Questions and Answers
What is the approximate amount of amino acids in a 70-kg man?
90-100 g
In healthy individuals, what is the state of the amino acid pool?
In a steady state
What happens to amino acids not used in biosynthetic reactions?
They are burned as a fuel
What is the purpose of protein degradation?
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What determines the concentration of protein in the cell for many proteins?
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What happens to the total amount of protein in the body in healthy adults?
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What type of modification can lead to preferential degradation of some proteins?
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What is the significance of N-terminal residues on protein half-life?
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What occurs when the carbonyl group of the peptide bond is contributed by arginine or lysine?
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What is the significance of PEST sequences in proteins?
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What is the function of enteropeptidase?
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Why must dietary proteins be hydrolyzed in the intestine?
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What is the result of enteropeptidase converting trypsinogen to trypsin?
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What is the role of cholecystokinin and secretin in pancreatic zymogen release?
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What is the typical daily intake of protein in the American diet?
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What is an exception to the rule that proteins are too large to be absorbed by the intestine?
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What is the purpose of synthesizing pancreatic proteases as inactive zymogens?
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What is the main function of trypsin in terms of activating other pancreatic zymogens?
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Where are D-amino acids found?
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What is the function of D-amino acid oxidase?
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What is the byproduct of the reaction catalyzed by D-amino acid oxidase?
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What is the purpose of glutamine synthetase?
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What is the fate of glutamine in the liver?
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What is the significance of D-amino acid oxidase activity?
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What is the purpose of the glucose-alanine cycle?
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What is the major disposal form of amino groups derived from amino acids?
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Where are the first two reactions of the urea cycle located?
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What is the source of the carbon and oxygen in urea?
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What is the enzyme responsible for the oxidative deamination of glutamate?
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Where is urea produced and then transported to for excretion?
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Study Notes
Amino Acid Pool and Nitrogen Metabolism
- The amino acid pool consists of approximately 90-100 g of amino acids, a small amount compared to the 12 kg of protein in the body of a 70-kg person
- In healthy, well-fed individuals, the input to the amino acid pool is balanced by the output, resulting in a steady state and nitrogen balance
Protein Turnover
- Proteins in the body are constantly being synthesized and degraded to remove abnormal or unneeded proteins
- Regulation of synthesis determines protein concentration in the cell, with degradation playing a minor role for some proteins
- In healthy adults, the rate of protein synthesis is sufficient to replace degraded protein, maintaining a constant total amount of protein in the body
Chemical Signals for Protein Degradation
- Proteins have different half-lives, indicating that degradation is influenced by structural aspects of proteins
- Chemical modifications, such as oxidation or ubiquitin tagging, can mark proteins for degradation
- The nature of the N-terminal residue affects protein half-life, with serine resulting in long-lived proteins and aspartate in short-lived proteins
- Proteins with PEST sequences (proline, glutamate, serine, and threonine) are rapidly degraded
Digestion of Dietary Proteins
- Most nitrogen in the diet comes from protein (70-100 g/day in the American diet)
- Proteins must be hydrolyzed to di- and tripeptides and amino acids for absorption
- Pancreatic proteases, such as trypsin, cleave proteins into smaller peptides and amino acids in the small intestine
Activation of Pancreatic Zymogens
- Pancreatic zymogens are activated by enteropeptidase, an enzyme on the luminal surface of intestinal mucosal cells
- Trypsin, once activated, can convert other trypsinogen molecules to trypsin, creating a cascade of proteolytic activity
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Description
This quiz covers the concept of the amino acid pool, its size, and its role in whole-body nitrogen metabolism, including the steady state and nitrogen balance in healthy individuals.
