Protein or the physical region of the protein that participates in chemical catalysis. In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules. Proteins can be informally divided into three main classes, which correlate with typical tertiary structures: globular proteins, fibrous proteins, and membrane proteins. Almost all globular proteins are soluble and many are enzymes. Fibrous proteins are often structural, such as collagen, the major component of connective tissue, or keratin, the protein component of hair and nails. Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass
Muehrcke's nails nail plate. As such, they do not move with nail growth, and disappear when pressure is applied to the nail (blanching the underlying nail bed): this distinguishes them from "true leukonychia striata" such as Mees' lines. As in Terry's and half-and-half nails, the pattern is thought to be formed by bands of localized edema exerting pressure on the surrounding capillaries. The appearance of Muehrcke's lines is associated specifically with marked hypoalbuminemia (serum albumin ≤ 2.2 g/dL) indicating decreased protein synthesis, which may occur during periods of metabolic stress (e.g. systemic infection, trauma, AIDS, chemotherapy), or in hypoalbuminemic states such as