Activated clotting enzyme

Fig. 9.4 The coagulation cascade in the horseshoe crab amoebocyte lysate. In the presence of LPS, the Factor C serine protease zymogen is autocatalytically activated to an active form, Factor C , which activates the proenzyme Factor B into Factor B . This in turn activates proclotting to active clotting enzyme. Clotting enzyme then converts coagulogen into a coagulin gel clot, which traps the invading bacteria...

The ability of blood to clot is so important that there are two separate pathways that converge to ensure sufficient activation of the final clotting enzyme, thrombin. It converts fibrinogen to fibrin which, along with the platelets, forms a clot that prevents further loss of blood and restricts entry of pathogens (Box 17.1) (Figure 17.2). 

The fibrin thrombus resulting from blood clotting (see p. 290) is dissolved again by plasmin, a serine proteinase found in the blood plasma. For this purpose, the precursor plasminogen first has to be proteolyti-cally activated by enzymes from various tissues. This group includes the plasminogen activator from the kidney (urokinase) and tissue plasminogen activator (t-PA) from vascular endothelia. By contrast, the plasma protein a2-antiplasmin, which binds to active plasmin and thereby inactivates it, inhibits fibrinolysis. 

Along with the production of insulin, many other medical uses have been achieved for recombinant DNA. This includes the production of erythropoetin, a hormone used to stimulate production of red blood cells in anemic people tissue plasminogen activator, an enzyme that dissolves blood clots in heart attack victims and antihemophilic human factor VIII, used to prevent and control bleeding for hemophiliacs. These three important genetically engineered proteins were all cloned in hamster cell cultures. 

M. miehei rennet is the most heat stable of all the commonly used milk-clotting enzymes (Thunell et al 1979). None is destroyed during Cheddar cheese manufacture although, like M. pusillus var. Lindt rennet, less than 2% remains active in the cheese (Harper and Lee 1975 Holmes et al. 1977). It remains active in the whey and is concentrated in condensed whey products. 

Each milk-clotting enzyme has an optimum pH at which it is most active. Moving the pH in either direction from that point decreases activity. Extremes of acid or base also denature the enzymes, but not as irreversibly as high temperature. In cheesemaking, the starter culture moves the pH down to the optimum of the milk-clotting enzyme. The different optimums for different enzymes must be considered in relation to pH at setting and culture activity (Brown 1981 Shalabi and Fox 1982). 

Thrombolytics facilitate the breakdown and dissolution of clots that have already formed. These drugs work by converting plasminogen (profibrinolysin) to plasmin (fibrinolysin).34 As shown in Figure 25-1, plas-min is the active form of an endogenous enzyme that breaks down fibrin clots. Thrombolytic drugs activate this enzyme by various mechanisms and can be used to dissolve clots that have already formed, thus reopening occluded blood vessels. 

Haruhiko, T., Kotani, S., Tanaka, S., Ogawa, T., Takahashi, I., Tsujimoto, M., Komuro, T., Shiba, T., Kusumoto, S., Kusunose, N., Hasegawa, A., Kiso, M. Structural requirements of lipid A species in activation of clotting enzymes from the horseshoe crab, and the human complement cascade. Eur J Biochem 175 (1988) 573-580. 

Lipase (Microbial) Activity for Medium- and Long-Chain Fatty Acids, (S3)105 Lysozyme Activity, (S3)106 Maltogenic Amylase Activity, 804 Milk-Clotting Activity, 805 Pancreatin Activity, 805 Pepsin Activity, 807 Phospholipase A2 Activity, 808 Phytase Activity, 808 Plant Proteolytic Activity, 810 Proteolytic Activity, Bacterial (PC), 811 Proteolytic Activity, Fungal (HUT), 812 Proteolytic Activity, Fungal (SAP), 813 Pullulanase Activity, 814 Trypsin Activity, 814 Enzyme Assays, 786 Enzyme-Hydrolyzed (Source) Protein,... 

F-1) Heparin interferes with the activity of a number of clotting enzymes in the clotting cascade. It is a useful anticoagulant drug (see also 151). 

The major function of the K vitamins is in the maintenance of normal levels of the blood clotting proteins, factors II, VII, IX, X and protein C and protein S, which are synthesized in the liver as inactive precursor proteins. Conversion from inactive to active clotting factor requires a post-translational modification of specific glutamate (E) residues. This modification is a carboxylation and the enzyme responsible for it requires vitamin K as a cofactor. 

ACC American College of Cardiology ACE angiotensin-converting enzyme ACS acute coronary syndrome ACT activated clotting time ADP adenosine diphosphate AHA American Heart Association aPTT activated partial thromboplastin time ARB angiotensin-receptor blocker... 

Although the general proteolytic activities of various milk clotting enzymes may vary, their milk clotting activities are apparently predicated on the same specific cleavage of the Phe-Met bond in -casein. Apparently rennin, pepsin, chymotrypsin, a microbial protease, proteases from Endo-thia parasitica, Mucor pusillus, and Mucor miehei exert the same type of activity on -casein (2, 169). Enzymes that are currently used commercially for cheesemaking in the United States include rennin, rennin-pepsin mixtures, and microbial proteases from Endothia parasitica, Mucor pusiUus, and Mucor miehei. 

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