What are the levels of protein structural complexity?

• Primary structure (AA sequence) • Secondary structure • Spatial arrangement of a polypeptide’s backbone atoms without regard to side-chain conformations • alpha, beta, coil, turns (Venkatachalam, 1968) • Super-secondary structure • alpha, beta, alpha/beta, alpha+Beta (Rao and Rassman, 1973) • Tertiary structure • 3-D structure of an entire polypeptide • Quaternary structure • Spatial arrangement of subunits (2 or more polypeptide chains)