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Questions and Answers
Which of the following is a principal animal source of dietary protein?
Which of the following is a principal animal source of dietary protein?
What is the main reason that proteins are not digested in the mouth?
What is the main reason that proteins are not digested in the mouth?
Which enzyme acts on the N-terminal amino acids during protein digestion?
Which enzyme acts on the N-terminal amino acids during protein digestion?
What role does hydrochloric acid (HCl) play in protein digestion in the stomach?
What role does hydrochloric acid (HCl) play in protein digestion in the stomach?
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Which of the following enzymes is not produced in the stomach?
Which of the following enzymes is not produced in the stomach?
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How is pepsinogen converted to pepsin?
How is pepsinogen converted to pepsin?
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What is the typical range for daily intake of dietary protein?
What is the typical range for daily intake of dietary protein?
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Which of the following statements about peptide bonds is true?
Which of the following statements about peptide bonds is true?
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What is the optimal pH range for the activity of pepsin?
What is the optimal pH range for the activity of pepsin?
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What is the role of rennin (chymosin) in digestion?
What is the role of rennin (chymosin) in digestion?
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How is trypsin activated from its zymogen form?
How is trypsin activated from its zymogen form?
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Which of the following statements about pepsin is incorrect?
Which of the following statements about pepsin is incorrect?
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What effect does cholecystokinin have on the digestive process?
What effect does cholecystokinin have on the digestive process?
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What products result from the action of pepsin on proteins?
What products result from the action of pepsin on proteins?
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What is the molecular weight of pepsin?
What is the molecular weight of pepsin?
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What is the primary function of gelatinase in gastric juice?
What is the primary function of gelatinase in gastric juice?
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What is the primary function of enterokinase in the digestive process?
What is the primary function of enterokinase in the digestive process?
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Which of the following enzymes can only hydrolyze peptide bonds connected to proline residues?
Which of the following enzymes can only hydrolyze peptide bonds connected to proline residues?
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Which factor is necessary for amino peptidases to effectively catalyze peptide hydrolysis?
Which factor is necessary for amino peptidases to effectively catalyze peptide hydrolysis?
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Where does the absorption of released amino acids primarily occur in the digestive system?
Where does the absorption of released amino acids primarily occur in the digestive system?
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What distinguishes tri-peptidase from di-peptidase in their enzymatic activity?
What distinguishes tri-peptidase from di-peptidase in their enzymatic activity?
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What characteristic defines the specificity of proteolytic enzymes during protein digestion?
What characteristic defines the specificity of proteolytic enzymes during protein digestion?
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What is a significant outcome of inadequate proteolytic enzyme activity during digestion?
What is a significant outcome of inadequate proteolytic enzyme activity during digestion?
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Which condition is necessary for the activation of trypsinogen into trypsin?
Which condition is necessary for the activation of trypsinogen into trypsin?
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Study Notes
Dietary Proteins
- Dietary proteins come from animal or vegetable sources
- Principal animal sources include milk, dairy products, meat, fish, liver, and eggs.
- Principal vegetable sources include cereals, pulses, peas, beans, and nuts.
- Dietary protein intake is roughly 50-100 g/day.
- Endogenous protein (digestive enzymes and worn-out cells) is about 28 g daily.
- Protein loss through feces is minimal, approximately 5-10 g/day.
Digestion in the Mouth
- No proteolytic enzymes are present in saliva.
- Chewing breaks down food into smaller pieces ("bolus").
- The bolus moves to the stomach to interact with gastric juice.
Protein Degradation
- Proteins are broken down by hydrolases.
- Hydrolases are enzymes classified as peptidases, cleaving peptide bonds.
- Endopeptidases (proteases) attack internal peptide bonds, and produce fragments
- Exopeptidases act on terminal amino acids
Digestion in the Stomach
- Gastric juice contains proteolytic enzymes (e.g., pepsin, renin, gastricin).
- Hydrochloric acid (HCl) is also present in gastric juice.
- HCl lowers stomach pH to less than 2
- HCl denatures proteins, making them more susceptible to proteases
- Pepsin works optimally in highly acidic conditions (pH 1.6-2.5)
- Pepsin is a proteinase that hydrolyses peptide bonds, forming smaller fragments
- Rennin is present in infants and children; converting milk protein casein to paracaseinate, which pepsin digests more easily.
- Gastricin is a protease active at a pH of 3 to 4.
- Gelatinase hydrolyzes gelatin in an acidic medium
Digestion in the Duodenum
- Food from the stomach enters the duodenum, where it mixes with pancreatic juice.
- Pancreatic juice contains proteases that are secreted as proenzymes (zymogens).
- Cholecystokinin and secretin stimulate release of pancreatic juice.
- Pancreatic proteases include trypsin, chymotrypsin, carboxypeptidases, and elastases.
- Trypsin, a proteinase, is secreted as trypsinogen (inactive form).
- Enterokinase (enteropeptidase) activate trypsinogen to trypsin.
- Trypsin is essential for activating other pancreatic proenzymes.
Digestion in the Small Intestine
- Intestinal juice contains proteolytic enzymes (enterokinase, prolidase, aminopeptidases, dipeptidases, tripeptidases).
- Enterokinase converts trypsinogen to trypsin.
- Prolidase acts on proline containing peptides.
- Aminopeptidases, dipeptidases, and tripeptidases help break down peptides into smaller fragments.
Absorption of Amino Acids
- Amino acids are absorbed into the bloodstream from the small intestine's ileum and distal jejunum.
- Some oligopeptides (dipeptides and tripeptides) are also absorbed, but then further broken down.
- L-amino acids and L-peptides are absorbed faster than their D-isomers.
- They typically enter the bloodstream passively via special transporters.
How amino acids reach the liver
- Amino acids and digested products are absorbed from the intestines.
- They are carried by the portal vein to the liver for further use in the body.
Overview of Protein Digestion
- Food proteins enter the digestive tract.
- Pepsin in the stomach breaks down proteins into smaller polypeptides.
- In the small intestine, proteases and other enzymes continue breaking them down into amino acids.
- Small fragments of peptides are also broken down into amino acids.
- Amino acids are absorbed into the bloodstream and used by the body.
Enzyme Specificity
- Enzymes have specificity, cleaving only certain peptide bonds.
- Pepsin is active at low pHs, preferring peptide bonds with aromatic amino acids.
- Trypsin prefers peptide bonds with Lys or Arg at the carboxyl terminal part of peptide bond.
- Chymotrypsin works well on peptide bonds with aromatic amino acids on the carboxyl terminus.
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Description
This quiz covers the sources of dietary proteins, including both animal and vegetable origins, and discusses their daily intake. It also examines the process of digestion in the mouth and stomach, including the role of different enzymes in protein degradation. Test your knowledge on these essential topics!
