Greatly simplified organizational hierarchy of fibrillar collagen structure (from polypeptide to fibril)

A. The collagen-forming polypeptide chains contain a large helix-forming domain with the repeat amino acid sequence Gly-X-Y, where X and Y are occupied by Pro or Hyp more frequently than other residues, but only account for approximately 1/6 of the total amino acid content (see for instance human sequence: ExPASy sequence data bank codes; P02452 and P08123). An arrow points to the figure element that shows that three polypeptides form the collagen monomer. The large triple-helix (super-helix) domain of approximately 300 nm in length is flanked by non-helical telopeptides (N and C, shown). The 6–8.6 nm dimension indicates the repeat of the triple-helix (36; 37). B. Collagen molecules are staggered approximately 67 nm from one another in the formation of microfibril aggregates. The microfibrils are D-periodic (D = 67 nm), and in each D-period, two monomers coil, or partially coil, around each other giving the appearance of another helix-like feature in the structural hierarchy (3). C. Cross-sectional view of the collagen molecular packing of a type I collagen fibril (11). Each circle represents one collagen molecule in cross-section (at the axial level of 0.44D). at the 0.44 D position. Next to B to C arrow, cross-section of an isolated microfibril. D) Archival image (Orgel laboratory) of the wide angle fiber diffraction pattern of type I collagen from rat tail tendon. The distinctly different but superimposed non-crystalline and crystalline diffraction patterns are indicated. Previous fiber diffraction studies of collagen's helical structure have concentrated on the non-crystalline part of the pattern, in this present study, we analyze crystalline diffraction data.