Fibrinogen function achieved through multiple covalent states

Disulfide bonds link pairs of cysteine amino acids and their formation is assumed to be complete in the mature, functional protein. We tested this assumption by quantifying the redox state of disulfide bonds in the thrombosis protein, fibrinogen. There is an extraordinary disulfide lability in fibrinogen, with 13 bonds in the protein ranging from 10 to 50% reduced in human donors, indicating that fibrinogen exists in hundreds of different disulfide-bonded states. The significance of this finding for fibrinogen conversion to fibrin was investigated. Fibrin polymer formation triggers formation of disulfides in fibrin molecules, which is required for a robust fibrin matrix that withstands the mechanical forces of flowing blood and resists premature fibrinolysis. The covalent states of fibrinogen are changed by fluid shear forces ex vivo and in vivo, indicating that the different states are dynamic. These findings demonstrate that proteins can exist and function as a multitude of covalent forms.