Enzymes chymotrypsinogen

The active center of the enzyme chymotrypsinogen consists of two histidine residues and a serine residue (Figure 30-2). The rest of the molecule maintains these three groups in a specific position relative to each other. Thus, only quite specific substrates can react with the active center, others do not match the active center, or cannot bring their reactive groups into the sterically or electronically most favorable position. The optimum reaction occurs when the substrate fits to the active center as a key fits to a lock. In such a case, the substrate is adsorbed up to saturation point by the enzyme consequently most enzyme reactions follow Michaelis-Menten kinetics (see also Chapter 19). 

Figure 11.7 Schematic diagram of the structure of chymotrypsin, which is folded into two antiparallel p domains. The six p strands of each domain are red, the side chains of the catalytic triad are dark blue, and the disulfide bridges that join the three polypeptide chains are marked in violet. Chain A (green, residues 1-13) is linked to chain B (blue, residues 16-146) by a disulfide bridge between Cys 1 and Cys 122. Chain B is in turn linked to chain C (yellow, residues 149-245) by a disulfide bridge between Cys 136 and Cys 201. Dotted lines indicate residues 14-15 and 147-148 in the inactive precursor, chmotrypsinogen. These residues are excised during the conversion of chymotrypsinogen to the active enzyme chymotrypsin.

The proteases are secreted as inactive zymogens the active site of the enzyme is masked by a small region of its peptide chain, which is removed by hydrolysis of a specific peptide bond. Pepsinogen is activated to pepsin by gastric acid and by activated pepsin (autocatalysis). In the small intestine, trypsinogen, the precursor of trypsin, is activated by enteropeptidase, which is secreted by the duodenal epithelial cells trypsin can then activate chymotrypsinogen to chymotrypsin, proelas-tase to elastase, procarboxypeptidase to carboxypepti-dase, and proaminopeptidase to aminopeptidase. 

Procarboxypeptidase A is activated by the removal of a peptide of some 64 residues from the N-terminus by trypsin.153 This zymogen has significant catalytic activity. As well as catalyzing the hydrolysis of small esters and peptides, procarboxypeptidase removes the C-terminal leucine from lysozyme only seven times more slowly than does carboxypeptidase. Also, the zymogen hydrolyzes Bz-Gly-L-Phe with kcsA = 3 s-1 and KM = 2.7 mM, compared with values of 120 s 1 and 1.9 mM for the reaction of the enzyme.154 In contrast to the situation in chymotrypsinogen, the binding site clearly pre-exists in procarboxypeptidase, and the catalytic apparatus must be nearly complete. 

Protein digestion occurs in two stages endopeptidases catalyse the hydrolysis of peptide bonds within the protein molecule to form peptides, and the peptides are hydrolysed to form the amino acids by exopeptidases and dipeptidases. Enteropeptidase initiates pro-enzyme activation in the small intestine by catalysing the conversion of trypsinogen into trypsin. Trypsin is able to achieve further activation of trypsinogen, i.e. an autocatalytic process, and also activates chymotrypsinogen and pro-elastase, by the selective hydro-... 

Hydrolysis of peptides or proteins with acid yields a mixture of free a-amino acids. When completely hydrolyzed, each type of protein yields a characteristic proportion or mixture of the different amino acids. The 20 common amino acids almost never occur in equal amounts in a protein. Some amino acids may occur only once or not at all in a given type of protein others may occur in large numbers. Table 3-3 shows the composition of the amino acid mixtures obtained on complete hydrolysis of bovine cytochrome c and chymotrypsinogen, the inactive precursor of the digestive enzyme chymotrypsin. These two proteins, with very different functions, also differ significantly in the relative numbers of each kind of amino acid they contain. 

For some enzymes, an inactive precursor called a zymogen is cleaved to form the active enzyme. Many proteolytic enzymes (proteases) of the stomach and pancreas are regulated in this way. Chymotrypsin and trypsin are initially synthesized as chymotrypsinogen and trypsinogen (Fig. 6-33). Specific cleavage causes conformational changes that expose the enzyme active site. Because this type of activation is irreversible, other... 

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... 

Some of the serine proteases are stored in the pancreas as inactive precursors that may be activated by proteolysis. Trypsinogen, for example, is converted to trypsin by the removal of the N-terminal hexapeptide on the cleavage of the bond between Lys-6 and Ile-7 by enterokinase. Chymotrypsinogen is activated by the tryptic cleavage of the bond between Arg-15 and He-16. (In this case, further proteolysis by the chymotrypsin that is released during the activation leads to the different forms of the enzyme—Figure 16.5.)... 

