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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Collagen triple-helix formation in all-trans chains proceeds by a nucleation/growth mechanism with a purely entropic barrier
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Collagen triple-helix formation in all-trans chains proceeds by a nucleation/growth mechanism with a purely entropic barrier

机译：全反式链中胶原三螺旋的形成是通过具有纯熵屏障的成核/生长机理进行的

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摘要

Collagen consists of repetitive Gly-Xaa-Yaa tripepticle units with proline and hydroxyproline frequently found in the Xaa and Yaa position, respectively. This sequence motif allows the formation of a highly regular triple helix that is stabilized by steric (entropic) restrictions in the constituent polyproline-II-helices and backbone hydrogen bonds between the three strands. Concentration-dependent association reactions and slow prolyl isomerization steps have been identified as major rate-limiting processes during collagen folding. To gain information on the dynamics of triple-helix formation in the absence of these slow reactions, we performed stopped-flow double-jump experiments on cross-linked fragments derived from human type III collagen. This technique allowed us to measure concentration-independent folding kinetics starting from unfolded chains with all pepticle bonds in the trans conformation. The results show that triple-helix formation occurs with a rate constant of 113 +/- 20 s(-1) at 3.7 degrees C and is virtually independent of temperature, indicating a purely entropic barrier. Comparison of the effect of guaniclinium chloride on folding kinetics and stability reveals that the rate-limiting step is represented by bringing 10 consecutive tripepticle units (3.3 per strand) into a triple-helical conformation. The following addition of tripepticle units occurs on a much faster time scale and cannot be observed experimentally. These results support an entropy-controlled zipper-like nucleation/growth mechanism for collagen triple-helix formation.
机译：胶原蛋白由重复的Gly-Xaa-Yaa杀菌剂单元组成，脯氨酸和羟脯氨酸分别经常出现在Xaa和Yaa位置。该序列基序允许形成高度规则的三重螺旋，该三重螺旋通过组成多脯氨酸-II-螺旋中的空间（熵）限制和三链之间的骨架氢键而稳定。浓度依赖性缔合反应和缓慢的脯氨酰异构化步骤已被确定为胶原蛋白折叠过程中的主要限速过程。为了获得在没有这些缓慢反应的情况下三螺旋形成动力学的信息，我们对衍生自人类III型胶原的交联片段进行了停流两次跳跃实验。这项技术使我们能够从反式构象中具有所有消化键的未折叠链开始测量与浓度无关的折叠动力学。结果表明，在3.7摄氏度下，三螺旋的形成速率常数为113 +/- 20 s（-1），实际上与温度无关，表明存在纯熵屏障。氯化胍对折叠动力学和稳定性的影响的比较表明，限速步骤是通过将10个连续的曲解单元（每链3.3个）引入三螺旋构象来表示的。随后添加的三足单位将在更快的时间范围内发生，并且无法通过实验观察到。这些结果支持胶原蛋白三螺旋形成的熵控制的拉链状成核/生长机制。

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