Kinetics of triple-helix formation

In native collagen, all Gly-Pro and Xaa-Hyp peptide bonds are in the trans conformation, whereas in the unfolded state, a significant fraction of cis isomers populates at each Gly-Pro and Xaa-Hyp peptide bond, cis-to-trans isomerization reactions at prolyl peptide bonds are the origin for the observed slow kinetics of triple helix formation" as shown by their high activation energy ( 72 kj moG )" and their acceleration by prolyl... 

Kinetic Aspects of Triple-Helix Formation of Peptide Models Compared with... 

This model does not say anything about the mechanism of triple-helix formation, because even in the case of an AON mechanism, nucleation may take place at many positions of the chains and may lead to products the chains of which are staggered. The AON model is based on the assumption that these products are too instable to exist in measurable concentration. As already mentioned, Weidner and Engel142 succeeded in proving by relaxation measurements of al CB2 that the kinetics of in vitro triple-helix formation is governed by more than one relaxation time. This rules out an AON mechanism, but the fitting to the experimentally found equilibrium transition curves nevertheless showed good accommodation and AH° computed from these curves could be confirmed by calorimetric measurement. 

Rougee, M., Faucon, B., Mergny, J.L., Barcelo, F., Giovannangeli, C., Montenay-Garestier, T. Helene, C. (1992) Kinetics and thermodynamics of triple helix formation effects of ionic strength and mismatches. Biochemistry, 31,92699278. 

This gives an important hint at what kind of model peptides are synthesized to obtain detailed information about the thermodynamics and kinetics of the collagen triple-helix formation. A first success was already achieved by synthesizing peptides of the following general structure37 ... 

Fig. 9. Effect of CsA on the rate of procollagen 1 triple helix formation in suspended chicken embryo tendon cells. The time course of procollagen 1 triple helix formation was monitored in a pulse-chase experiment by separation of protease-resistant al(l) and a2(I) chains by SDS-polyacrylamide gel electrophoresis. The fluorograms (upper panel) show the appearance of protease-resistant and hence triple-helical collagen I in the absence (—) or in the presence (-I-) of 5 fiM CsA. The kinetics are shown in the lower panel (O) no CsA ( ) 5 ixM CsA. Best fits are drawn according to the model of Bruckner and Eikenberry (1984). From Steinmann et al. (1991).

In the case of cooperative processes, the formation of a nucleus, already discussed from the kinetical point of view, plays a crucial role. The steady state described by Eq. (1) depicts the formation of a triple helix as the simplest model by the formation of a nucleus Hx through fast pre-equilibria and subsequent propagation steps, Hx in this case is a triple-helical intermediate with x tripeptide units (that means x hydrogen bonds) in the helical state. The final product H3n 2 possesses two hydrogen bonds less than tripeptide units because the three single chains are staggered at one amino add residue each. 

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