The inactive precursors are called trypsinogen, pepsinogen, chymotrypsino-gen, and procarboxypeptidase. These precursors are converted to the active enzymes by hydrolytic cleavage of a few specific peptide bonds under the influence of other enzymes (trypsin, for example, converts chymotrypsinogen to chymotrypsin). The digestive enzymes do not appear to self-destruct, probably because they are so constructed that it is sterically impossible to fit a part of one enzyme molecule into the active site of another. In this connection, it is significant that chymotrypsin attacks denatured proteins more rapidly than natural proteins with their compact structures of precisely folded chains. 

Schematic diagrams of the amino acid sequences of chymotrypsin, trypsin, and elastase. Each circle represents one amino acid. Amino acid residues that are identical in all three proteins are in solid color. The three proteins are of different lengths but have been aligned to maximize the correspondence of the amino acid sequences. All of the sequences are numbered according to the sequence in chymotrypsin. Long connections between nonadjacent residues represent disulfide bonds. Locations of the catalytically important histidine, aspartate, and serine residues are marked. The links that are cleaved to transform the inactive zymogens to the active enzymes are indicated by parenthesis marks. After chymotrypsinogen is cut between residues 15 and 16 by trypsin and is thus transformed into an active protease, it proceeds to digest itself at the additional sites that are indicated these secondary cuts have only minor effects on the enzymes s catalytic activity. (Illustration copyright by Irving Geis. Reprinted by permission.)...

The trypsin family of proteases plays a role in acute and chronic pancreatitis, as well as leads to its ultimate destruction [4, 105]. In pancreatitis, active exocrine enzymes are prematurely released inside the pancreatic duct. Various factors can contribute to the development of acute pancreatitis. Trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase are inactive proforms of proteolytic enzymes produced by the pancreatic acinar cells. Following secretion these enzymes are activated in a cascade that converts trypsinogen to trypsin in the duodenum and/or small intestine. 

Many enzymes are synthesized as inactive zymogens and are activated only after secretion from their site of synthesis and storage. Activation is achieved by cleavage of one or more peptide bonds. A standard example is the secretion of trypsinogen and chymotrypsinogen from the pancreas into the gas-... 

Chymotrypsin Chymotrypsinogen Pancreas Two dipeptides, creating a three-subunit enzyme Trypsin and chymotrypsin Carboxyl group contributed by aromatic amino acids... 

In the duodenum, the pancreatic zymogens, trypsinogen, chymotrypsinogen, proelastase and procarboxypeptidase are converted into active enzymes by enteropeptidase and trypsin, as shown in Fig. 15-6. The activation of all the zymogens involves cleavage of peptide bonds and removal of peptides, enabling conformational changes and formation of a functional active site. 

Q8 The normal pancreas is not damaged by the enzymes it produces because they are produced and stored in an inactive form, for example trypsinogen and chymotrypsinogen. 

When trypsinogen enters the small intestine, it is converted to trypsin by enterokinase. The trypsin produced then converts chymotrypsinogen and other proteolytic enzymes to their active form. 

Dietary proteins, with very few exceptions, are not absorbed rather they must be digested into amino acids, or di- and tripeptides. Protein digestion begins in the stomach, where proenzyme pepsinogen is autocatalytically converted to pepsin A. Most proteolysis takes place in the duodenum via enzymes secreted by the pancreas, including trypsinogen, chymotrypsinogen and pro-carboxypeptidase A. These serine and zinc proteases are produced in the form of their respective proenzymes they are both endopeptidase and exopeptidase, and their combined action leads to the production of amino acids, dipeptides and tripeptides. 

Another example of post-translational modification is the cleavage of the polypeptide chain. Chymotrypsin is produced in the inactive form, the proenzyme, as chymotrypsinogen. This type of inactive precursor to an enzyme is known as a zymogen. 

Chymotrypsinogen and chymotrypsin Activation of zymogen —> Tyr A-spectrum autolysis or urea treatment of enzyme —> Try A-speotrum Chervenka (1959)... 

X-ray studies have also shown that substrate binding capability is a key distinction between the active enzyme and its zymogen (20, 21). Chymotrypsinogen is unable to form a substrate binding pocket. The... 

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