Protein is a large complex molecule that must undergo a series of processes during digestion. During digestion and absorption, protein passes through many organs. Once protein is digested, the body can utilize its nutrients to build and repair many of the cells in the body. The body also uses the calories from protein which is released during the digestion process for energy, when carbohydrates and fats are not available. The protein subjected to digestion and absorption are obtained from two sources: Exogenous and endogenous. Whole proteins are not absorbed; they are too large to pass through cell membrane. Digestive enzymes like hydrolases break the peptide bond of protein and secreted as inactive pre-enzymes. Some seven or more factors influence how fast the enzyme act on the proteins. These factors includes the concentration of the enzymes i.e. how much  of it is present; the amount of the protein feed needing action, the acidity of the food and stomach, the temperature of the food, time and the presence of any digestive inhibitors such as antacids.

        I.            Protein digestion in the stomach:

Once food is chewed and swallowed, HCl and pepsin begin protein digestion in the stomach. HCl helps to kill bacteria in food that could cause infection and breaking the bond between the proteins which are disintegrated into amino acid and plays an important role in metabolism. It also makes the stomach very acidic with a pH of 1.5. This acidic environment is necessary for HCl to react with pepsinogen to form pepsin so that it can break the central peptide bond in proteins. Renin is an enzyme i.e. present in infants to help breakdown milk protein.

a.       Role of gastric HCl: It causes denaturation of protein, convert proteins into metaproteins, which are easily digested, activates pepsinogen to pepsin and makes the pH in the stomach suitable for the action of pepsin.

b.      Pepsin: It is an endopeptidase acting on central peptide bond in which amino group is belongs to aromatic amino acid. Eg: Phenylalanine, tyrosine and tryptophan. It is secreted in an inactive form called pepsinogen. Its optimum pH- 1.5-2.2. It is activated by HCl. It also helps in digesting collagen which is one of the main constituent of connective tissue in animals.

c.       Renin: It is a milk clotting enzyme. It is present in stomach of infants and young animals. Its optimum pH is 4. It acts on casein converting it to soluble paracasein which in turns bind calcium ions forming insoluble Ca-paracaseinate which is then digested by pepsin.

d.      Gelatinase: Its an enzyme that liquefies gelatine. The end products of protein digestion in the stomach are proteoses, peptones and large polypeptides.


      II.            Protein digestion in the small intestine:

Protein digestion continues in the upper portion of the small intestine under the action of the pancreatic protein enzymes; trypsin and chymotrypsin. The pancreas release digestive enzymes into the small intestine. In the duodenum, the first section of the small intestine, trypsin breaks down protein into single amino acid by a process called hydrolysis. During hydrolysis, a water molecule is placed between two amino acids and breaking the bond between them. Trypsin also activates the chymotrypsin, carboxypeptidase and elastase that are released into the small intestine for amino acid chain breakdown.

·         Trypsinogen converted into trypsin by enteropeptidase which cleaves carboxy terminal of following amino acid; Phe, Tyr, Trp. Trypsin continues the disintegration of protein into amino acid by hydrolysis which involves insertion of a water molecule between two amino acid which forces the bond between them to break because amino acid have very small dimensions, they are able to penetrate the intestinal lining. Now they enter the bloodstream through tiny veins which are called capillaries. Once in the bloodstream amino acid are transported by liquid blood plasma and RBC to various tissues, depending on where cell structure need to be created on repaired.

·         Procarboxypeptidase converted into carboxypeptidase by exopeptidase which removes carboxy terminal residue.

Pancreatic juice: It is an endopeptidase that hydrolyzes central peptide bond in which the carboxyl group belongs to basic amino acid. Eg: Arginine, lysine, histamine. It is secreted in an inactive form called trypsinogen.

Protein Absorption:

·         Protein absorption takes place in the jejunum and ileum portions of the small intestine. This process requires energy ATP is the energy, the body utilizes during protein synthesis. The body uses the carrier protein transport system to absorb amino acid. Each amino acid group has carrier protein i.e. responsible for transporting it from the intestine to the mucosal cells. Sodium and potassium are minerals needed for the amino acid to pass from the intestine through the villi and into the bloodstream.

·         Amino acids are absorbed by the blood capillaries of the small intestine, carried through the liver and then go into the blood of the general circulation.

·         Dietery proteins are very large complex molecule that cannot be absorbed from the intestine.

·         To be absorbed, dietery proteins must be digested to small simple molecule (amino acid) which is easily absorbed from the intestine.

·         The brush border of the small intestine is equipped with a family of peptidases like lactase and maltase, these peptidase are integral membrane proteins rather than soluble enzyme. They perform the hydrolysis of luminal peptide, converting them to free amino acid and very small peptides. These end products of digestion found on the surface of the enterocyte are rapidly for absorption.

·         Villi, line the walls of our small intestine while even smaller structure called microvilli, line your villi. The villi and microvilli are series of folds that serve to increase the surface area available for absorption. The digested amino acid from our food protein passes from the inside of our small intestine through the epithelial cells of our villi and microvillli and into our capillaries. They travel through epithelial cells with the help of protein called amino acid transporters, which shuttle the amino acid from the gut side of our cell to the capillary side of the cell. Once in capillaries the amino acid moves through our body via bloodstream.

·         Peptides are the forms in which majority of proteins are absorbed, more rapid than absorption of free amino acid.  Energy is required for the active transport which metabolized protein into amino acid which is ready for absorption into blood.

Protein metabolism and synthesis:

The single molecule amino acid or free amino acids that are absorbed through the wall of the small intestine are now used for the last part of protein metabolism, protein synthesis. The protein ingested from animal and plant protein sources are made into new tissues or used for tissues repairs in the body or they are broken down and used for energy, when there is too much protein in the body the excess gets converted into fat for storage.

Abnormalities in the protein digestion and amino acid absorption:

Defect in the pancreatic secretion cause cystic fibrosis which results in incomplete digestion of fat and protein results in abnormal appearance of lipids (steatorrhea) and protein in faeces due to defective carrier system.

Hartwig’s disease: Inability of intestinal and epithelial cells to absorb neutral amino acid. Tryptophan absorption is severely affected resulting in pellagra.

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