Some proteins also incorporate phosphorous, iron, zinc, and copper.
Every protein molecule has a characteristic three-dimensional shape, or conformation. Fibrous proteins, such as collagen and keratin, consist of polypeptide chains arranged in roughly parallel fashion along a single linear axis, thus forming tough, usually water-insoluble, fibers or sheets. Globular proteins, e.g., many of the known enzymes, show a tightly folded structural geometry approximating the shape of an ellipsoid or sphere. Because the physiological activity of most proteins is closely linked to their three-dimensional architecture, specific terms are used to refer to different aspects of protein structure. The term primary structure denotes the precise linear sequence of amino acids that constitutes the polypeptide chain of the protein molecule.