Structure of t h e Collagen Microjibril. A Four-Strand Overlap Model
The problems of t.he assembly of collagen molecules during fibril formation and the internal structure of collagen fibrils have been under investigation for many years but several aspects of these problems have yet to be completely resolved. This is in spite of recent high-resolut ion and quite sophisticated X-ray diffraction analyses. -4
The first proposals on the relative arrangeinent of collagen molecules came from electron-microscopic studies. Hodge and Schmittj postulated a two-dimensional "quarterstagger" arrangement of collagen molecules to account for the native period of collagen fibrils. Petruska and Hodge6 extended this model to include the concept of "holes" occasioned by :he fact that each molecule is 4.4 times the length of the fundamental repeat D of -670 A. These two-dimensional quarter-stagger models are, as shown by Smith,' not applicable to a three-dimensional hexagonal packing of collagen molecides. In three dimensions, only two-thirds of the lateral contacts between molecules can be quarterstaggered. Veis et a1.8 postulated that collagen fibrils are made from limiting microfibrils comprised of unit assemblies of four molecules arranged in a square space grollp. However, in order to preserve the Hodge-Petrriska holes, and build a constant diameter fibril, only four of every five contacts of their model were quarter-staggered. Using the same kind of limiting microfibril concept, Smithg postulated that Ihe limiting microfibrils are comprised of unit assemblies of five molecules rolled to form a smooth surfaced closedcylinder in which all molecules obey the quarter-st agger rule. Other molecular arrangements have been postulated in the form of a spiral or rolled-up monomolecr~lar sheet.10-l2 Itecent low-angle X-ray d i f f r a c t i ~n ' . ~, ~ and elertron-microscopi~'~ studies have provided evidence supporting a limiting microfibril structure.
Miller and Parry2 concluded from the difraction data of Miller and U'rayl that the