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.