ABSTRACT
This chapter discusses unstructured regions of polypeptide chain are often used to connect the binding sites for proteins that function together to catalyze a biological reaction. Proteins constitute most of a cell's dry mass. Proteins embedded in the plasma membrane form channels and pumps that control the passage of small molecules into and out of the cell. Other proteins carry messages from one cell to another, or act as signal integrators that relay sets of signals inward from the plasma membrane to the cell nucleus. From a chemical point of view, proteins are by far the most structurally complex and functionally sophisticated molecules known. Proteins are therefore also known as polypeptides. Proteins come in a wide variety of shapes, and they are generally between 50 and 2000 amino acids long. Regions of a helix are especially abundant in proteins located in cell membranes, such as transport proteins and receptors.
