Calcium Ion Binding Within Fibrinogen Fragment D

Fibrinogen contains a number of strongly bound calcium. ions and a considerable body of evidence new exists to show that the plasmin degradation product fragment D contains one strongly bound calciumion. It is also established that this calcium ion has a notable effect on the plasmin resistance of the molecule. Previous work from this laboratory strongly suggests that the binding site is located towards the C-terminus of the γ chain. We have also investigated the influence of calcium. ions on the conformation of fragment D by ultracentrifugation and chemical cross-linking. In the presence of calcium ions there is a preponderance of intra-molecular cross-linking even at high concentrations of fragment D using bisimidates such as dimethyl suberimidate and dimethyl adipimidate. In the absence of calcium. ions there is an increase in the extent of inter-molecular cross-linking. From such evidence we would propose that calcium ions stabilise a compact conformation within fragment D. The presence of calcium ions also affects the stability of the D:E complex.