Okuyama Kenji
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan.
Connect Tissue Res. 2008;49(5):299-310. doi: 10.1080/03008200802325110.
The triple helix is a specialized protein motif found in all collagens. Although X-ray diffraction studies of collagen began in the 1920s, the very small amount of data available from fiber diffraction of native collagen caused the determination of its molecular conformation to take a very long time. In the early 1950s, two plausible fiber periods of about 20 and 30 A were proposed, together with corresponding single-strand models having 7/2- and 10/3-helical symmetry, respectively. The first framework of the triple helix was proposed by Ramachandran and Kartha in 1955. In the same year, Rich and Crick proposed another structure with the same framework that avoided some of the steric problems of the first model. Their framework, which involved a triple-helical structure with a fiber period of 28.6 A and 10/3-helical symmetry, was exactly the same as one of two single-strand models for collagen proposed at that time, except for the number of strands. At that time, however, nobody considered the triple-strand model with the other framework, with a fiber period of 20 A and 7/2-helical symmetry, until Okuyama et al. detected this structure in the single crystal of (Pro-Pro-Gly)(10) in 1972. Although they proposed this structure as a new structural model for collagen in 1977, it has not been acknowledged as such, but instead has been regarded only as a model for a collagen-like peptide. In 2006, it was shown that both 7/2- and 10/3-helical models could explain X-ray diffraction data from native collagen quantitatively. Furthermore, during the past decade, many single crystals of collagen-model peptides have been analyzed at high resolution. The helical symmetries observed in these model peptides are very close to the ideal 7/2-helical symmetry, whereas no supporting data were found for the 10/3-helical model. This evidence strongly suggests that an average molecular structure of native collagen is the 7/2-helical model rather than the prevailing Rich and Crick (10/3-helical) model. Knowing the correct molecular structure, the driving force for the formation of a quarter-staggered structure in collagen fibrils will be elucidated in the near future by analysis incorporating the molecular structure of collagen and its amino acid sequence.
三螺旋是在所有胶原蛋白中发现的一种特殊蛋白质基序。尽管对胶原蛋白的X射线衍射研究始于20世纪20年代，但天然胶原蛋白纤维衍射获得的可用数据非常少，这使得确定其分子构象花费了很长时间。在20世纪50年代早期，人们提出了两个合理的纤维周期，分别约为20埃和30埃，以及相应的单链模型，分别具有7/2螺旋和10/3螺旋对称性。1955年，拉马钱德兰和卡尔塔提出了三螺旋的第一个框架。同年，里奇和克里克提出了另一种具有相同框架的结构，该结构避免了第一个模型中的一些空间问题。他们的框架涉及一个纤维周期为28.6埃、具有10/3螺旋对称性的三螺旋结构，与当时提出的两种胶原蛋白单链模型之一完全相同，只是链的数量不同。然而，当时没有人考虑另一个框架的三链模型，即纤维周期为20埃、具有7/2螺旋对称性的模型，直到1972年奥山等人在(脯氨酸-脯氨酸-甘氨酸)(10)的单晶中检测到这种结构。尽管他们在1977年将这种结构作为胶原蛋白的一种新结构模型提出，但它并未得到认可，反而仅被视为一种类胶原蛋白肽的模型。2006年，研究表明7/2螺旋和10/3螺旋模型都能定量解释天然胶原蛋白的X射线衍射数据。此外，在过去十年中，许多胶原蛋白模型肽的单晶都得到了高分辨率分析。在这些模型肽中观察到的螺旋对称性非常接近理想的7/2螺旋对称性，而没有发现支持10/3螺旋模型的数据。这一证据有力地表明，天然胶原蛋白的平均分子结构是7/2螺旋模型，而不是普遍存在的里奇和克里克(10/3螺旋)模型。了解正确的分子结构后，通过结合胶原蛋白的分子结构及其氨基酸序列进行分析，在不久的将来将阐明胶原蛋白原纤维中四分之一交错结构形成的驱动力。
