Triple-helix-coil

Thermodynamic Aspects of the Triple Helix-Coil Transition. . 186... 

In 1% aqueous acetic acid, the peptides of the sequence (Ala-Gly-Pro)n, bridged with Lys-Lys and beginning with n = 8 show a cooperative transition, which was interpreted as a triple helix-coil transition (see Figs. 35, 36). 

Because of these observations, comparative experiments with peptides of different proline content in a solvent less polar than water, are recommended. (Pro-Pro-Gly)n and (Pro-Ala-Gly)n, in methanol/acetic add (volume ratio 9 1) show a temperature-induced triple helix-coil transition which is characterized by the following parameters92,150) (Pro-Pro-Gly)n AH°s = -1.9 kJ/rnol tripeptide AS° = -5.4 J tnor1 K (Pro-Ala-Gly) A HI = -0.9 kJ/mol tripeptide A 5° = -3.8 J mol-1 K ... 

Thus, a peptide sequence that was originally designed to form a two-stranded parallel coiled coill50 was later identified from X-ray crystallographic data as a triple-helix coiled coil)51 A synthetic peptide with lie in all five d positions and Leu in all five a positions was a four-stranded coiled coil)12 while a similar peptide with only three lie residues in the middle d positions was two-stranded)32 In addition, a 35-residue peptide with Leu at all the a and d positions was two-stranded in the reduced form, but the disulfide-bridged coiled coil became four-stranded)32 Analogues of the Rop protein, where the hydrophobes in the interface were... 

The individual chains are synthesized as procollagen a chains, which, with a mass of 150,000 Da, have additional extension peptides at both the amino and carboxyl termini. The amino- and carboxyl-terminal regions from the three a chains each fold to form globular structures, which then interact to guide the formation of the triple helix. Interchain disulfide bonds stabilize the carboxyl-terminal domain and the triple helix coils up from this domain (Fig. 5-15). 

Roth W, Heidemann E. Triple helix-coil transition of covalently bridged collagenlike peptides. Biopolymers 1980 19 1909-1917. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

The interpretation of these results is, however, problematic since no data on the absolute enthalpy and entropy of the respective triple helix and coiled state are available. Though it may be taken as an established fact that the entropy of conformation of a (Pro-Pro-Gly) coil is lower than in the case of a (Pro-Ala-Gly)n coil, we are not sure whether the entropy of the triple helix depends on the imino acid content. 

Since 1973, several authors have proved that there is a relationship between thermostability of collagen and the extent of hydroxylation of the proline residues31,34). Equilibrium measurements of the peptides al-CB 2 of rat tail and rat skin revealed a higher rm, for al-CB 2 (rat skin)157). The sequence of both peptides is identical except that in the peptide obtained from rat skin, the hydroxylation of the proline residues in position 3 has occurred to a higher extent than in the case of al-CB 2 (rat tail). Thus, a mere difference of 1.8 hydroxy residues per chain causes a ATm of 26 K. Obviously, there are different stabilizing interactions in the triple-helical state, that means al-CB 2 (rat skin) forms more exothermic bonds than al-CB 2 (rat tail) in the coil triple-helix transition. This leads to an additional gain of enthalpy which overcompensates the meanwhile occurring losses of entropy. 

When the collagen rod can be extracted in the native form it is soluble in acidic solutions, at room temperature. If the solutions are heated, the collagen is denaturated the chains lose their helical conformation. The characteristic temperature of this helix —> coil transition is around 36 C. The solution then contains principally single chains, but also some double and triple chains which were initially covalently bound and some sub-units of the single chains. This product is gelatin. 

There are several types of collagen. In one type, two identical chains of one kind are coiled together with a third dissimilar chain to form the triple helix. Several of these triple helices associate to form 8-nm microfibrils (Fig 2-23D).199 Once synthesized, collagen is extensively modified and crosslinked. See Chapter 8. 

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