hitocontruct osp0eethgnscnobceomea8aienm -Pleated sheets impart cons ble rigidity to the syste P o h aniets of thehelare IR68g股e8 cem ohgaG :DeUhaebndoes g。very wne2ne eR 8g 5 5 In the pleated sheet, the two chains line up with the carboxy groups of one chain opposite the amino groups of another. Additional chains may be bonded to either side to construct a “sheet” of chains connected by hydrogen bonds. This arrangement of chains can also be formed by a single polypeptide chain folding back and forth on itself several times. β-Pleated sheets impart considerable rigidity to the system. The α-helix is formed by hydrogen bonds between carbonyl groups and amino groups 3.6 amino acids apart in the amino acid sequence. The carbonyl group of one amino acid hydrogen bonds with the amino group of the amino acid four residues ahead in the sequence. Two equivalent points in adjacent turns of the helix are 5.4 Å apart. Too much charge of the same kind or the presence of the amino acid proline may disrupt secondary structure. The final overall folding of the entire polypeptide chain is called the tertiary structure of the chain. A variety of forces are involved in stabilizing the tertiary structure. •Disulfide bridges •Hydrogen bonds •London forces •Electrostatic attraction and repulsion •Micellar effects (hydrophobic effect) Pronounced folding is found in the globular proteins (chemical transport, catalysis, etc.) In fibrous proteins (myosin, fibrin, α-keratin), several α-helices are coiled together to form a superhelix. Enzymes and transport proteins fold up in such a way to produce three dimensional pockets or groves on their surfaces called active sites or binding sites. The size and shape of these sites provide a very specific fit for the intended substrate or ligand. The inner surface of an active site generally contains a specific arrangement of side chains of polar amino acids that attract functional groups on the substrate by hydrogen bonding or ionic interactions. Active sites align the functional groups on the enzyme and substrates in such a way as to promote the associated chemical reaction. A typical example of an enzyme is chymotprysin which catalyses the hydrolysis of specific peptide bonds (adjacent to phenylalanine, tyrosine or tryptophan) at physiological temperature and pH. Exposure of a protein to extremes of heat or pH usually causes denaturation, or breakdown, of the tertiary structure of a protein. In some proteins, several polypeptide chains, each with its own tertiary structure, assemble to form a larger structure called a quaternary structure