Digestion and absorption of proteins The intake of dietary protein is in the range of 50 100 g day The digestion and absorption of proteins is very efficient in healthy humans hence very little protein about 5 10 g day is lost through feces Dietary proteins are denatured on cooking and therefore easily digested Proteins are degraded by a class of enzymes namely hydrolases which specifically cleave the peptide bonds hence known as peptidases They are divided into two groups Endopeptidases proteases which attack the internal peptide bonds and release peptide fragments e g pepsin trypsin Exopeptidases which act on the peptide bonds of terminal amino acids Exopeptidases are subdivided into carboxypeptidases act on C terminal amino acid and aminopeptidases act on N terminal amino acid The proteolytic enzymes responsible for the digestion of proteins are produced by the stomach the pancreas and the small intestine Proteins are not digested in the mouth due to the absence of proteases in saliva The digestion of protein is effected by enzymes in Stomach Pancreas Intestinal cells A Gastric Digestion of Proteins In the stomach hydrochloric acid is secreted It makes the pH optimum for the action of pepsin and also activates pepsin The acid also denatures the proteins Rennin Rennin otherwise called Chymosin is active in infants and is involved in the curdling of milk It is absent in adults Milk protein casein is converted to paracasein by the action of rennin This denatured protein is easily digested further by pepsin Pancreatic Digestion of Proteins The optimum pH for the activity of pancreatic enzymes pH 8 The secretion of pancreatic juice is stimulated by the peptide hormones Cholecystokinin and Pancreozymin Pancreatic juice contains the important endopeptidases namely Trypsin Chymotrypsin Elastase and Carboxypeptidase These enzymes are also secreted as zymogens trypsinogen chymotrypsinogen and pro elastase Pepsin It is secreted by the chief cells of stomach as inactive pepsinogen The conversion of pepsinogen to pepsin is brought about by removal of 44 amino acids from the N terminal end by the hydrochloric acid The optimum pH for activity of pepsin is around 2 Pepsin is an endopeptidase Pepsin catalyses hydrolysis of the bonds formed by carboxyl groups of Phe Tyr Trp and Met By the action of pepsin proteins are broken into proteoses and peptones Trypsin Trypsinogen is activated by enterokinase enteropeptidase present on the membranes Once activated the trypsin activates other enzyme molecules Trypsin is activated by the removal of a hexapeptide from N terminal end Trypsin catalyses hydrolysis of the bonds formed by carboxyl groups of Arg and Lys Acute pancreatitis Premature activation of trypsinogen inside the pancreas itself will result in the auto digestion of pancreatic cells The result is acute pancreatitis It is a life threatening condition intestinal microvillus Chymotrypsin Trypsin will act on chymotrypsinogen in such a manner that A B and C peptides are formed These 3 segments are approximated so that the active site is formed Thus selective proteolysis produces the catalytic site Carboxypeptidases Trypsin and chymotrypsin degrade the proteins into small peptides these are further hydrolysed into dipeptides and tripeptides by carboxypeptidases present in the pancreatic juice The procarboxy peptidase is activated by trypsin They are metalloenzymes requiring zinc Intestinal Digestion of Proteins Complete digestion of the small peptides to the level of amino acids is brought about by enzymes present in intestinal juice succus entericus Luecine aminopeptidases It releases the N terminal basic amino acids and glycine Proline amino peptidase It removes proline from the end of polypeptides Dipeptidases and tripeptidases They will bring about the complete digestion of proteins ABSORPTION Amino acids are primarily absorbed by a similar mechanism as described for the transport of D glucose It is basically a Na dependent active process linked with the transport of Na As the Na diffuses along the concentration gradient the amino acid also enters the intestinal cell Both Na and amino acids share a common carrier and are transported together Meister cycle Another transport system to explain the mechanism of amino acid transfer across membrane in the intestine and kidney has been put forth gamma glutamylcysteinylglycine This is known as gamma glutamyl cycle or Meister cycle and involves a tripeptide namely glutathione Three ATP are utilized for the transport of a single amino acid by this cycle For this reason Meister cycle is not a common transport system for amino acid However this cycle is operative for rapid transport of cysteine and glutamine The gamma glutamyl cycle appears to be important for the metabolism of glutathione Clinical application The allergy to certain food proteins milk fish is believed to result from absorption of partially digested proteins Defects in the intestinal amino acid transport systems are seen in inborn errors of metabolism such as Hartnup s disease It is characterized by the inability of intestinal and renal epithelial cells to absorb neutral amino acids tryptophan alanine serine threonine valine Tryptophan absorption is most severely affected with a result that typical symptoms of pellagra are observed in the patients of Hartnup s disease Partial gastrectomy pancreatitis carcinoma of pancreas and cystic fibrosis may affect the digestion and absorption of proteins Protein losing enteropathy There is an excessive loss of serum proteins through the gastrointestinal tract