Left-handed helix

Figure 10.1 Basic polypeptide geometry. The upper panel shows a short peptide sequence of three amino acids joined by two peptide bonds. A relatively rigid planar structure, indicated by dashed lines, is formed by each peptide bond. The relative positions of two adjacent peptide bond planes is determined by the rotational dihedral angles

Many of the polysulfides described above have been investigated by X-ray diffraction on either powders or single crystals. In all cases the more sulfur-rich anions (n>3) form unbranched chains the symmetry of which varies between Ci, C2, and Cs. According to Fig. 1 the symmetry C2 results if all torsion angles have the same sign (right-handed helix + + +... left-handed helix ----...). If the different torsion angles of the anion vary between + and... 

Right-handed helix Left-handed helix... 

Abbreviations a-helix, right-handed a-helix aL-helix, left-handed a-helix /3-sheet, antiparallel /3-sheet. 

Helices can be coiled in two different ways, and are referred to as right- or left-handed. In Figure 6.5, compare the n=3 helix (right-handed) with the n = -3 helix (left-handed). 

The heart of the Watson-Crick model is the postulate that a molecule of DNA is a complementary double helix. It consists of two antiparallel polynucleotide strands coiled in a right-handed manner about the same axis to form a double helix. As illustrated in the ribbon models in Figure 28.6, chirality is associated with a double helix left-handed and right-handed double helices are related by reflection just as enantiomers are related by reflection. 

The structure proposed by Watson and Crick was modeled to fit crystallographic data obtained on a sample of the most common form of DNA called B DNA Other forms include A DNA which is similar to but more compact than B DNA and Z DNA which IS a left handed double helix... 

Short segments of poly(dG—dC) incorporated within plasmids, or citcular DNA, adopt the Z-conformation under negative superhehcal stress. This left-handed DNA may be important in genetic control. On the other hand, the stmctural alteration of the helix requited in a B-to-Z transition within a plasmid is radical, and would involve either a multistep mechanism or the complete melting and reformation of helix. The improbability of such transitions has led to questions concerning the feasibility of a biological role for Z-DNA. 

An a helix can in theory be either right-handed or left-handed depending on the screw direction of the chain. A left-handed a helix is not, however, allowed for L-amino acids due to the close approach of the side chains and the CO group. Thus the a helix that is observed in proteins is almost always right-handed. Short regions of left-handed a helices (3-5 residues) occur only occasionally. 

Figure 2.18 The P-a-P motif can in principle have two "hands." (a) This connection with the helix above the sheet is found in almost all proteins and is called right-handed because it has the same hand as a right-handed a helix, (b) The left-handed connection with the helix below the sheet.

The answer is quite clear. His 64, which is part of the catalytic triad, is in the first turn of helix Ob (Figure 11.13). This helix would be on the other side of the P sheet, far removed from the active site if the motif P2-o.b-P3 were right-handed. Therefore, to produce a proper catalytic triad of Asp 32, His 64, and Ser 221, helix Ob must be on the same side of the p sheet as Ser 221 consequently, the motif has evolved to be left-handed. 

Chirality axis (Section 10.8) Line drawn through a molecule that is analogous to the long axis of a right-handed or left-handed screw or helix. 

D- or L-amino acids, but a given helix must be composed entirely of amino acids of one configuration. a-Helices cannot be formed from a mixed copolymer of D- and L-amino acids. An a-helix composed of D-amino acids is left-handed. 

FIGURE 6.18 Poly(Gly-Pro-Pro), a collagen-like right-handed triple helix composed of three left-handed helical chains. (Adaptedfrom Miller Scheraga, H. A., 1976, Calculation of the... 

Antiparallel tt-helix proteins are structures heavily dominated by a-helices. The simplest way to pack helices is in an antiparallel manner, and most of the proteins in this class consist of bundles of antiparallel helices. Many of these exhibit a slight (15°) left-handed twist of the helix bundle. Figure 6.29 shows a representative sample of antiparallel a-helix proteins. Many of these are regular, uniform structures, but in a few cases (uteroglobin, for example) one of the helices is tilted away from the bundle. Tobacco mosaic virus protein has small, highly... 

FIGURE 10.41 (a) Gramicidin forms a double helix in organic solvents a helical dimer is the preferred strnctnre in lipid bilayers. The strnctnre is a head-to-head, left-handed helix, with the carboxy-termini of the two monomers at the ends of the strnctnre. (b) The hydrogen-bonding pattern resembles that of a parallel /3-sheet. 

Helix rotation sense Right-handed Right-handed Left-handed... 

Fig. 8. Space Ailing model of a poly-L-lysine a-helix with CIO4 anions inserted between the NH3 -side groups forming a left-handed superhelix 911...

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