Proenzymes, trypsin

Trypsinogen, the inactive proenzyme form of trypsin has no water molecules in its unordered active site cf. Fehlhammer, H., Bode, W., Huber, R. ibid. Ill, 415 (1977)... 

These proteolytic enzymes are all endopeptidases, which hydrolyse links in the middle of polypeptide chains. The products of the action of these proteolytic enzymes are a series of peptides of various sizes. These are degraded further by the action of several peptidases (exopeptidases) that remove terminal amino acids. Carboxypeptidases hydrolyse amino acids sequentially from the carboxyl end of peptides. They are secreted by the pancreas in proenzyme form and are each activated by the hydrolysis of one peptide bond, catalysed by trypsin. Aminopeptidases, which are secreted by the absorptive cells of the small intestine, hydrolyse amino acids sequentially from the amino end of peptides. In addition, dipeptidases, which are structurally associated with the glycocalyx of the entero-cytes, hydrolyse dipeptides into their component amino acids. 

Trypsinogen plays a key role among the proenzymes released by the pancreas. In the bowel, it is proteolytically converted into active trypsin (see p. 176) by enteropeptidase, a membrane enzyme on the surface of the en-terocytes. Trypsin then autocatalytically activates additional trypsinogen molecules and the other proenzymes (left). 

Pancreatic secretions are especially rich in the phospholipase A2 proenzyme, which is activated by trypsin and requires bile salts for activity. 

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... 

Figure 12-8 Cascade of reactions that activate pancreatic proteases. Enteropeptidase, or trypsin, cleaves the proenzyme (zymogen) at specific sites.

Chymotrypsinogen consists of a single 245-residue chain. The amino acid residues in chymotrypsin, trypsin, and elastase are usually all numbered according to their position in this zymogen. Inactive proenzymes are formed as precursors to enzymes of many different classes and are activated in a variety of ways. A part of the polypeptide chain of the proenzymes is often folded over the active site, interacting in a nonsubstrate-like fashion and blocking the site.197a... 

Renaturation of denatured protein is dictated by the primary structure of the protein. The trypsin family of enzymes and carboxypeptidase A are synthesized as proenzymes that are proteolytically activated. The proteolyzed, active enzymes have primary structures different from the gene product and are not active upon renaturation. In addition, zinc is a cofactor required for carboxypeptidase A activity. 

Latent forms of a neutral protease and an acid protease (pH optimum 5.3) from cartilage are activated by trypsin hydrolysis to give Ca2+-dependent enzymes that catalyze the hydrolysis of proteoglycan.406 Some Ca2+-dependent proteinases are isolated as proenzymes that can be converted to the active form by high [Ca2+] or by low [Ca2+] in the presence of a digestable substrate.407... 

The 19th century witnessed the beginning of studies on the pathophysiology of acute pancreatitis. In 1867 Kiine isolated an enzyme catalyzing the cleavage of peptide bonds, which was later termed trypsin (1877), and a few years later Heidenhain found trypsin in pancreatic cells in the form of inactive proenzyme. 

The pancreatic enzyme it stored and secreted as a proenzyme with an additional seven residues at the N-tcmiinus, The proenzyme serves, like the other pancreatic proteinase zymogens, to prevent aotodigestion of the pancreatic cells. Upon secretion into the gastrointestinal tract, trypsin cleaves off these seven residues to produce the enzymatic form with full activity on insoluble substrates. On monomeric substrates, however, there is little difference between the catalytic activity of the proenzyme and of the activated enzyme [34],... 

The activation of enzyme precursors is likely to be of central importance. A significant fraction of the total SCCE present in the stratum corneum is in the form of inactive proenzyme.53,54 A change in the ratio of precursor to active enzyme may be expected to cause marked changes in the rate of corneodesmosomal degradation. In vitro pro-SCCE can be activated by pancreatic trypsin.39 As mentioned earlier SCTE has been suggested to act as an SCCE activator, but this remains to be elucidated. It is possible that SCCE is just one of a number of enzymes constituting a proteolytic cascade in the stratum corneum, in which one enzyme serves as activator of another enzyme. 

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]. 

Bovine carboxypeptidase A is produced in the pancreas as a zymogen, procarboxypeptidase A, MW = 87,000. The proenzyme is composed of three polypeptide chains (151, schematically shown in Figure 15). On limited digestion with trypsin one or more peptide bonds in subunit II is split resulting in its conversion to an enzyme (ATEEase) having activity on acetyl-L-tyrosine ethyl ester similar to that of chymotrypsin. Continued... 

Our knowledge of porcine pancreatic phospholipase comes from the laboratory of G. H. de Haas and his co-workers. The enzyme is quite different from pancreatic lipase (Table VIII). Its molecular weight is quite small, it is a metalloenzyme that requires Ca ion as a cofactor, and it is excreted from the pancreas as a proenzyme which is then activated by trypsin with removal of a heptapeptide. The molecule has six... 

S-S bridges and is very stable, even at 100 °C. Figure 5 shows the complete primary structure of the proenzyme with the point of attack of trypsin at residue 7 and with the S-S bridges (20). 

Trypsin also activates trypsinogen as well as all the other proenzymes in the pancreatic secretion, producing the active proteases and peptidases that hydrolyze dietary polypeptides. 

Pancreatic lipase, in the presence of bile salts and coUpase, acts at the oil-water interface of the triglyceride emulsion to produce fatty acids and 2-monoacylglycerols. Cohpase is secreted in pancreatic juice as an inactive proenzyme, which is converted to the active form by trypsin. Other significant enzymes involved in the breakdown of fats within the intestinal lumen are cholesterol ester hydrolase, phospholipase A, and a nonspecific bile salt-activated lipase. 

All human metzincins are secreted as proenzymes. Astacins and adamalysins are mostly activated by calcium-ion-dependent serine proteases pro-protein convertases) that meet up with their substrates in trans-Golgi and secretory vacuoles. These proenzymes are known as furin-like convertases because of their homology to a serine protease called furin and a bacterial endoprotease called subtilisin. The furin-like enzymes require calcium ions to maintain structural stability whereas other serine proteases, represented by trypsin and chymotrypsin, do not. The furin-like pro-protein convertases autocleave their own N-terminal domain propeptide (self-activate) during secretion and then convert the N-terminal domains of co-secreted metzincins. Activation cascades also occur among the... 

Pancreatic juice contains the proenzymes trypsinogen, chymotrypsinogen, procarboxypeptidases, and proelas-tase. All are activated by trypsin in the intestinal lumen. Enteropeptidase located in the brush border of the jejunal mucosa converts trypsinogen to trypsin. A trypsin inhibitor in pancreatic juice protects against indiscriminate autodigestion from intraductal activation of trypsinogen. Other enzymes of pancreatic juice and their substrates are listed below. 

The answer is d. (Murray, pp 48-62. Scriver, pp 3 5. Sack, pp 1-3. Wilson, pp 101-120.) Pepsin is secreted in a proenzyme form in the stomach. Unlike the majority of proenzymes, it is not activated by protease hydrolysis. Instead, spontaneous acid hydrolysis at pH 2 or lower converts pepsinogen to pepsin. Hydrochloric acid secreted by the stomach lining creates the acid environment. All the enzymes secreted by the pancreas are activated at the same time upon entrance into the duodenum. This is accomplished by trypsin hydrolysis of the inactive proenzymes trypsino-gen, chymotrypsinogen, procarboxypeptidase, and proelastase. Primer... 

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