Structure, primary supersecondary

The gene-encoded primary structure of a polypeptide is the sequence of its amino acids. Its secondary structure results from folding of polypeptides into hydrogen-bonded motifs such as the a helix, the P-pleated sheet, P bends, and loops. Combinations of these motifs can form supersecondary motifs. 

The architecture of protein molecules is complex and can be described according to structural organization as primary structure (amino acid sequence), secondary structure (regular structures such as helical, pleated sheet, and coil stractures), tertiary structure (fold in three-dimensional space), quaternary structure (subunit structure) and quintemary structure (biomacromolecular complexes). Usually the overall three-dimensional (3D) architecture of a protein molecule is termed as its conformation, which refers to its secondary and tertiary structures. Between these two stractures, motifs (supersecondary structures) refer to the packing of adjacent secondary stractures into distinct structural elements and domains refer to identifiable 3D structural units that may correspond to functional units. The structures of most proteins with more than 200 amino acid residues appear to consist of two or more domains. 

A description of the protein-structure hierarchy is incomplete without a discussion of structural motifs, which are critical to an understanding of protein structure [17]. Identification of recurring motifs in protein structures has refined our knowledge of the protein-structure hierarchy these motifs occur at all levels from primary to tertiary. The Phe-Asp-Thr-Gly-Ser sequence found in the active site of all aspartic acid proteinases, and the Gly-Gly-X-Leu sequence (where X represents any amino acid residue) that predicts a 3-strand for the last two residues [17], are examples of sequence motifs a-helices, P-strands, and turns are examples of secondary-structural motifs PaP and PxP units, P-hairpins, and Greek keys are examples of supersecondary-structural motifs and four-a-helix bundles and TIM barrels are examples of tertiary-structural motifs. The tertiary fold of a protein is characterized by its tertiary-structural motif. 

According to X-ray and NMR analysis, a number of proteins which regulate gene expression possess a supersecondary structure, known as a helix-turn-helix (HTH) motif. The HTH motif consists of two symmetrically arranged a-helical structures, each containing about 20 residues with similar primary structures. The helices cross at about 120°, and are spaced in such a way that they can bind two sequential turns of the major groove of the DNA helix, Samples of regulatory proteins that possess a HTH motif are ... 

---