamentous structure. This structure is typically composed of repetitive sequences of amino acids that aggregate to form strong, stable fibers. The insolubility is primarily due to hydrophobic interactions and the presence of structural motifs such as alpha-helices and beta-sheets, which create a stable, rigid framework.

Fibrous proteins play crucial roles in providing structural support to cells and tissues, exemplified by collagen in connective tissues and keratin in hair and nails. Their insolubility is essential for their structural functions, as it allows them to form supportive matrices and structures that withstand mechanical forces.

In contrast, spherical or globular proteins have hydrophilic regions that allow them to be soluble in water, as they are often involved in cellular functions such as enzyme activity or transport. Complex proteins typically refer to those that consist of multiple subunits or diverse components, which can include both soluble and insoluble varieties, thus making their solubility variable based on composition and structure.