Factor XIII tetrameric structure and activation.

Plasma factor XIII is a heterologous tetramer consisting of 2 A- and 2 B-subunits. The A-subunits contain the enzyme's active site, and the B-subunits serve a carrier function of the hydrophobic A-subunit in the aqueous environment of human plasma. Activation of factor XIII involves cleavage of the activation peptides from the A-subunit, which then may or may not dissociate from the complex. In a second step, calcium and fibrin induce the dissociation of the B-subunits from A to expose the active site's thiol group.