Poly-proline helix

Fig. 11. Ribbon diagram of the GFCC /1-sheet face in Dl. The tt cation stacking motif and structural differences observed in the edge /3-strand are shown. /3-bulges (B1 or B2) and the left handed poly proline helix (PPII) in GHR D2 are denoted with arrows. Residues at the end of the /3-strand GDI that participated in cytokine contacts are labeled on the figure. IFNAR2 Gys-12 which forms a disulfide bond with cysteine on /3-strand is also labeled.

Figure 1.4. Chiral supramolecular structures (a) a-helix of polypeptides, (b) poly-prolin-helix of collagene, and (c) deoxyribonucleic acid (DNA) Z = D-Deoxyribose, P = phosphoric acid, A — adenine, G = guanine, C = cytosine, T = thymine (from [8]).

Polypeptides undergo a conformational change from helix to random coil when the solvent composition, pH or temperature is varied. Adsorption of polypeptides such as poly(/3-benzyl-L-glutamate), poly-proline, and polyhydroxyproline on glass was studied... 

Poly Proline Helices. Poly proline (PP) displays some unusual structural properties as it is die only naturally occuring tertiary amide found in polypeptides. The inability of this residue to act as a hydrogen-bond donor leads to two types of extended helical structures, neither of which resembles the a helix. The first, called PP I, is a right-handed helix which possesses cis amide bonds (left-handed helix with trans amide bonds ( d = 180°). Both occupy a fairly limited region of < >,y space. The two forms interconvert slowly over time. 

With heating from 5 to 45°C, thermal changes in conformation in the major /3-casein are observed by spectral methods (Garnier 1966). From measurements of the optical density at 286 nm and of the specific optical rotation at 436 nm, a rapidly reversible endothermic transition (AH 30 kcal/mole) with a half-transition temperature of 23-24°C is observed. The optical rotatory dispersion data suggest a decrease in the poly-L-proline II structure (12 to 5%) and a slight increase in a-helix (11 to 16%) with increasing temperature. This transition probably occurs prior to association, since it is rapid, and the carboxyacyl derivative of the monomer, which does not polymerize with increasing temperature, also demonstrates the optical rotatory disperson thermal transition. 

FIGURE 3.4.2 Increments on circular dichroism spectra of secondary structure elements cx-helix (black solid line), (3-sheet (black dashed-dotted line), (3-turn (gray dashed line), poly-L-proline (gray solid line), and random coil (black dashed line). 

Despite different sequences and repetitive motives, all gliadins have the same secondary structure of loose spirals which are a balanced compromise between the p-spiral and poly-L-proline structure (polyproline helix II) (Parrot et al., 2002), the balance is dependent on temperature, type of solvent, and hydration level (Miles et al., 1991). Similar sequences can be found in other proteins, mainly animal proteins such as elastin and collagen, and they are responsible for particular biomechanical properties connected to reverse P-spirals or p-sheet structures (Tatham and Shewry, 2000). 

This K is considerably smaller than that of the single-strand polymer at neutral pH, and suggests that the molecules have a rather thick structure. Adopting the interrupted-helix model, we may apply Eq. (97) as in the case of poly-L-proline. Noting that the unit translation distance b0 is 1.70 A for the double-stranded Watson-Crick helix (263 ) [i. e., a distance of 3.40 A for two residues, one in each chain], and taking Mu = 328 for poly(adenylic acid), we obtain r = 22 residues per helical... 

Poly-L-proline can form a single-strand helix that is similar to that of a single strand of the collagen triple helix, but it cannot form a triple helix. Why not ... 

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