Proteins

Proteins Digestion and absorption

ProteinsMost of the nitrogen in the diet is consumed in the form of protein, typically amounting to 70-100g/day in the American diet. Proteins are generally too large to be absorbed by the intestine. An example of an exception to this rule is that new borns can take up maternal antibodies in breast milk. They must therefore be hydrolyzed to yeild their constituent amino acids, which can be absorbed.. 1ProteinsProteolytic enzymes responsible for degrading proteins are produced by three different organs, the stomach, the pancreas and the small intestine2Proteins3Proteolytic enzymesExopeptidases Polypeptidases AminopeptidasescarboxypeptidasesTripeptidasesDipeptidases Endopeptidases PepsinTrypsinChymotrysinElastase ProteinsEndopeptidasesPolypeptidasesAminopeptidasesCarboxypeptidases Tripeptidases Dipeptidases

ExopeptidasesPepsinTrypsin Chymotrypsin elastase4Stomach

H+ProteinsThe digestion of proteins begins in the stomach, which secretes gastric juice----a unique solution containing hydrochloric acid and the proenzyme, pepsinogen.HCl: stomach acid is too dilute (pH 2-3) to hydrolyze proteins. The acid functions instead to kill some bacteria and to denature proteins, thus making them more susceptible to subsequent hydrolysis by proteases. Pepsin: this acid stable endopeptidase is secreted by the serous cells of the stomach as an inactive zymogen (or proenzyme), pepsinogen. In general, zymogens contain extra amino acids in their sequences which prevent them from being catalytically active. [note: removal of these amino acids permits the proper folding required for an active enzyme.] pepsinogen is activated to pepsin, either by HCl, or autocatalitically by other pepsin molecules that have already been activated . Pepsin releases peptides and a few free amino acids from dietary proteins.

AcidicAromatic

Pepsin cuts before Leu, Phe, Trp or Tyr, unless preceded by Pro. Also others, quite nonspecific; works best at pH 2.5Exocrine pancreas CCKZymogens Secretin ProteinsOn entering the small intestine, large polypeptides produced in the stomach by the action of pepsin are further cleaved to oligopeptides and amino acids by a group of pancreatic proteases.Release of zymogens:The release and activation of the pancreatic zymogens is mediated by the secretion of cholecystokinin and secretin, two polypeptide hormones of the digestive tract.

The small intestine is periodically assaulted by a flood of acid from the stomach, and it is important to put out that fire in a hurry to avoid acid burns. Secretin functions as a type of fireman: it is released in response to acid in the small intestine, and stimulates the pancreas and bile ducts to release a flood of bicarbonate base, which neutralizes the acid. Secretin is also of some historical interest, as it was the first hormone to be discovered.Cholecystokinin plays a key role in facilitating digestion within the small intestine. It is secreted from mucosal epithelial cells in the first segment of the small intestine (duodenum), and stimulates delivery into the small intestine of digestive enzymes from the pancreas and bile from the gallbladder. Cholecystokinin is also produced by neurons in the enteric nervous system, and is widely and abundantly distributed in the brain.The most potent stimuli for secretion of cholecystokinin are the presence of partially-digested fats and proteins in the lumen of the duodenum (a particularly potent stimulus is pictured above). An elevation in blood concentration of cholecystokinin has two major effects that facilitate digestion: Release of digestive enzymes from the pancreas into the duodenum. Older literature refers to cholecystokinin as pancreozymin, a term coined to describe this effect.Contraction of the gallbladder to deliver bile into the duodenum. The name cholecystokinin (to "move the gallbladder") was given to describe this effect. Cholecystokinin is also known to stimulate secretion of bile salts into the biliary system

6Activation of pancreatic zymogens Carboxypeptidase ProteinsActivation of zymogens:Enteropeptidase (formerly called enterokinase) ---an enzyme synthesized by and present on the luminal surface of the intestinal mucosal cells of the brush border membrane---converts the pancreatic zymogen trypsinogen to trypsin by removal of a hexapeptide from the amino terminus of trypsinogen. Trypsin subsequently converts other trypsinogen molecules to trypsin by cleaving a limited number of specific peptide bonds in the zymogen. Enteropeptidase thus unleashes a cascade of proteolytic activity, because trypsin is the common activator of all the pancreatic zymogens. TRYPSINOGEN ENTEROKINASE TRYPSIN

CHYMOTYPSINOGEN CHYMOTRYPSINPROELASTASE ELASTASEPROCARBOXYPEPTIDASE CARBOXYPEPTIDASE

7Trypsin AriginineLysine Dr Fatahiya Kashif

ProteinsProteinsArginine Lysine

Trypsin: cuts after Arg or Lys, unless followed by Pro. Very strict. Works best at pH 8.

