30 Questions
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?
To remove abnormal or unneeded proteins
What determines the concentration of protein in the cell for many proteins?
Rate of synthesis only
What happens to the total amount of protein in the body in healthy adults?
It remains constant
What type of modification can lead to preferential degradation of some proteins?
Ubiquitination
What is the significance of N-terminal residues on protein half-life?
They influence protein half-life
What occurs when the carbonyl group of the peptide bond is contributed by arginine or lysine?
Trypsin cleaves the peptide bond
What is the significance of PEST sequences in proteins?
They target proteins for rapid degradation
What is the function of enteropeptidase?
To convert trypsinogen to trypsin
Why must dietary proteins be hydrolyzed in the intestine?
Because they are too large to be absorbed
What is the result of enteropeptidase converting trypsinogen to trypsin?
An increase in proteolytic activity
What is the role of cholecystokinin and secretin in pancreatic zymogen release?
They mediate the release and activation of pancreatic zymogens
What is the typical daily intake of protein in the American diet?
70-100 g/day
What is an exception to the rule that proteins are too large to be absorbed by the intestine?
Newborns' ability to absorb maternal antibodies
What is the purpose of synthesizing pancreatic proteases as inactive zymogens?
To prevent the digestion of pancreatic cells
What is the main function of trypsin in terms of activating other pancreatic zymogens?
It converts them to their active forms
Where are D-amino acids found?
In plants and microorganisms
What is the function of D-amino acid oxidase?
To degrade D-amino acids
What is the byproduct of the reaction catalyzed by D-amino acid oxidase?
α-keto acids, ammonia, and hydrogen peroxide
What is the purpose of glutamine synthetase?
To transport ammonia to the liver
What is the fate of glutamine in the liver?
It is cleaved by glutaminase to produce glutamate and ammonia
What is the significance of D-amino acid oxidase activity?
It is linked to increased susceptibility to schizophrenia
What is the purpose of the glucose-alanine cycle?
To transport amino acids from muscle to liver
What is the major disposal form of amino groups derived from amino acids?
Urea
Where are the first two reactions of the urea cycle located?
Mitochondria
What is the source of the carbon and oxygen in urea?
CO₂
What is the enzyme responsible for the oxidative deamination of glutamate?
Glutamate dehydrogenase
Where is urea produced and then transported to for excretion?
Liver and kidneys
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
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.
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