Trypsinogen, activation hydrolysis

Trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase are all synthesized as single polypeptide chains of around 25-30 kDa. The initial step in the activation is the hydrolysis of a hexapeptide from the N terminus of trypsinogen. This hydrolysis produces trypsin and is catalyzed by enteropeptidase (Fig. 14-8), an enzyme on the membranes of brash border cells of the small intestine. 

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. 

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-... 

Their mode of appearance in the lumen of the intestine is rather complicated and involves activation of trypsinogen secretion by enterokinase. Once trypsin is formed it activates chymotrypsinogen. Pancreatic lipase is also secreted into the lumen with the pancreatic fluid. The digestion process of fatty acids by their lipase-mediated hydrolysis is completed by bile salts, which are also secreted in the duodenum and are crucial for micellization of lipophilic compounds. The micelles formed in the duodenum enable the absorption of hydro-phobic drugs such as steroids. They pose, however, a serious constraint for the stability of drug delivery carriers such as liposomes and emulsions. 

The low pH of the stomach contents stimulates the secretion of the hormone secretin, as it passes into the small intestine. This hormone is secreted into the bloodstream, and when it reaches the pancreas, it stimulates that organ to secrete bicarbonate into the gut which neutralizes the low pH of the entering stomach contents. Entering amino acids stimulate the secretion by intestinal cells of the specialized enzyme enterokinase. It specificity is directed toward the conversion of the zymogen, trypsinogen, secreted by the pancreas, to the active proteolytic enzyme, trypsin. Trypsin then converts chymotrypsinogen to the active chymotrypsin. These two enzymes reduce polypeptides to small peptides. The further hydrolysis of the small peptides to their constituent amino acids is accomplished by two other enzymes secreted by intestinal cells, carboxypeptidase and aminopeptidase. The mixture of amino acids is then transported across the intestinal cells, enters the blood, and is transported to the liver. 

Comparison of the structures of bovine trypsinogen and chymotrypsinogen A. Solid arrows show the hydrolysis sites that result in the activation of trypsinogen to trypsin, and chymotrypsinogen to chymotrypsin. Broken arrows are additional hydrolysis sites, leading to formation of a-chymotrypsin. Shaded circles represent amino acids that are Identical or similar in the two proteins. Disulfide bridges are lettered A to G. H represents the histidine residues, and S the serine residue of the active site this serine residue reacts with diisopropylfluorophosphate. Deletions are shown by lines between the circles. To aid comparison, the residue numbers of both structures are based on those of chymotrypsinogen. [B.S.Hartley etal. Nature 207 (1965) 1157-1159]... 

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