Terms: acid, amino acid, basolateral, di- or tripeptides, cytosolic, pepsin lumi

Question

Terms: acid, amino acid, basolateral, di- or tripeptides, cytosolic, pepsin luminal peptidase, endopeptidase, enterokinase, -inogen, internal, lumen, pancreas, pepsinogen, pepsin, pro-, stomach, zymogens Proteins (i.e. ____________ polymers that contain > 100 monomer subunits) and peptides (i.e. similar polymers but containing < or = 100 monomer subunits) of varying sizes are consumed in the diet. However, with rare exception, they cannot be absorbed intact across the wall of the human GI tract. Rather, they must be broken down to individual amino acids or into _________________ to be absorbed effectively across the wall of the small intestine. Because there are no protein-digesting enzymes in saliva, enzymatic modification of these macronutrient molecules doesn't begin until they reach the ____________. Specialized cells in the gastric mucosa (i.e. stomach lining) secrete an enzyme precursor called ______________ that undergoes a spontaneous cleavage reaction to generate the protein-digesting enzyme known as ____________. ________ production by parietal cells of the gastric lining promotes more rapid pepsinogen cleavage and pepsin generation. Pepsin, once formed, functions as an _____________ - meaning that it cleaves ____________ peptide linkages, which converts larger proteins and peptides into smaller proteins and peptides., but generates few, if any absorpbable end products in the stomach. So, the products produced by pepsin action in the stomach generally require further modification in the small intestine before they are finally converted to absorbable end products. Spell check Spell check Spell check Spell check Spell check Spell check Spell check Spell check The exocrine _____________ secretes a highly alkaline fluid into the duodenum that contains many digestive enzymes or enzyme precursors that help to complete the digestion of all macronutrients, including proteins, in the small intestine. Among these are chymotrypsinogen, trypsinogen, proelastase, and procarboxypeptidases A and B. The suffix _____________ at the end of the names of the first two substances listed and the prefix ______ at the beginning of the names of the latter 3 substances listed indicate that all of these pancreatic products are inactive enzyme precursors - or ____________ that, in their original form have no digestive enzyme activity, but that can be modified to become active digestive enzymes. However, they are not activated in the pancreas itself. Rather, they are sent as enzyme precursors into the small intestine. In the small intestine, the intestinal brush border enzyme known as ____________ cleaves trypsinogen to its active form - called trypsin. Trypsin, once formed, then drives more trypsinogen cleavage to form more trypsin and also cleaves and activates all the other pancreatic zymogens. These enzymes work in the ___________ of the small intestine to convert the end products of pepsin action and any remaining undigested proteins/peptides to small oligopeptides that contain between 3 - 10 amino acid subunits. Spell check Spell check Spell check Spell check Spell check Spell check In the small intestine, as was the case for carbohydrate digestion, there are also a number of intestinal brush border enzymes attached to the villous epithelium that function in the final digestion of proteins. These brush border peptidases continue to cleave the peptide linkages in the small oligopeptides that had been generated by the luminal actions of pepsins in the stomach and/or the luminal actions of activated pancreatic digestive enzymes present in small intestine lumen until the amino acid polymers reach the absorbable di- or tripeptide sizes or are cleaved as individual amino acids. These final absorbable end products are then taken up from the intestinal lumen with the aid of specialized transport proteins. However, while some di- and tripeptides can be absorbed from the lumen of the small intestine into the enterocytes lining the small intestine, they cannot be released from these cells into the interstitium and made available to the body in this form. Rather, they must be broken down to individual amino acids within the enterocytes by the actions of __________ peptidase enzymes in these cells. Then, specialized transport molecules facilitate diffusion of the individual amino acids across the ____________ cell membranes and into the interstitial fluids that bath the gut. Spell check Spell check

Explanation / Answer

Terms:

Proteins polymers that contain > 100 monomer subunits) and peptides (i.e. similar polymers but containing < or = 100 monomer subunits) of varying sizes are consumed in the diet.

However, with rare exception, they cannot be absorbed intact across the wall of the human GI tract. Rather, they must be broken down to individual amino acids or into di- or tripeptides to be absorbed effectively across the wall of the small intestine.

Because there are no protein-digesting enzymes in saliva, enzymatic modification of these macronutrient molecules doesn't begin until they reach the Stomach. Specialized cells in the gastric mucosa (i.e. stomach lining) secrete an enzyme precursor called pepsinogen that undergoes a spontaneous cleavage reaction to generate the protein-digesting enzyme known as Pepsin. Enterokinase production by parietal cells of the gastric lining promotes more rapid pepsinogen cleavage and pepsin generation.

Pepsin, once formed, functions as an –inogen meaning that it cleaves internal peptide linkages, which converts larger proteins and peptides into smaller proteins and peptides, but generates few, if any absorbable end products in the stomach. So, the products produced by pepsin action in the stomach generally require further modification in the small intestine before they are finally converted to absorbable end products.

The exocrine pancreas secretes a highly alkaline fluid into the duodenum that contains many digestive enzymes or enzyme precursors that help to complete the digestion of all macronutrients, including proteins, in the small intestine.

Among these are chymotrypsinogen, trypsinogen, proelastase, and procarboxypeptidases A and B. The suffix inogen at the end of the names of the first two substances listed and the prefix Pro at the beginning of the names of the latter 3 substances listed indicate that all of these pancreatic products are inactive enzyme precursors - or Zymogens that, in their original form have no digestive enzyme activity, but that can be modified to become active digestive enzymes.

However, they are not activated in the pancreas itself. Rather, they are sent as enzyme precursors into the small intestine. In the small intestine, the intestinal brush border enzyme known as pepsin luminal peptidase cleaves trypsinogen to its active form - called trypsin.

Trypsin, once formed, then drives more trypsinogen cleavage to form more trypsin and also cleaves and activates all the other pancreatic zymogens. These enzymes work in the Lumen of the small intestine to convert the end products of pepsin action and any remaining undigested proteins/peptides to small oligopeptides that contain between 3 - 10 amino acid subunits.

In the small intestine, as was the case for carbohydrate digestion, there are also a number of intestinal brush border enzymes attached to the villous epithelium that function in the final digestion of proteins.

These brush border peptidases continue to cleave the peptide linkages in the small oligopeptides that had been generated by the luminal actions of pepsins in the stomach and/or the luminal actions of activated pancreatic digestive enzymes present in small intestine lumen until the amino acid polymers reach the absorbable di- or tripeptide sizes or are cleaved as individual amino acids.

These final absorbable end products are then taken up from the intestinal lumen with the aid of specialized transport proteins. However, while some di- and tripeptides can be absorbed from the lumen of the small intestine into the enterocytes lining the small intestine, they cannot be released from these cells into the interstitium and made available to the body in this form.

Rather, they must be broken down to individual amino acids within the enterocytes by the actions of endopeptidase peptidase enzymes in these cells. Then, specialized transport molecules facilitate diffusion of the individual amino acids across the internal cell membranes and into the interstitial fluids that bath the gut.

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