These dimers and monomers are not known to participate in coagulation, whereas the tetramers do.A units have a mass of about 83 kDa, 731 amino acid residues, 5 protein domains (listed from the N-terminal to C-terminal, residue numbers are in brackets):
activation peptide (1–37)
β-sandwich (38–184)
catalytic domain (185–515), in which the residues C314, H373, D396 and W279 partake in catalysis
β-barrel 1 (516–628)
β-barrel 2 (629–731)B units are glycoproteins. Each has a mass of about 80 kDa (8.5% of the mass is from carbohydrates), 641 residues and 10 sushi domains. Each domain has about 60 residues and 2 internal disulfide bonds. Physiology
A subunits of human factor XIII are made primarily by platelets and other cells of bone marrow origin. B subunits are secreted into blood by hepatocytes. A and B units combine within blood to form heterotetramers of two A units and two B units. Blood plasma concentration of the heterotetramers is 14–48 mg/L and half-life is 9–14 days.A clot that has not been stabilized by FXIIIa is soluble in 5 mol/L urea, while a stabilized clot is resistant to this phenomenon. Factor XIII deficiency
Deficiency of Factor XIII (FXIIID), while generally rare, does occur, with Iran having the highest global incidence of the disorder with 473 cases. The city of Khash, located in Sistan and Balochistan provinces, has the highest incidence in Iran, with a high rate of consanguineous marriage. Diagnostic use
Factor XIII levels are not measured routinely, but may be considered in patients with an unexplained bleeding tendency.
Genetics
Human factor XIII consist of A and B subunits. A subunit gene is F13A1. It is on the chromosome 6 at the position 6p24–25. It spans over 160 kbp, has 14 introns and 15 exons. Its mRNA is 3.9 kbp. It has a 5 UTR of 84 bp and a 3 UTR of 1.6 kbp. F13A1 exon(s)
1 code 5 UTR
2 code activation peptide
2–4 code β-sandwich
4–12 code catalytic domain
12–13 code β-barrel 1
13–15 code β-barrel 2B subunit gene is F13B. It is on the chromosome 1 at the position 1q31–32.1. It spans 28 kpb, has 11 introns and 12 exons. Its mRNA is 2.2 kbp. Exon 1 codes 5 UTR. Exons 2–12 code the 10 different sushi domains. Structure
Factor XIII of human blood is a heterotetramer of two A and two B linear polypeptides or "units". A units are potentially catalytic; B units are not. A units form a dimeric center. Non-covalently bound B units form a ring-like structure around the center. B units are removed when XIII is activated to XIIIa. Dimers containing only A units also occur within cells such as platelets. Large quantities of singular B units (monomers) also occur within blood.