Enzymic Control of Collagen Fibril Shape

The shape of collagen fibrils growing in vitroin a cell-free enzyme/sub strate system is shown to be dependent on the enzyme/substrate ( E/ S) ratio. Long fibrils with tapered ends were generated by exposing pCcollagen (procollagen from which the N-propeptides had been removed) to procollagen C-proteinase (which acts by cleaving the C-propeptides from the pCcollagen, converting it to insoluble fibril-form ing collagen). Tip shape profiles, established quantitatively by scanning transmission electron microscopy, depended critically on the C-protein ase/pCcollagen ratio. The finest tips occurred at low ratios, the coarsest at high ratios. All fibrils had molecules oriented with amino termini closest to the pointed ends, i.e. N,N-bipolar fibrils in which molecules change orientation abruptly at one location along the fibril. Fibrils had maximal diameter at this molecular switch region. Shape asymmetric fibrils occurred at low E/ Sratios, near-shape symmetric fibrils occurred at high ratios. Fibrils generated at low E/ Sratios bore the closest resemblance to those formed in vivoexcept that the central shaft regions of fibrils formed in vitroshowed no tendency to be limited to a uniform diameter.