Structures Tertiary structure

Sequence analysis is a core area of bioinformatics research. There are four basic levels of biological structure (Table 1), termed primary, secondary, tertiary, and quaternary structure. Primary structure refers to the representation of a linear, hetero-polymeric macromolecule as a string of monomeric units. For example, the primary structure of DNA is represented as a string of nucleotides (G, C, A, T). Secondary structure refers to the local three-dimensional shape in subsections of macromolecules. For example, the alpha- and beta-sheets in protein structures are examples of secondary structure. Tertiary structure refers to the overall three-dimensional shape of a macromolecule, as in the crystal structure of an entire protein. Finally, quaternary structure represents macromolecule interactions, such as the way different peptide chains dimerize into a single functional protein. 

Coils such as those found in alpha-keratin are not the only structural motifs present in fibrous proteins. Silk, for example, is largely composed of fibrous proteins whose structures resemble interleaved sheets, see also Quaternary Structure Secondary Structure Tertiary Structure. 

Certain RNAs also possess substantial amounts of tertiary structure. Tertiary structure in RNA refers to the folding of secondary structural elements, such as double helical regions or hairpin stem-loops, into discrete three-dimensional structures. Forces involved in stabilizing such interactions are diverse, involving hydrogen-bonding, base... 

Four levels of structure are frequently cited in discussions of protein architecture. So far, we have considered three of them. Primary structure is the amino acid sequence. Secondary structure refers to the spatial arrangement of amino acid residues that are nearby in the sequence. Some of these arrangements are of a regular kind, giving rise to a periodic structure. The a liel ix and (3 strand are elements of secondary structure. Tertiary structure refers to the spatial arrangement of amino acid residues that are far apart in the sequence and to the pattern of disulfide bonds. We now turn to proteins containing more than one polypeptide chain. Such proteins exhibit a fourth level of structural organization. Each polypeptide chain in such a protein is called a subunit. 

There are four recognized levels of protein structure primary, secondary, tertiary, and quaternary. The primary structure refers to the amino acid sequence of a protein. The primary structure is important to the protein s unique three-dimensional structure, its mechanism of action, and its relationship to other proteins with similar physiological roles. The amino acids in a protein are linked together by a specific type of covalent bond, called a peptide bond, that exists between adjacent amino acids in the polypeptide chain. Another important aspect of the primary structure is the sequence or order of amino acids in the polypeptide chain. The sequence of amino acids in a protein is specified by the nucleotide sequence of the segment of DNA containing the gene that codes for that protein. Each protein has a characteristic number and sequence of amino acid residues. The primary structure of a protein determines how the protein folds into a unique three-dimensional structure (further described by the secondary, tertiary, and quaternary structures), which in turn determines the biological function of the protein, see also Peptide Bond Proteins Quaternary Structure Secondary Structure Tertiary Structure. 

Proteins can also be dangerous or unhealthy. For many who suffer from allergies to agents like pollen, it is proteins on the surface of the pollen that cause an immune response that triggers the allergic reaction. More seriously, many natural toxins are proteins. Snake venom is one example of a naturally occurring protein-based toxin, see also Active Site Amino Acid Denaturation Enzymes Fibrous Proteins Globular Proteins Neurotransmitters Peptide Bond Protein Solubility Protein Synthesis Protein Translation RNA Synthesis Secondary Structure Tertiary Structure Transmembrane Proteins Venom. 

PRIMARY STRUCTURE SECONDARY STRUCTURE TERTIARY STRUCTURE... 

Primary structure For a nucleic acid or a protein, the sequence of the bases or amino acids in the polynucleotide or polypeptide. Compare quaternary structure second structure tertiary structure. 

Quaternary structure (1) The three-dimensional structure of a protein molecule formed by the association of two or more polypeptide chains. (2) For a protein, the level of structure that results when separate, folded polypeptide chains (subunits) associate in a specific way to produce a complete protein. Compare primary structure, secondary structure tertiary structure. 

Secondary structure (1) Local folding of the backbone of a linear polymer to form a regular, repeating structure. The B- and Z-forms of the DNA helix and the a-helix and p-sheet structures of polypeptides are examples. Compare primary structure, quaternary structure, tertiary structure. (2) The folding or coiling of a polypeptide chain into a particular pattern such as a helix or pleated sheet. 

For example, insulin is a protein that promotes the absorption of glucose out of the blood and into muscle cells where glucose is needed for energy. Insulin recognizes muscle cells because their surfaces contain insulin receptors, molecules that fit a specific portion of the insulin protein. If insulin were a different shape, it would not latch onto insulin receptors on muscle cells and therefore would not do its job. So the shape, or conformation, of proteins is crucial to their function. We can xmderstand protein structure by exploring it on four levels primary structure, secondary structure, tertiary structure, and quaternary structure (T Figure 19.7). 

Primary structure Secondary structure Tertiary structure Quaternary stmcture... 

Hemoglobin is a complex protein made of hundreds of amino acids. Its 3-dimensional shape is called a tertiary structure. Tertiary structures break down when a protein is denatured. 

Protein structure is analyzed at four levels primary structure, secondary structure, tertiary structure, and quaternary structure (Figure 21.9 t). We examine each of these categories separately. 

Tertiary structures, like primary and secondary structures, are observed within single peptide chains. Unlike primary or secondary structures, tertiary structures can exhibit primary and/or secondary structures all within the same molecule. Tertiary... 

---