The Triple Helix

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. 

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

Since these investigations could be carried out only in the crystalline state, the question of the dynamics of the triple-helix formation and of the correlation of its stability with the amino acid sequence could be answered only with the help of other methods working in solution. 

This experiment shows that folding of the non-helix or poor helix-forming sequence is encouraged in the triple helix too, even if this part of the helix contributes to a decrease of the folding strength. Sedimentation and hydrodynamic measurements clearly show the formation of a trimer aggregation and of an extended shape. 

One may conclude that the rate-determining step of the renaturation is at least partly influenced by the cis-trans isomerization of the peptide bond the secondary nitrogen atom of which arises from proline. Otherwise, only the entropy-controlled slow nuclea-tion should be observed kinetically. The covalent bridging through Lys-Lys, therefore, gives rise not only to thermodynamic stabilization of the triple helix but also to kinetic properties which have hitherto been observed in the case of type III procollagen146) and its aminoterminal fragment Col 1-3144). 

According to Eq. (1), the triple-helix formation can be imagined as a process which may be divided into 3n - 2 steps of equilibrium with the same values of Aand AS each. This means that AG° is a linear function of 3n - 2 for the whole process. 

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. 

The amino acids proline and hydroxyproline exert a stabilizing influence on the triple helix as described in detail in Sect. 4.5. By examining the CB peptides of collagen, a structural stability which is directly proportional to the itnino acid content may thus be found. It has, however, not been possible to synthesize model peptides displaying structural stability comparable to that of the native peptides having corresponding amino acid contents. 

In gelatin, when the triple helixes are heated in water, they open up. Some of the hydrolyzed ends fray out to tangle with other ropes,... 

The 18 hydroxymethyl groups in one turn of the triple helix have quite different orientations. b The two residues per turn are conformationally independent. 

Three classes of nucleic acid triple helices have been described for oligonucleotides containing only natural units. They differ according to the base sequences and the relative orientation of the phosphate-deoxyribose backbone of the third strand. All the three classes involve Hoogsteen or reverse Hoogsteen-like hydrogen bonding interaction between the triple helix form-... 

Bachinger, H.P., Bruckner, P., Timpl, R., Prockop, D.J. and Engel, J. (1980) Folding mechanism of the triple helix in type-III collagen and type-III pN-collagen role of disulfide bridges and peptide bond isomerisation. European Journal of... 

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