Digestion serine proteases

Thrombin is a proteolytic enzyme and has a remarkable similarity in its overall three-dimensional structure to the digestive serine proteases, trypsin, and chymotrypsin [11-13]. Trypsin and thrombin share a common primary specificity for proteolysis next to arginine or lysine residues. Structural data of thrombin and trypsin have demonstrated strong resemblance in their substrate sites, and many small organic inhibitors are comparably active against both the enzymes [14,15]. For this reason, no or low inhibition of trypsin is viewed as a required condition for a compoimd to be a successful orally bioavailable thrombin inhibitor [16]. 

Digestive serine proteases are present in the pyloric ceca, the pancreatic tissues, and the intestines of animals and have been reported in numerous species of Archaea (Eichler, 2001). Serine proteases are inactive at acidic pH and have high activity under neutral to slightly alkaline conditions (Simpson, 2000). Although fish serine proteases are quite similar to their mammalian counterparts, they have been reported to be more active under alkaline rather than neutral conditions (Shahidi and Janak Kamil, 2001). Some of the most well-known serine proteases from marine sources include trypsins, chymotrypsins, collagenases, and elastases. 

Proteolytic maturation is a common theme among phage proteins involving either phage-derived proteases or autocatalytic processes [32,138-141]. In contrast to pro-enzymes like mammalian digestive serine proteases, the function of... 

The serine proteases are the most extensively studied class of enzymes. These enzymes are characterized by the presence of a unique serine amino acid. Two major evolutionary families are presented in this class. The bacterial protease subtilisin and the trypsin family, which includes the enzymes trypsin, chymotrypsin, elastase as well as thrombin, plasmin, and others involved in a diverse range of cellular functions including digestion, blood clotting, hormone production, and complement activation. The trypsin family catalyzes the reaction ... 

The catalytic mechanism of the subtilisins is the same as that of the digestive enzymes trypsin and chymotrypsin as well as that of enzymes in the blood clotting cascade, reproduction and other mammalian enzymes. The enzymes are known as serine proteases due to the serine residue which is crucial for catalysis (Kraut, 1977 and Polgar, 1987)... 

The serine proteases cleave amide (peptide) bonds in peptides and have a wide variety of functions, including food digestion, blood clotting, and hormone production. They feature as one of the best-understood groups of enzymes as far as mechanism of action is concerned. We are able to ascribe a function to many of the amino acid residues in the active site, and we also understand how they determine the specificity of the various enzymes in the group. 

A large group of proteinases contain serine in their active center. The serine proteases include, for example, the digestive enzymes trypsin, chymotrypsin, and elastase (see pp. 94 and 268), many coagulation factors (see p. 290), and the fibrinolytic enzyme plos-min and its activators (see p. 292). 

Fibrinolysis refers to the process of fibrin digestion by the fibrin-specific protease, plasmin. The fibrinolytic system is similar to the coagulation system in that the precursor form of the serine protease plasmin circulates in an inactive form as plasminogen. In response to injury, endothelial cells synthesize and release tissue plasminogen activator (t-PA), which converts plasminogen to plasmin (Figure 34-3). Plasmin remodels the thrombus and limits its extension by proteolytic digestion of fibrin. 

Trypsin is the most frequently used serine protease. Generally, porcine or bovine pancrease is used as the source of pure trypsin. Trypsin usually digests proteins at their lysine and arginine residues. The superiority of trypsin is that it displays good activity both in solution and in-gel digestion protocols. 

The digestive enzymes trypsin, chymotrypsin, elastase, and proteinase E are related serine proteases. All three are synthesized in the pancreas which secretes 5-10 g per day of proteins, mostly the inactive proenzymes (zymogens) of digestive enzymes.191,192... 

Acyl-enzyme intermediates. Serine proteases are probably the most studied of any group of enzymes.229 Early work was focused on the digestive enzymes. The pseudosubstrate, p-nitrophenyl acetate, reacts with chymotrypsin at pH 4 (far below the optimum pH for hydrolysis) with rapid release of p-nitro-phenol and formation of acetyl derivative of the enzyme. 

