Relationships of the Catalytic Properties of Human Plasma and Platelet Transglutaminases (Activated Blood Coagulation Factor XIII) to Their Subunit Structures

Abstract The subunit structures of the transglutaminases from human plasma and platelets have been studied by gel filtration, ultracentrifugation, and chemical cross-linking, both in the presence and absence of the catalytically essential metal ion, Ca2+. The plasma enzyme in the absence of Ca2+ is composed of four subunits and may be designated (a'2b2). This tetrameric structure is similar to that of its zymogen (a2b2), except that the a chains have been proteolytically modified by the activating enzyme, thrombin. Addition of Ca2+ causes a reversible dissociation of the plasma enzyme molecule into a catalytic dimer (a'2) and a noncatalytic dimer (b2). The platelet enzyme with or without Ca2+ exists as a catalytic dimer (a'2) derived by thrombin activation from its two-subunit zymogen (a2). Isolated b2 from plasma enzyme may be combined with platelet enzyme in the absence of Ca2+ to give a four-subunit molecule indistinguishable from the plasma enzyme. This supports earlier evidence that the catalytic chains of plasma zymogen are very similar or identical with those of platelet zymogen. Each of the plasma protransglutaminase a chains contains six —SH groups and no disulfide bonds; each of its b chains has 16 or 17 intramolecular disulfide bonds and no —SH groups. The platelet zymogen contains six —SH groups in each of its a chains. Inactivation of both the plasma and platelet enzymes by iodoacetamide occurs only in the presence of Ca2+ and is a result of selective alkylation of a single cysteine —SH group in only one of the catalytic subunits. This finding is consistent with half-of-the-site reactivity for both of the enzymes. The protransglutaminase from platelets (a2) is activated by thrombin at a significantly faster rate than the zymogen from plasma (a2b2). A proenzyme prepared by combining the isolated b chain dimer from plasma enzyme with the platelet zymogen was found to be activated by thrombin at the same rate as the plasma zymogen.