Digestion of the protein

The digestion of proteins begins in the stomach, with the involvement of acid component, which has two functions here. The first is to activate the zymogen form pepsin, the second is to promote the denaturation of proteins.
Pepsin is a key enzyme that initiates the process of protein hydrolysis. The mucous cells secrete pepsinogen, and HCl in the stomach stimulates the conversion of pepsinogen into pepsin. This enzyme breaks down proteins and peptides in specific sites in the peptide bond, as the carboxyl group of some amino acids, phenylalanine, tryptophan and tyrosine, and perhaps other amino acids leucine and acidic.
When the protein, partially split, pass the small intestine, pancreatic enzymes trypsin, chymotrypsin and carboxypeptidases A and B are responsible for further digestion. Trypsinogen, chymotrypsinogen and procarboxypeptidase A and B are the zymogen forms of trypsin, chymotrypsin and carboxypeptidases A and B, respectively. Intestinal mucosal cells secrete the enzyme enterokinase, which unfolds a hexapeptide of trypsinogen to form active trypsin. Once formed, trypsin can also perform a division hexapeptide of trypsinogen, providing more trypsin. This enzyme in turn converts inactive forms of pancreatic enzymes in their active forms. Trypsin acts on peptide bonds involving the carboxyl groups of arginine and lysine. It is also an endopeptidase that cleaves peptide since the interior of the protein chain. Chymotrypsinogen is a Endopeptidase. Carboxypeptidases A and B are considered as exopeptidase that cleave amino acids from the carboxyl end of polypeptides. Aminopeptidases, which are considered to be exopeptidase, cleave the peptides into amino acids and oligopeptides.
The final hydrolysis of the peptides produced by pancreatic enzymes occurs at the surface of the membranes of the microvilli of intestinal mucosa cells. In short, the end result of luminal digestion of proteins in the small intestine is to obtain fragments of oligopeptides, dipeptides and amino acids.
The absorption of the protein is mainly in the form of individual amino acids, and the ileal part of the small intestine. This is done by a mechanism that uses energy-dependent transporters, which are located in the membrane of the microvilli. These transporters, which are for four different groups of amino acids: I) Neutral: a) aromatic (tyrosine, tryptophan, phenylalanine, b) aliphatic (alanine, serine, threonine, valine, leucine, isoleucine, glycine), and methionine, histidine, glutamine, asparagine, cysteine, II) Basic (lysine, arginine, ornithine, cystine), III) dicarboxylic acids (glutamic and aspartic), IV) Amino acids: proline, hydroxyproline, glycine can also use this carrier that used by neutral amino acids, other amino acids (taurine, D-alanine, gamma-aminobutyric acid.

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