digestion

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Digestion and absorption of Proteins 2

Digestion is the disintegration of complex nutrients into simple, soluble and assimilable form. Most of the nitrogen in the diet is consumed in the form of proteins. Proteins are too large to be absorbed. The dietary proteins are hydrolyzed to amino acids by proteolytic enzymes, which can be easily absorbed. Proteolytic enzymes responsible for degrading proteins are produced by three different organs; The stomach, pancreas and the small intestine. :

Digestion is the disintegration of complex nutrients into simple, soluble and assimilable form. Most of the nitrogen in the diet is consumed in the form of proteins. Proteins are too large to be absorbed. The dietary proteins are hydrolyzed to amino acids by proteolytic enzymes, which can be easily absorbed. Proteolytic enzymes responsible for degrading proteins are produced by three different organs; The stomach, pancreas and the small intestine. 3 Digestion of dietary proteins

1)The y are hydrolases. 2)Secreted in the zymogen (Inactive) form, converted to active form in the intestinal lumen. The active site of the enzyme is masked by a small region of the peptide chain that is removed by hydrolysis of a specific peptide bond. 3) They may be Exopeptidases or Endopeptidases. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of peptides. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments. :

1)The y are hydrolases. 2)Secreted in the zymogen (Inactive) form , converted to active form in the intestinal lumen. The active site of the enzyme is masked by a small region of the peptide chain that is removed by hydrolysis of a specific peptide bond. 3) They may be Exopeptidases or Endopeptidases. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of peptides. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments. Characteristics of proteolytic enzymes 4

There are no proteolytic enzymes present in the saliva. The function of the saliva is to lubricate the food, this helps in making food soluble for the action of proteolytic enzymes. After mastication and chewing, the bolus of food enters stomach where it is acted upon by gastric juice. :

There are no proteolytic enzymes present in the saliva . The function of the saliva is to lubricate the food, this helps in making food soluble for the action of proteolytic enzymes. After mastication and chewing, the bolus of food enters stomach where it is acted upon by gastric juice. Digestion in the oral cavity 5

Gastric juice contains HCL and 4 proteolytic enzymes: Pepsin, Rennin, Gastriscin and Gelatinase. Functions of HCL -Too dilute to hydrolyze but can cause denaturation of dietary proteins -Required for activation of inactive proteolytic enzymes secreted by the gastric mucosa. -Has a strong bacteriostatic action. :

Gastric juice contains HCL and 4 proteolytic enzymes: Pepsin, Rennin, Gastriscin and Gelatinase. Functions of HCL - Too dilute to hydrolyze but can cause denaturation of dietary proteins -Required for activation of inactive proteolytic enzymes secreted by the gastric mucosa. -Has a strong bacteriostatic action. Digestion by gastric juice 6

-Secreted in the form of pepsinogen, activated by HCL and pepsin itself (Autocatalysis) -Acid stable endopeptidase, optimum pH lies between 1.6-2.5 ; becomes inactive if the pH is > 5.0 -Active for a peptide bond where amino group is contributed by aromatic amino acids like Phenyl Alanine, Tyrosine and Tryptophan. -Has a strong action on milk protein casein. -Hydrolyzes proteins to proteoses, peptones and small peptides. :

-Secreted in the form of pepsinogen, activated by HCL and pepsin itself (Autocatalysis) - Acid stable endopeptidase, optimum pH lies between 1.6-2.5 ; becomes inactive if the pH is > 5.0 - Active for a peptide bond where amino group is contributed by aromatic amino acids like Phenyl Alanine, Tyrosine and Tryptophan. -Has a strong action on milk protein casein . -Hydrolyzes proteins to proteoses, peptones and small peptides. Functions of Proteolytic enzymes Pepsin 7

-Present in infants but absent in adult gastric juice. - Secreted as pro-rennin -Optimum p H 4.0 -Action is similar to pepsin -Acts on casein of milk and is involved in the curdling of milk -Commercially, ‘rennet’ tablets are used in the making of cheese. :

- Present in infants but absent in adult gastric juice. - Secreted as pro-rennin -Optimum p H 4.0 -Action is similar to pepsin -Acts on casein of milk and is involved in the curdling of milk -Commercially , ‘rennet’ tablets are used in the making of cheese. Functions of proteolytic enzymes Rennin 8

-Both are secreted in zymogen form and are acid-stable. Activated in the presence of HCL. -Gelatinase acts on Gelatin to hydrolyze it to short peptides. :

-Both are secreted in zymogen form and are acid-stable. Activated in the presence of HCL. -Gelatinase acts on Gelatin to hydrolyze it to short peptides. Functions of proteolytic enzymes Gastriscin and Gelatinase 9

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Digestion by gastric juice- Over view 10

