Chapters 1 and 2 introduced a-helices and b-sheets (Secondary Struture), and some common motifs composed of 2 or 3 of these elements (Super-secondary Struture). Tertiary structure describes the folding of the polypeptide chain to assemble the different secondary structure elements in a particular arrangement.As helices and sheets are units of secondary structure, so the domain is the unit of tertiary structure.In multi-domain proteins, tertiary structure includes the arrangement of domains relative to each other as well as that of the chain within each domain.There is a blurred distinction between super-secondary structure and tertiary structure.lthough the term motif is often used to describe super-secondary structures (e.g. Branden and Tooze, 1991), it may also be used to describe a consensus sequence of amino acids identified in a number of different proteins, rather than a repeated three-dimensional conformation.