Holmes D F, Mould A P, Chapman J A
Department of Medical Biophysics, University of Manchester, England.
J Mol Biol. 1991 Jul 5;220(1):111-23. doi: 10.1016/0022-2836(91)90385-j.
At high concentrations, type I pN-collagen, pC-collagen and procollagen (the first 2 generated from procollagen by enzymic cleavage of C-propeptides and N-propeptides, respectively) can all be made to assemble in vitro into thin D-periodic sheets or tapes. Scanning transmission electron microscopy mass measurements show that the pN-collagen sheets and procollagen tapes have a mass per unit area corresponding to that of approximately 6.8 monolayers of close-packed molecules. pN-collagen sheets are extensive and remarkably uniform in mass thickness (fractional S.D. 0.035); procollagen tapes are neither as extensive nor as uniform in thickness. The mean thickness of pC-collagen tapes is less and the variability is greater. In pN-collagen sheets, the overlap: gap mass contrast in a D-period is increased from 5:4 (the ratio in a native collagen fibril) to 6:4, showing that the N-propeptides do not project into the gap but are folded back over the overlap zone. Assuming all N-propeptides to be constrained to the two surfaces of a sheet, their surface density can be found from the mass thickness of the sheet. In a lateral direction (i.e. normal to the axial direction where the spacing is D-periodic), the N-propeptide domains are calculated to be spaced, centre to centre, by 2.23 (+/- 0.1) nm on both surfaces. This value (approx. 1.5 x the triple-helix diameter) implies close-packing laterally with adjacent domains in contact. Sheet formation and the "surface-seeking" behaviour of propeptides can be understood in terms of the dual character of the molecules, evident from solubility data, with propeptides possessing interaction properties very different from those displayed by the rest of the molecule. The form and stability of sheets (and of first-formed fibrils assembling in vivo) could, it is suggested, depend on the partially fluid-like nature of lateral contacts between collagen molecules.
在高浓度下，I型前胶原、前胶原N端肽（pN - 胶原）和前胶原C端肽（pC - 胶原）（前两者分别是前胶原通过C - 前肽和N - 前肽的酶切产生的）都可以在体外组装成薄的具有D周期的片层或条带。扫描透射电子显微镜质量测量表明，pN - 胶原片层和前胶原条带的单位面积质量对应于约6.8个紧密堆积分子单层的质量。pN - 胶原片层范围广泛，质量厚度非常均匀（分数标准差为0.035）；前胶原条带的范围和厚度都不如pN - 胶原片层。pC - 胶原条带的平均厚度较小，变异性较大。在pN - 胶原片层中，D周期内的重叠：间隙质量对比度从5:4（天然胶原纤维中的比例）增加到6:4，这表明N - 前肽不会伸入间隙，而是折叠回重叠区域。假设所有N - 前肽都被限制在片层的两个表面上，那么可以从片层的质量厚度得出它们的表面密度。在横向方向（即垂直于间距为D周期的轴向方向）上，计算得出两个表面上N - 前肽结构域的中心间距为2.23（±0.1）nm。这个值（约为三螺旋直径的1.5倍）意味着相邻结构域在横向紧密堆积并相互接触。片层的形成以及前肽的“表面寻找”行为可以根据分子的双重特性来理解，这从溶解度数据中很明显，前肽具有与分子其余部分非常不同的相互作用特性。有人认为，片层（以及体内组装的最初形成纤维）的形式和稳定性可能取决于胶原分子之间横向接触的部分类似流体的性质。