The bolus of food after leaving stomach reaches duodenum and is acted upon by the pancreatic juice. All the enzymes are active only in the alkaline medium and alkalinity is provided by Bile juice and bicarbonates present in pancreatic juice. The proteolytic enzymes present in pancreatic juice are: -Trypsin -Chymotrypsin -Elastase -Collagenase - Carboxypeptidases :

The bolus of food after leaving stomach reaches duodenum and is acted upon by the pancreatic juice. All the enzymes are active only in the alkaline medium and alkalinity is provided by Bile juice and bicarbonates present in pancreatic juice. The proteolytic enzymes present in pancreatic juice are: -Trypsin -Chymotrypsin -Elastase -Collagenase - Carboxypeptidases Digestion by Pancreatic Juice 11

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Role of Trypsin 12 -Secreted in the zymogen form (Trypsinogen) and activation is brought about by Enterokinase (secreted by the intestinal mucosa ) and by Trypsin itself. Trypsin is specific for cleaving peptide bonds contributed by basic amino acids. Required for activation of Chymotrypsin Required for conversion of pro- elastase to elastase It is required for conversion of fibrinogen to fibrin It has weak action on casein

-Activated by Trypsin and Chymotrypsin itself (auto catalytically ) -Cleaves peptide bonds contributed by aromatic amino acids. - Can hydrolyze milk protein. :

- Activated by Trypsin and Chymotrypsin itself (auto catalytically ) -Cleaves peptide bonds contributed by aromatic amino acids. - Can hydrolyze milk protein. Role of Chymotrypsin 13

Two types of Carboxypeptidase are there Carboxypeptidase A:Metalloenzyme , contains Zinc, Activation brought about by Trypsin and auto catalytically by itself, Exopeptidase, Hydrolyzes the terminal peptide bond connected to an end amino acid (Aromatic) bearing free carboxyl group Carboxypeptidase B: Also an Exopeptidase, hydrolyzes terminal peptide bonds connected by basic amino acids :

Two types of Carboxypeptidase are there Carboxypeptidase A :Metalloenzyme , contains Zinc, Activation brought about by Trypsin and auto catalytically by itself, Exopeptidase, Hydrolyzes the terminal peptide bond connected to an end amino acid (Aromatic) bearing free carboxyl group Carboxypeptidase B : Also an Exopeptidase, hydrolyzes terminal peptide bonds connected by basic amino acids Role of Carboxy-peptidases 14

Elastase- Activated by Trypsin Has maximum activity on peptide bonds contributed by carbonyl groups of neutral aliphatic amino acids- Alanine, Glycine, Serine etc. Collagenase- Acts on Collagen :

Elastase- Activated by Trypsin Has maximum activity on peptide bonds contributed by carbonyl groups of neutral aliphatic amino acids- Alanine, Glycine, Serine etc. Collagenase- Acts on Collagen Role of Elastase and Collagenase 15

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Enzymes present in intestinal juice are: -Enterokinase -Amino peptidases -Prolidase -Di and Tri peptidases :

Enzymes present in intestinal juice are: -Enterokinase -Amino peptidases -Prolidase -Di and Tri peptidases Digestion by intestinal Juice 17

Enterokinase - required for activation of Trypsin, also called Enteropeptidase, present in the epithelial cells of brush border of duodenal mucosa. Bile salts help in its liberation in to intestinal lumen. Amino peptidases – are exopeptidases, remove the amino acid one by one from amino terminal end of peptide chains. Prolidase - also an exopeptidase, removes proline residues from terminal end of peptide chains. :

Enterokinase - required for activation of Trypsin, also called Enteropeptidase, present in the epithelial cells of brush border of duodenal mucosa. Bile salts help in its liberation in to intestinal lumen. Amino peptidases – are exopeptidases , remove the amino acid one by one from amino terminal end of peptide chains. Prolidase - also an exopeptidase, removes proline residues from terminal end of peptide chains. Functions of the Intestinal enzymes 18

Di and Tripeptidases - Digestion of di and tri peptides is brought about by di and tri peptidases present in brush border membrane of epithelial cells as well as present in the interior of the cell. Tripeptidases convert tripeptides into a dipeptide and a free amino acid, then dipeptidases convert dipeptides to free amino acids. :

Di and Tripeptidases - Digestion of di and tri peptides is brought about by di and tri peptidases present in brush border membrane of epithelial cells as well as present in the interior of the cell. Tripeptidases convert tripeptides into a dipeptide and a free amino acid, then dipeptidases convert dipeptides to free amino acids. Functions of Intestinal enzyme(Contd.) 19

Acute Pancreatitis Premature activation of pancreatic proteolytic enzymes in the pancreas itself causes digestion of the secretory mucosa causing Acute pancreatitis. It is a life-threatening condition. :

Acute Pancreatitis Premature activation of pancreatic proteolytic enzymes in the pancreas itself causes digestion of the secretory mucosa causing Acute pancreatitis. It is a life-threatening condition. Diseases associated with digestion of proteins 20 Biochemistry for Medics

