The High Affinity Calcium Ion Bending Sites of Fibrinogen
There is considerable evidence that fibrinogen contains a number of strongly bound calcium ions and these appear to have a significant role in the structure and properties of the molecule. Most of the evidence suggests that there are three such strongly bound calcium ions in fibrinogen and each of the two fragments D contains one of these. It has been suggested that the section of the (A) α chain which is the region of the molecule first attacked by plasnin is involved in binding calcium ions. Should this constitute the third site it follows that this calcium ion must link the two (A) α chains and the site may well be destroyed by minimal plasnin attack. The figure of three calcium ions bound, however, must be open to sane doubt due to the difficulty in evaluating data from Scatchard plots prepared from. a system, such as fibrinogen, which contains a number of identical ligands with more than one binding affinity. Accordingly we have developed methods to prepare fibrinogen in as intact a form as possible, and used such fibrinogen in flow dialysis systems. Studies of calcium ion release during proteolytic degradation of fibrinogen lead us to conclude that there are probably only two high affinity, calcium ion specific binding sites in fibrinogen.