Digested dietary proteins

Ah 20 types of amino adds are required for protein synthesis. These amino adds can be derived from digesting dietary protein and absorbing their constituent amino acids or, alternatively, by synthesizing them de novo. 

Most dietary proteins are known not to be absorbed in humans as intact forms. Instead, they are usually broken down into amino acids or di- and tripeptides first in the GI tract. The stomach secretes pepsinogen, which is converted to the active protease pepsin by the action of acid. Pepsins, which are most active at pH 2-3, hydrolyze partially digested dietary proteins. The partially digested dietary proteins are further broken down by proteolytic enzymes (peptidases) produced by the pancreas and secreted in the duodenum of the small intestine. The peptidases that break the internal peptide linkages are known as endopeptidases, whereas those that attack the terminal, or end, groups of amino acids are called exopeptidases. 

Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 17). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. The diet must contain these amino acids in the proper proportions. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Protein constitutes about 10-15% of the average total energy intake. 

Patients with cystic fibrosis, such as Sissy Fibrosa, have a genetically determined defect in the function of the chloride channels. In the pancreatic secretory ducts, which carry pancreatic enzymes into the lumen of the small intestines, this defect causes inspissation (drying and thickening) of pancreatic exocrine secretions, eventually leading to obstruction of these ducts. One result of this problem is the inability of pancreatic enzymes to enter the intestinal lumen to digest dietary proteins. 

Pepsin Animal stomach Digestion of dietary protein... 

Amide hydrolysis is common in biological chemistry. Just as the hydrolysis of esters is the initial step in the digestion of dietary fats, the hydrolysis of amides is the initial step in the digestion of dietary proteins. The reaction is catalyzed by protease enzymes and occurs by a mechanism almost identical to that we just saw for fat hydrolysis. That is, an initial nucleophilic acyl substitution of an alcohol group in the enzyme on an amide linkage in the protein gives an acyl enzyme intermediate that then undergoes hydrolysis. 

CARMONA A (1996) Tannins thermostable pigments which complex dietary proteins and inhibit digestive enzymes. Latinoam Nutr. 44 31S-35S. 

M Yoshioka, RH Erikson, JF Woodly, R Gulb, D Guam, YS Kim. Role of rat intestinal brush-border membrane angiotensin-converting enzyme in dietary protein digestion. Am J Physiol 253 G-781-G-786, 1987. 

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." ... 

Various excellent reviews are available on phenolic compounds, their chemistry and analysis, content in foods and nutritional significance (Bravo, 1998 Dykes and Rooney, 2006 Manach et al., 2004 Naczk and Shahidi, 2006 Robbins, 2003). From a nutritional perspective, phenolic compoimds (especially tannins) are regarded as antinutritional factors due to their ability to form complexes with dietary proteins and minerals and digestive enzymes (Bravo, 1998). However, lately there has been increasing focus on the positive aspects of phenolics due to their ability to act as antioxidants which may offer potential health benefits such as prevention of diseases such as cancer and cardiovascular disease. 

All human tissues are capable of synthesizing the nonessential amino acids, amino acid remodelling and conversion of non-amino-acid carbon skeletons into amino acids and other derivatives that contain nitrogen. However, the liver is the major site of metabolism of nitrogenous compounds in the body. Dietary proteins are the primary source of essential amino acids (or nitrogen). Digestion of dietary proteins produces amino acids, which are absorbed through epithelial cells and enter the blood. Various cells take up these amino acids that enter the cellular pools. 

Digestion of dietary proteins by the proteolytic enzymes of the gastrointestinal tract. 

Takada, K., et al. 1991. Effect of pH, dietary proteins and trypsin inhibitors on hydrolytic rate of human granulocyte colony-stimulating factor (G-CSF) by rat digestive enzymes. J Pharmacobio-dyn 14 363. 

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