Specificity: Each of these enzymes has a different specificity for the amino acid R groups adjacent to the susceptible peptide bond. For example trypsin cleaves only when the carbonyl group of the peptide bond is contributed by arginine or lysine. These enzymes, like pepsin are synthesized and secreted as inactive zymogens.

8Chymotrypsin Acidic Hydrophobic

ProteinsProteinsAcidicHydrophobic

Chmotrypsin 9Elastase Alanine Glycine Serine

ProteinsProteinsAlanineGlycine Serine

Elastase cuts after Ala, Gly, Ser, or Val, unless followed by Pro10Carboxypeptidase Hydrophobic Basic

ProteinsProteinsA.HydrophobicB.Basic

Carboxypeptidase, which is a digestive enzyme present in pancreatic juice, will cleave a single amino acid from the carboxylic end of the peptide.

11Intestinal enzymes Exopeptidases Brush border Within the cell Enterokinase ProteinsProteinsEXOPEPTIDASESBRUSH BORDERWITHIN THE CELLENTEROKINASE

Digestion of oligopeptides by enzymes of the small intestineThe luminal surface of the intestine contains aminopeptidase an exopeptidase that repeatedly cleaves the N terminal residue from oligopeptides to produce free amino acids and smaller peptides.12Aminopeptidase

ProteinsAND ALLAH DOTH INCREASE IN GUIDANCE THOSE WHO SEEK GUIDANCE; AND THE THINGS THAT ENDURE, GOOD DEEDS, ARE BEST IN THE SIGHT OF THY LORD, AS REWARDS, AND THE BEST IN RESPECT OF EVENTUAL RETURNS.

76-MARYAM; AL-QURANProteinsProteinsAbsorption of amino acids Proteins15Transport systems Free amino acidsSodium linked secondary transportDi and tri peptidesH+ linked transport system

Portal veinLiverGeneral circulationProteinsProteinsNa DEPENDENTNEUTRALPROLINE HYDROXYPROLINEACIDICBASIC AND CYSTINE

Na INDEPENDENTFACILITATED TRANSPORT

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Na+

3Na+2K+ProteinsFree amino acids are taken into enterocytes up by a Na linked secondary transport system. Di and tri peptides, however, are taken up by a H+ linked transport system. There the peptides are hydrolyzed in the cytosol to amino acids before being released into the portal system. Thus, only free amino acids are found in the portal vein after a meal containing protein. These amino acids are either metabolized by the liver or released into the general circulation. Branched chain amino acids are important examples of amino acids that are not metabolized by the liver, but instead are sent from the liver into the blood.