In conditions of deficient pancreatic secretions like cystic Fibrosis, chronic pancreatitis or surgical removal of pancreas, the digestion and absorption of fats and proteins is left incomplete with the resultant appearance of lipids and undigested proteins in the feces. This condition is called Steatorrhea :

In conditions of deficient pancreatic secretions like cystic Fibrosis, chronic pancreatitis or surgical removal of pancreas, the digestion and absorption of fats and proteins is left incomplete with the resultant appearance of lipids and undigested proteins in the feces. This condition is called Steatorrhea Steatorrhea 21

Proteins are completely digested to constituent amino acids. But some amounts of oligopeptides may remain undigested. The products of digestion are rapidly absorbed. Site of Absorption- Oligopeptides are absorbed from duodenum. and proximal Jejunum, while amino acids are absorbed from ileum and distal jejunum . :

Proteins are completely digested to constituent amino acids. But some amounts of oligopeptides may remain undigested. The products of digestion are rapidly absorbed. Site of Absorption- Oligopeptides are absorbed from duodenum. and proximal Jejunum, while amino acids are absorbed from ileum and distal jejunum . Absorption of amino acids 22

Mechanism of Absorption- The absorption takes place by active transport (same as that of glucose). Natural L-amino acids are actively transported. D- amino acids are absorbed by simple diffusion. Vitamin B6 is involved in the active transfer of amino acids. Energy requiring process, ATP is required as a source of energy A carrier protein is also required which may be Na+ dependent or independent. Different carrier proteins are there specific for different amino acids. :

Mechanism of Absorption- The absorption takes place by active transport (same as that of glucose). Natural L-amino acids are actively transported. D- amino acids are absorbed by simple diffusion. Vitamin B6 is involved in the active transfer of amino acids. Energy requiring process, ATP is required as a source of energy A carrier protein is also required which may be Na+ dependent or independent. Different carrier proteins are there specific for different amino acids. Absorption of amino acids 23 Biochemistry for Medics

Absorbed by active transport Intracellular peptidases hydrolyze them to amino acids Hydrolysis is rapid to keep peptide concentration low in the cell Transport mechanism is independent of L- amino acids :

Absorbed by active transport Intracellular peptidases hydrolyze them to amino acids Hydrolysis is rapid to keep peptide concentration low in the cell Transport mechanism is independent of L- amino acids Absorption of Oligopeptides 24 Biochemistry for Medics

Glutathione participates in the Active group translocation of L- amino acids in to the cells of small intestine, kidneys, seminal vesicles, epididymis and brain. A cyclic pathway, operates in which Glutathione is regenerated again, it is named as Gamma Glutamyl Cycle or Meister cycle( Based on the name of Scientist). :

Glutathione participates in the Active group translocation of L- amino acids in to the cells of small intestine, kidneys, seminal vesicles, epididymis and brain . A cyclic pathway, operates in which Glutathione is regenerated again, it is named as Gamma Glutamyl Cycle or Meister cycle( Based on the name of Scientist). Role of glutathione 25

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Meister cycle 26

Cystinuria-Common transporter for cystine, ornithine, arginine and lysine(COAL) is present in gut and renal tubules. Deficiency of transporter results in loss of these amino acids in the feces and urine. Hart- Nup Disease-There is deficiency of transporter for tryptophan and neutral amino acid. no absorption of tryptophan takes place ,tryptophan deficiency produce neurological and skin manifestation (pellagra-like rashes). :

Clinical Significance 27 Cystinuria- Common transporter for cystine, ornithine, arginine and lysine(COAL) is present in gut and renal tubules. Deficiency of transporter results in loss of these amino acids in the feces and urine. Hart- Nup Disease- There is deficiency of transporter for tryptophan and neutral amino acid. no absorption of tryptophan takes place ,tryptophan deficiency produce neurological and skin manifestation (pellagra-like rashes).

Food allergies are due to absorption of undigested peptides by pinocytosis. The immune response may be local or systemic . Examples- Gluten induced enteropathy Fish allergies or egg allergies Clinical Manifestations may be due to – local damage in the intestine in the form of nausea, vomiting, diarrhea, abdominal cramps: or Systemic manifestations - cutaneous rashes, running nose, facial flushing, broncho-constriction or anaphylactic shock in severe cases :

Food allergies are due to absorption of undigested peptides by pinocytosis. The immune response may be local or systemic . Examples- Gluten induced enteropathy Fish allergies or egg allergies Clinical Manifestations may be due to – local damage in the intestine in the form of nausea, vomiting, diarrhea, abdominal cramps: or Systemic manifestations - cutaneous rashes, running nose, facial flushing, broncho-constriction or anaphylactic shock in severe cases Food Allergies 28

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Summary 29

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