Polypeptides, flexibility

Structural features that make molecules more rigid reduce rotational and vibrational contributions to entropy. Thus the formation of a double bond or ring decreases the entropy even when the molecular weight is unchanged. The formation of comparatively rigid macromolecular structures from flexible polypeptide or polynucleotide chains also requires an entropy decrease, although this can be offset by increases in the entropy of the surrounding water molecules (see chapter 4). 

While the evidence is undeniable for electron transfer via the pterin system for enzymes in the XO/XDH and AOR families, comparable structural features are not observed in SO. The additional electron-transfer group, the heme, is quite distant from the pterin ring system (Mo Fe 32 A) prohibiting an efficient electron transfer between these cofactors in the solid state. Because a flexible polypeptide chain connects the two domains housing the heme and the Moco, one postulation under investigation is that in solution the heme domain moves to position the heme closer to the pterin system to receive electrons during catalysis. 

Furthermore, the F. and the two F units of the intact IgG are joined by flexible polypeptide regions that allow... 

The hormones of the anterior pituitary gland fall into three groups on the basis of structural similarities. Corticotropin (ACTH), the melanotropins (a-MSH and 6-MSH), the lipotropins (B-LPH and y-LPH) and endorphin (b—EP) comprise the first group of closely related flexible polypeptides. 

SDS-PAGE is performed in polyacrylamide gels containing SDS. SDS and proteins form complexes with structures composed of protein-decorated micelles coimected by short flexible polypeptide segments [63]. The result of this strucmre is that large amounts of SDS are incorporated into the SDS-protein complex in a ratio of approximately 1.4 g SDS/g protein. SDS masks the charge of the proteins... 

Another way to look at the molecule is to focus on the porphyrin it forms a rather rigid structure wrapped in a flexible polypeptide (the 1-47 amino acid sequence on the right side, the 48-91 sequence on the left), and only an edge of the porphyrin ring is exposed to the exterior. 

Such zipper-like electrostatic attraction could be one of the reasons for condensation of DNA molecules and of nucleosome core particles in solutimis of some multivalent counterions (for a discussion see [25,223]). Another possibility is inter-nucleosomal attraction mediated by bridging interacticMis of flexible polypeptide histone tails [213, 224, 225]. The theoretical basis for such attractirMi driven by sharing of polyelectrolyte chains adsorbed simultaneously on two nucleosomes/ macroions has been developed in [46, 175, 226, 227]. 

The molecular basis of contractility is now quite well understood. It is generally assumed that elastic proteins and rubber-like materials are built of flexible polypeptide chains. The similarity in the molecular mechanism of such materials is indicated by the fact that all yield S-shaped curves. Such curves are obtained for gelatin and glue, and by Treitel (95) for cellulose in the living plant. 

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