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Na+

Proteins

Na+

Proteins

Na+

Proteins

Na+

Proteins

Na+

ProteinsGamma glutamyl cycle Glu Cys Gly Glu Proteins23In cells of the intestine and kidney, amino acids can be transported across the cell membrane by reacting with glutathione ( glutamyl-cysteinyl-glycine) to form a glutamyl amino acid. The amino acid is released into the cell, and glutathione is resynthesized. However the major role of this cycle is glutathione synthesis, and many tissues lack the transpeptidase and 5-oxoprolinase activities. The extracellular amino acid reacts with glutathione in a reaction catalyzed by transpeptidase present in the cell membrane.A glutamyl amino acid is formed, which travels across the cell membrane and releases the amino acid into the cell.The other products of these two reactions are reconverted to glutathione.The reactions converting glutamate to glutathione in the glutamyl cycle are the same reactions required for the synthesis of glutathione. The enzymes for glutathione synthesis, but not the transpeptidase, are found in most tissues. The oxoprolinase is also missing from many tissues, such that the major role of this pathway is one of glutathione synthesis from glutamate, cysteine and glycine. The transpeptidase is the only protease in the cell that can break the glutamyl linkage in glutathione. Glutathione is also involved in reducing compounds such as hydrogen peroxide. It also protects cells, in particular erythrocytes, from oxidative damage, through formation of oxidized glutathione, two glutathione residues connected by a s-s bond.Gamma glutamyl cycle Cys Gly Glu 5-oxoproline Proteins24In cells of the intestine and kidney, amino acids can be transported across the cell membrane by reacting with glutathione ( glutamyl-cysteinyl-glycine) to form a glutamyl amino acid. The amino acid is released into the cell, and glutathione is resynthesized. However the major role of this cycle is glutathione synthesis, and many tissues lack the transpeptidase and 5-oxoprolinase activities. The extracellular amino acid reacts with glutathione in a reaction catalyzed by transpeptidase present in the cell membrane.A glutamyl amino acid is formed, which travels across the cell membrane and releases the amino acid into the cell.The other products of these two reactions are reconverted to glutathione.The reactions converting glutamate to glutathione in the glutamyl cycle are the same reactions required for the synthesis of glutathione. The enzymes for glutathione synthesis, but not the transpeptidase, are found in most tissues. The oxoprolinase is also missing from many tissues, such that the major role of this pathway is one of glutathione synthesis from glutamate, cysteine and glycine. The transpeptidase is the only protease in the cell that can break the glutamyl linkage in glutathione. Glutathione is also involved in reducing compounds such as hydrogen peroxide. It also protects cells, in particular erythrocytes, from oxidative damage, through formation of oxidized glutathione, two glutathione residues connected by a s-s bond.Gamma glutamyl cycle Cys Gly 5-oxoproline ATPADPGluProteins25In cells of the intestine and kidney, amino acids can be transported across the cell membrane by reacting with glutathione ( glutamyl-cysteinyl-glycine) to form a glutamyl amino acid. The amino acid is released into the cell, and glutathione is resynthesized. However the major role of this cycle is glutathione synthesis, and many tissues lack the transpeptidase and 5-oxoprolinase activities. The extracellular amino acid reacts with glutathione in a reaction catalyzed by transpeptidase present in the cell membrane.A glutamyl amino acid is formed, which travels across the cell membrane and releases the amino acid into the cell.The other products of these two reactions are reconverted to glutathione.The reactions converting glutamate to glutathione in the glutamyl cycle are the same reactions required for the synthesis of glutathione. The enzymes for glutathione synthesis, but not the transpeptidase, are found in most tissues. The oxoprolinase is also missing from many tissues, such that the major role of this pathway is one of glutathione synthesis from glutamate, cysteine and glycine. The transpeptidase is the only protease in the cell that can break the glutamyl linkage in glutathione. Glutathione is also involved in reducing compounds such as hydrogen peroxide. It also protects cells, in particular erythrocytes, from oxidative damage, through formation of oxidized glutathione, two glutathione residues connected by a s-s bond.Gamma glutamyl cycle Cys GlyATPADPGluProteins26In cells of the intestine and kidney, amino acids can be transported across the cell membrane by reacting with glutathione ( glutamyl-cysteinyl-glycine) to form a glutamyl amino acid. The amino acid is released into the cell, and glutathione is resynthesized. However the major role of this cycle is glutathione synthesis, and many tissues lack the transpeptidase and 5-oxoprolinase activities. The extracellular amino acid reacts with glutathione in a reaction catalyzed by transpeptidase present in the cell membrane.A glutamyl amino acid is formed, which travels across the cell membrane and releases the amino acid into the cell.The other products of these two reactions are reconverted to glutathione.The reactions converting glutamate to glutathione in the glutamyl cycle are the same reactions required for the synthesis of glutathione. The enzymes for glutathione synthesis, but not the transpeptidase, are found in most tissues. The oxoprolinase is also missing from many tissues, such that the major role of this pathway is one of glutathione synthesis from glutamate, cysteine and glycine. The transpeptidase is the only protease in the cell that can break the glutamyl linkage in glutathione. Glutathione is also involved in reducing compounds such as hydrogen peroxide. It also protects cells, in particular erythrocytes, from oxidative damage, through formation of oxidized glutathione, two glutathione residues connected by a s-s bond.Gamma glutamyl cycleGlyATPADP Glu Cys Proteins27In cells of the intestine and kidney, amino acids can be transported across the cell membrane by reacting with glutathione ( glutamyl-cysteinyl-glycine) to form a glutamyl amino acid. The amino acid is released into the cell, and glutathione is resynthesized. However the major role of this cycle is glutathione synthesis, and many tissues lack the transpeptidase and 5-oxoprolinase activities. The extracellular amino acid reacts with glutathione in a reaction catalyzed by transpeptidase present in the cell membrane.A glutamyl amino acid is formed, which travels across the cell membrane and releases the amino acid into the cell.The other products of these two reactions are reconverted to glutathione.The reactions converting glutamate to glutathione in the glutamyl cycle are the same reactions required for the synthesis of glutathione. The enzymes for glutathione synthesis, but not the transpeptidase, are found in most tissues. The oxoprolinase is also missing from many tissues, such that the major role of this pathway is one of glutathione synthesis from glutamate, cysteine and glycine. The transpeptidase is the only protease in the cell that can break the glutamyl linkage in glutathione. Glutathione is also involved in reducing compounds such as hydrogen peroxide. It also protects cells, in particular erythrocytes, from oxidative damage, through formation of oxidized glutathione, two glutathione residues connected by a s-s bond.Gamma glutamyl cycle Glu Cys Gly Cys Gly Glu Proteins28In cells of the intestine and kidney, amino acids can be transported across the cell membrane by reacting with glutathione ( glutamyl-cysteinyl-glycine) to form a glutamyl amino acid. The amino acid is released into the cell, and glutathione is resynthesized. However the major role of this cycle is glutathione synthesis, and many tissues lack the transpeptidase and 5-oxoprolinase activities. The extracellular amino acid reacts with glutathione in a reaction catalyzed by transpeptidase present in the cell membrane.A glutamyl amino acid is formed, which travels across the cell membrane and releases the amino acid into the cell.The other products of these two reactions are reconverted to glutathione.The reactions converting glutamate to glutathione in the glutamyl cycle are the same reactions required for the synthesis of glutathione. The enzymes for glutathione synthesis, but not the transpeptidase, are found in most tissues. The oxoprolinase is also missing from many tissues, such that the major role of this pathway is one of glutathione synthesis from glutamate, cysteine and glycine. The transpeptidase is the only protease in the cell that can break the glutamyl linkage in glutathione. Glutathione is also involved in reducing compounds such as hydrogen peroxide. It also protects cells, in particular erythrocytes, from oxidative damage, through formation of oxidized glutathione, two glutathione residues connected by a s-s bond.GLUTATHIONE GSH GSH GSSGProteins29GLUTATHIONE is important inDetoxification of free radicals and peroxides formed within the cellMaintenance of reduced states of sulfhydral groups in proteins, including hemoglobin (oxidation of those sulfhydral groups leads to the formation of denatured proteins that form insoluble masses called Heinz bodies that attach to the red cell membranes. Additional oxidation of the membrane proteins causes the red cells to be rigid and nondeformable and they are removed from the circulation by macrophages in the spleen and liver.Reduced glutathione is itself regenerated by NADPH In RBC the only source of NADPH is the HMP shuntIn other tissues, NADPH is also generated by NADP-dependent malate dehydrogenase/malic enzyme, and extramitochondrial isocitrate dehydrogenase (more important in ruminants).Digestion and absorption of proteinsDisorders ProteinsDisorders Cystic fibrosis

