University Clinic Bonn Institute of Experimental Haematology and Transfusion Medicine Director: Prof. Dr. Johannes Oldenburg
FXIII Proteins

In plasma, FXIII pro – enzyme circulates in form of a tetramer composed of two catalytic A subunits bound to two carrier B subunits (A2B2) (Lorand et al 1980). Intracellularly, FXIII is found as a homodimer of the two A subunits (A2) (Schwartz et al 1973).
The A subunit constitutes the catalytic moiety and the B subunit is thought to play a role in stabilization of the A subunit. On activation by thrombin and Ca2+, the A and B subunits dissociate. The A subunit is then cleaved to produce the catalytically active form of the protein, A2* (Takagi&Doolittle 1974). A2* catalyzes the Ca2+ - dependent formation of glutamyl-lysine bonds between fibrin and other protein molecules (see figure 1).

Figure 1. Factor XIII tetrameric structure and activation.