sentences:  Exosite 1 mediates the binding of thrombin to the Aα- and Bβ-chains, and exosite 2 causes an interaction with a second fibrinogen molecule at the C-terminus of the y’-chain.Consequently, when thrombin binds a yA/yA fibinogen only exosite 1 is occupied, and when it binds yA/y’ both exosites are bound tightly.So fibrinogen yA/y’ is a competitor to yA/yA, which decrease the amount of clotting.