Cystinuria

Hartnup disease

KwashiorkorProteinsProteinsAbnormalities in protein digestion:In individuals with a deficiency in pancreatic secretion (for example, due to chronic pancreatitis, cystic fibrosis or surgical removal of the pancreas), the digestion and absorption of fat and protein is incomplete. This results in an abnormal appearance of lipids (called steatorrhea) and undigested protein in the feces.31Cystic Fibrosis

ProteinsCystic Fibrosis Defect in function of chloride channels in pancreatic secretory ductsInspissation of pancreatic exocrine secretionsEventual obstruction of ductsProteinsProteinsCystic FibrosisRecurrent infections

Poor growth

Foul smelling bulky stools

ProteinsProteinsCystic Fibrosis Commercial enzyme preparations ProteinsProteinsHartnup diseaseAutosomal recessive disorder

Transport of neutral aminoacids across both intestinal and renal epithilial cellsDr Fatahiya KashifProteinsProteinsMomi momi boho bohoLa la la la lav36Hartnup disease Essential IsoleucineLeucinePhenylanalineThreonine TryptophanValine

Nonessential Alanine SerineTyrosine ProteinsISOLEUCINELEUCINE PHENYLALANINETHREONINETRYPTOPHANVALINE

NONESSENTIALALANINESERINETYROSINE

37Hartnup disease C/FAsymptomaticPallegra like symptomsNeuropsychiatric

TreatmentNiacin

ProteinsProteinsC/FASYMPTOMATICPALLEGRA LIKE SYMPTOMSNEUROPSYCHIATRIC

TREATMENTNIACIN38Cystinuria Defect in absorption ofCystineOrnithineArginineLysineC/FAppearance of these amino acids in urineCystine stones Treatment Oral hydration

ProteinsProteinsDEFECT IN ABSORPTION OFCYSTINEORNITHINEARGININE LYSINEC/FAPPEARANCE OF THESE AA IN URINECYSTINE STONESTREATMENTORAL HYDRATION

39Kwashiorkor Deficiency of protein in diet which is adequate in calories

C/FMuscle wastingEdema

ProteinsProteinsCAUSEDEFICIENCY OF PROTEIN IN DIET WHICH IS ADEQUATE IN CALORIESCLINICAL FEATURESMUSCLE WASTINGEDEMA

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