Sequence regularities and packing of collagen molecules |
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Authors: | Karl A Piez Benes L Trus |
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Institution: | Laboratory of Biochemistry National Institute of Dental Research National Institutes of Health Bethesda, MD 20014, U.S.A. |
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Abstract: | Fourier analysis of sequences along edges of the type I collagen molecule constructed from two α1(I) and one α2 chains shows that the molecule is two-sided if the supercoil pitch of the α chains along the molecular axis, P, is 39 residues (, where D = 234 residues or 67 nm). One side has alternating charged and hydrophobic regions with spacings of , while the other side has an excess of hydrophobic residues with a spacing of . These characteristics arise from sequence regularities in the α chains and the geometric relationship between the chains. The pattern is marginally strongest with α2 as chain 1. The sides could form the inside of a helical microfibril where contacts between molecules would fall P apart along the α chains. The sides could form the outside of the microfibril where contacts between microfibrils would be spaced apart by the α chain supercoil along the microfibril axis, P′. If the microfibril is a 54 helix of D-staggered collagen molecules with a left-handed supercoil of pitch , P′ is close to (43 residues). subsets in the α chains give rise to the spacing along the molecule. The microfibril has 41 screw symmetry satisfying X-ray diffraction evidence that microfibrils pack in a tetragonal unit cell.This model is the same as proposed previously by us (Trus & Piez, 1976: Piez & Trus, 1977) except that P = 39 rather than 30 residues. Contrary to our earlier assumption, P = 39 residues is within the range allowed by X-ray diffraction measurements. The present results favor P = 39 since it relates regularities in the α chain sequences to helical parameters in a direct way. Furthermore, model studies show that geometric arguments which support P = 30 are equally strong at P = 39 residues. |
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