The serine proteases are a large family of proteolytic ( enzymes that use the reaction mechanism for nucleophilic catalysis outlined in equations (3) and (4), with a serine residue as the reactive nucleophile. The best known members of the family are three closely related digestive enzymes trypsin, chymotrypsin, and elastase. These enzymes are synthesized in the mammalian pancreas as inactive precursors termed zymogens. They are secreted into the small intestine, where they are activated by proteolytic cleavage in a manner discussed in chapter 9. 

Pancreatic fluid, secreted in the duodenum is composed of digestive enzymes and bicarbonate. The two major pancreatic proteases are the serine proteases trypsin and chymotrypsin. 

Serine proteases (SP) are a family of enzymes that use a uniquely activated serine residue in the substrate-binding pocket to catalytically hydrolyze peptide bonds [66], SP carry out a diverse array of physiological functions, of which the best known are digestion, blood clotting, fibrinolysis, fertilization, and complement activation during immune responses [67], They have also been shown to be abnormally expressed in many diseases including cancer, arthritis, and emphysema [42, 43, 67-70],... 

Interactions between serine proteases are common, and substrates of serine proteases are usually other serine proteases that are activated from an inactive precursor [66]. The involvement of serine proteases in cascade pathways is well documented. One important example is the blood coagulation cascade. Blood clots are formed by a series of zymogen activations. In this enzymatic cascade, the activated form of one factor catalyzes the activation of the next factor. Very small amounts of the initial factors are sufficient to trigger the cascade because of the catalytic nature of the process. These numerous steps yield a large amplification, thus ensuring a rapid and amplified response to trauma. A similar mechanism is involved in the dissolution of blood clots. A third important example of the coordinated action of serine proteases is the intestinal digestive enzymes. The apoptosis pathway is another important example of coordinated action of other types of proteases. 

The properties and spatial arrangement of the amino acid residues forming the active site of an enzyme will determine which molecules can bind and be substrates for that enzyme. Substrate specificity is often determined by changes in relatively few amino acids in the active site. This is clearly seen in the three digestive enzymes trypsin, chymotrypsin and elastase (see Topic C5). These three enzymes belong to a family of enzymes called the serine proteases - serine because they have a serine residue in the active site that is critically involved in catalysis and proteases because they catalyze the hydrolysis of peptide bonds in proteins. The three enzymes cleave peptide bonds in protein substrates on the carboxyl side of certain amino acid residues. 

The serine proteases are a dass of proteolytic enzyme (they catalyze the hydrolysis of either ester or peptide bonds in proteins) that require an active site residue for covalent catalysis. The active site residue, the catalytic Ser-195, is particularly activated by hydrogen-bonding interactions with His-57 and Asp-102. Crystal structures show that Ser-195, His-57, and Asp-102 are dose in space. Together these three residues, which are located in the substrate binding (SI) pocket, form the famed catalytic triad of the serine proteases. In humans and mammals serine proteases perform many important functions, especially the digestion of dietary protein, in the blood-dotting cascade, and in the complement system ... 

The destructive potential of proteases means that their activity has to be tightly regulated to prevent autolysis of protease-producing cells. Thus digestive proteases such as chymotrypsin, pepsin and trypsin are produced as inactive zymogens (proenzymes) and are subsequently activated after secretion. The serine protease-catalysed process of blood clotting involves a cascade of successive proteolytic activations of the blood clotting factor proteases involved [2-6]. 

The cereal dual function a-amylase/trypsin inhibitor proteins are cysteine-rich, disulphide-rich, double-headed, 13-16 kDa, dual function inhibitor proteins that inhibit both of the digestion enzymes a-amylase and trypsin [290-325] (Table 11). Thus the Zea (com) member of this family, com Hageman factor inhibitor (CHFI), is a double-headed 14 kDa protein that inhibits a-amylase and the serine proteases trypsin and blood clotting Factor Xlla [323-324] (Table 11). The structures of the bifunctional a-amylase/trypsin inhibitor proteins from Eleusine (ragi) (RBI) [292-295] and Zea (com) (CHFI) [325] have been determined. These proteins are structurally similar to the lipid transfer proteins, being composed of a bundle of 4 a-helices together with a short [3-sheet element connected by loops, the a-amylase- and protease-inhibitory domains being separately located [325]. 

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