referred to as polypeptide chains. Proteins are functional units composed of one or more of these chains. In proteins made up of more than one polypeptide chain, each polypeptide chain is called a subunit. Proteins are stabilized by electrostatic interactions and hydrogen bonds between their amino acids, and by covalent disulfide bonds formed between the sulfur atoms of two cysteine amino acids. The linear sequence of amino acids in a protein's polypeptide chains is referred to as the primary structure of the protein. A protein's secondary structure refers to the spatial arrangement of nearby amino acid residues in each polypeptide chain, and the locations of disulfide bonds between the polypeptide chains. There are common secondary structural motifs: for example, some regions of polypeptides twist into helices, while others fold into pleated sheets. The overall spatial arrangement of amino acid residues in a polypeptide chain constitutes tertiary structure. For a multimeric protein, the arrangement of different polypeptide subunits and the interactions between them, define quaternary structure.
