Helicity flip

These results indicate that is it possible to change the fold of a protein by changing a restricted set of residues. They also confirm the validity of the rules for stability of helical folds that have been obtained by analysis of experimentally determined protein structures. One obvious impliction of this work is that it might be possible, by just changing a few residues in Janus, to design a mutant that flip-flops between a helical and p sheet structures. Such a polypeptide would be a very interesting model system for prions and other amyloid proteins. 

Z-form DNA is a more radical departure from the B structure the most obvious distinction is the left-handed helical rotation. There are 12 base pairs per helical turn, and the structure appears more slender and elongated. The DNA backbone takes on a zigzag appearance. Certain nucleotide sequences fold into left-handed Z helices much more readily than others. Prominent examples are sequences in which pyrimidines alternate with purines, especially alternating C and G or 5-methyl-C and G residues. To form the left-handed helix in Z-DNA, the purine residues flip to the syn conformation, alternating with pyrimidines in the anti conformation. The major groove is barely apparent in Z-DNA, and the minor groove is narrow and deep. 

The helical hand of the individual stems in chiral but racemic polymers is a very severe and therefore critical criterion in the crystallization process. The constraints apply for each stem and are dictated by the symmetry of the unit-cell. Contrary to the stem length (which, if incorrect, can be healed or adjusted by later structural reorganization), helix chirality involves a flip of a coin type of decision - right- or left-handedness. 

The racemate of 140 has been partially separated on a TAP A (772)-impregnated silicagel column The rate of racemization (k = 1.9 x 10" min ) and the half-life (tiy2 = 38 min at 210 °C) are of the same order as those of hexahelicene (t./j = 48 min at 205 °C). This means that the conformational flipping of the one helical part of the molecule is not strongly hindered by the other. An intermediate of this conformational process therefore should be the symmetrical meso form. 

V-shaped protein encloses a "chamber within the bilayer where it is hypothesized that bound substrates are flipped across the membrane, as depicted in Figure 7-12. Each identical subunit in this homodimeric protein has one transmembrane domain, comprising six a. helices, and one cytosolic domain where ATP binding occurs. [Adapted from G. Chang and C. Roth, 2001, Science 293 1793.]... 

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