Proteins are made of a polypeptide backbone with attached side chains. Each type of protein differs in its amino acid sequence. Thus the sequential position of the chemically distinct side chains gives each protein its individual properties. The two ends of each polypeptide chain are chemically different: the end that carries the free amino group (NH₃⁺, also written NH₂) is called the amino, or N-, terminus; and the end carrying the free carboxyl group (C00^–, also written COOH) is the carboxyl, or C-, terminus. The amino acid sequence of a protein is always presented in the N to C direction, reading from left to right.  Fair Use and Copyright info