Proteinase precursors

Proteinase precursors, also called zymogens or proenzymes, become catalytically active enzymes upon specific proteolytic cleavage. The precursor molecules circulate as inactive forms that do not exhibit enzymatic activity. The activation processes are irreversible. Extensive structural similarities are found among all proteinase precursors. The regions of the molecules that express proteinase activity after activation are found in the C-terminal one-half to one-third of each molecule. The portion of each precursor... 

Proteinase Precursors Malignant tumors have high concentrations of fibrinolytic system components ... 

The N-terminal domains of the hemostatic proteinase precursors and proteinases are constructed of several protein structural motifs (Figure 36-5). These motifs, in various combinations and by placement in different positions,... 

The four common motifs found within the amino terminal regions of the proteinase precursors and proteinases are kringles, epidermal growth factor (EGF)-like motifs, fibronectin motifs, and apple motifs (Figure 36-5). The kringle is so named because in two dimensions, i.e., on paper prior to determination of its three-dimensional structure, it has the shape of a Danish pretzel. Similarly, apple motifs resemble drawings of apples. 

The proteolytic reactions of the hemostatic system are neither catalytically efficient nor localized when proteinase and proteinase precursor only are present. The rapid, localized proenzyme activation required for normal hemostatic response occurs only in a complex of proteinase, proteinase precursor, and cofactor protein assembled on the surface of a damaged cell membrane, or in vitro, on the surface of phospholipid bilayers. The catalytic efficiency of an enzyme-catalyzed reaction is expressed by the ratio of the kinetic constants /ic and kJKm). In the activation complexes, kJKm values can be greater than lO M s . With proteinase and proenzyme alone, the kJKm values are only approximately 100 M s and thus the reactions are 10 times less efficient. Expressed in terms of the same amount of product formed in the two situations, a 10 increase represents the difference between requiring 1 minute and about 6 months to form the product ... 

It is helpful in the effort to understand activation complexes to consider complex formation, the reactions that occur in the complexes, and the demise of the complexes as proceeding in a sequence. First, a reversible, noncovalent association of proteinase, cofactor protein (strictly, activated cofactor protein), proteinase precursor, and membrane surface occurs to form the activation complex. This spontaneous association occurs as the result of complementary interaction sites on the protein molecules, e.g., the binding sites between proteinase and protein substrate, proteinase and cofactor protein, substrate and cofactor protein, and all three proteins with the membrane surface. Tissue factor normally exists as an integral membrane protein and is always associated with the membranes of cells in the vessel wall. The same processes are involved in the anticoagulant subsystem and, with a different surface, fibrin in the fibrinolytic system as well. 

Second, irreversible proteolytic action in the complex converts the proteinase precursor in the complex into an active proteinase. This is followed by dissociation of the product proteinase from the complex in which it has been formed. Association of this proteinase with the next cofactor protein and the next protein substrate to form the next complex in the coagulation cascade then occurs. 

The fibrinolytic system removes the fibrin of the hemostatic plug and thus is responsible for the temporary existence of the fibrin clot. The proteolytic action of plasmin on fibrin and fibrinogen is extensive and more like the digestive proteolysis catalyzed by trypsin and chy-motrypsin than the proteolysis involved in proteinase precursor activation. The fibrinolytic subsystem includes the reactions of plasminogen activation, plasmin inactivation, and fibrin digestion. As is common throughout the hemostatic system, irreversible activation reactions of the fibrinolytic system are opposed by irreversible proteinase inactivation. 

CHAPTER 36, FIGURE 5 Motif structures within coagulation proteins. Common motifs are found in the amino terminal regions of the proteinase precursor molecules. Shown are the kringle motifs and EGF-like motifs found in the vitamin K-dependent proteins and in plasminogen. Fibronectin (types I and II) motifs and apple motifs (named from their two-dimensional representations) are also present but not shown. Some epidermal growth factor-like domains contain P-hydroxylated Asp residues. The cartoon structures for the motifs are derived from three-dimensional structures determined by x-ray crystallography or by two-dimensional NMR spectroscopy. 

Gerlach D, Knoll H, Kohler W, Ozegowski J, Hribalova V Isolation and characterization of erythrogenic toxins. 5. Communication identity of erythrogenic toxin type B and streptococcal proteinase precursor. Zentralbl Bakteriol Mikrobiol Hyg [A] 1983 255 221-233. 

Hauser, A.R. and Schlievert, P.M. 1990. Nucleotide sequence of the streptococcal pyrogenic exotoxin type B gene and toxin relationship to streptococcal proteinase precursor. J. Bacteriol. 172 4536-4542. 

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