Digestion of protein

Dietary protein is the principal source of fixed nitrogen in higher animals. In digestion, proteins are hydrolyzed by a series of hydrolytic enzymes in the stomach and the small intestine to peptides and amino acids, which are absorbed from the lumen of the gastrointestinal tract. These enzymes are known collectively as proteolytic enzymes, or proteases, and belong to the class of enzymes called hydrolases (Chap. 8). 

In the disease pancreatitis, that occasionally follows a bout of mumps, the proteolytic enzymes secreted by the pancreas are prematurely activated and digest the cells of the pancreas. 

The entry of protein into the stomach stimulates the release of a hormone, gastrin, which then causes the release of hydrochloric acid from the parietal cells, and pepsinogen from the chief cells (Fig. 15-5). Pepsinogen is another zymogen (they all start with pro- or end in -ogen) that is converted in the gastric juice to the active enzyme pepsin. 

Question What is the function of hydrochloric acid in digestion  

The hydrochloric acid lowers the pH of the stomach contents to pH2, which kills most microorganisms and denatures proteins, making their peptide bonds more accessible to enzymatic hydrolysis. 

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Cathepsin D Spleen, liver, and many other animal tissues Lysosomal digestion of proteins... 

Several Groups of Enzymes Catalyze the Digestion of Proteins... 

There are problems as well in the absorption of certain drugs in the presence of specific food components. L-Dopa absorption may be inhibited in the presence of certain amino acids formed from the digestion of proteins [43], The absorption of tetracycline is reduced by calcium salts present in dairy foods and by several other cations, including magnesium and aluminum [115-117], which are often present in antacid preparations. In addition, iron and zinc have been shown to reduce tetracycline absorption [118], Figure 17 illustrates several of these interactions. These cations react with tetracycline to form a water-in-soluble and nonabsorbable complex. Obviously, these offending materials should not be co-administered with tetracycline antibiotics. 

Zhou, X. H. Gonnet, G. Hallett, M. Munchbach, M. Folkers, G. James, P. Cell fingerprinting An approach to classifying cells according to mass profiles of digests of protein extracts. Proteomics 2001,1, 683-690. 

Tbe Digestion of Protein and Nitrogenous Compounds in Ruminants Margaret I. Chalmers and R. L. M. Synge... 

The problem to be solved with respect to the chemical reactions that constitute metabolism and sustain life is that, without the action of catalysts, they are far too slow. Let s consider the digestion of the proteins themselves, an important constituent of our diet. In an enviromnent similar to that of our digestive system, several tens of thousand years would be required to digest half of the protein content of a typical meal in the absence of a catalyst. Clearly, this will not do. In reality, the stomach secretes one protein catalyst, the enzyme pepsin, and the pancreas secretes several enzymes that catalyze the digestion of proteins. In the presence of these enzymes, dietary proteins are fully digested and reduced to their basic constituents, the amino acids, in a matter of hours. Obviously, these enzymes are enormously potent catalysts." ... 

Temporary storage endocrine and paracrine communication digestion of protein. 

Figure 8.13 The central role of transdeamination in metabolism of amino adds and further metabolism of the oxoacids in the liver. The box contains the reactions for conversion of the amino acids to their respective oxoacids. Processes are as follows (1) digestion of protein in the intestine and absorption of resultant amino acids, (2) degradation of endogenous protein to amino acids (primarily but not exclusively muscle protein), (3) protein synthesis, (4) conversion of amino acid to other nitrogen-containing compounds (see Table 8.4), (5) oxidation to CO2, (6) conversion to glucose via gluconeogenesis, (7) conversion to fat.

The origin of the idea that a ribosome might be a ribozyme is derived from the experiment in which peptidyl transferase activity was observed even after digestion of protein components of the ribosome [15]. This was surprising because the most important biological function involved in the synthesis of proteins is catalyzed by RNA. Recently, a large ribosomal subunit from Haloarcula marismortui was determined at a resolution of 2.4 A [16, 155]. Importantly, because of the absence of proteins at the active site, it was concluded that the key peptidyl transferase reaction is accomplished by the ribosomal RNA (rRNA) itself, not by proteins. How does it work ... 

In the stomach, pepsin and related enzymes initiate the enzymatic digestion of proteins, which takes 1-3 hours. The acidic gastric contents are then released into the duodenum in batches, where they are neutralized by alkaline pancreatic secretions and mixed with cystic bile. 

Effect of predigestion factors on the apparent digestibility of protein for swine determined by the mobile nylon bag technique. Journal of Animal Science 66 (8), 1963-1968. 

Asafetida, administered orally to albino rats at a dose of 250 mg% for 8 weeks, enhanced pancreatic lipase activity, stimulated pancreatic amylase and chymotrypsin. The stimulatory influence was not observed when their intake was restricted to a single oral dose " " ". Protein digestibility. Asafetida did not affect the digestibility of protein in sorghum " . ... 

Mecfianism of Action Digestive enzymes that replace endogenous pancreatic enzymes. Tfierapeutic Effect Assist in digestion of protein, starch, and fats. Pharmacokinetics Not absorbed systemically. Released at the duodenojejunal junction. 

Rosenfeld J, Capdevielle J, Guillemot JC, Ferrara P. (1992) In-gel digestion of proteins for internal sequence analysis after one- or two-dimensional gel electrophoresis. Anal Biochem 203, 173-9. 

Certain polypeptides resulting from protease digestion of proteins contribute to the typical taste of savoury foods. The DNA sequence coding for an octapep-tide known as beefy meaty peptide was cloned into yeast as a fusion with the yeast a factor to be secreted as free octapeptide into the medium which facilitated its recovery [37]. Alternatively, intracellular expression of tasty peptides... 

Figure 8.15 Cartoon showing how proteins, polysaccharides and surfactants (emulsifiers) might be distributed at the triglyceride-water interface. Inter-facial complexation in vivo between adsorbed protein and charged polysaccharide in the gastrointestinal tract could affect digestion of protein and fat by forming structures that inhibit the accessibility and activity of enzymes (proteases and lipases). Reproduced from Dickinson (2008) with permission.

As the acidic stomach contents pass into the small intestine, the low pH triggers secretion of the hormone secretin into the blood. Secretin stimulates the pancreas to secrete bicarbonate into the small intestine to neutralize the gastric HC1, abruptly increasing the pH to about 7. (All pancreatic secretions pass into the small intestine through the pancreatic duct.) The digestion of proteins now continues in the small intestine. Arrival of amino acids in the upper part of the intestine (duodenum) causes release into the blood of the hormone... 

The digestion of proteins begins in the stomach, which secretes gastric juice—a unique solution containing hydrochloric acid and the proenzyme, pepsinogen ... 

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