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Questions and Answers
What is the equilibrium constant of most transamination reactions?
What is the equilibrium constant of most transamination reactions?
Which two amino acids do not participate in transamination?
Which two amino acids do not participate in transamination?
What is the main action of alanine aminotransferase (ALT)?
What is the main action of alanine aminotransferase (ALT)?
What is the relationship between aspartate and the urea cycle?
What is the relationship between aspartate and the urea cycle?
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Which enzyme catalyzes oxidative deamination in hepatocytes?
Which enzyme catalyzes oxidative deamination in hepatocytes?
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What are the main products of oxidative deamination by glutamate dehydrogenase?
What are the main products of oxidative deamination by glutamate dehydrogenase?
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What is the combined action of aminotransferase and glutamate dehydrogenase called?
What is the combined action of aminotransferase and glutamate dehydrogenase called?
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Which molecules act as inhibitors of glutamate dehydrogenase?
Which molecules act as inhibitors of glutamate dehydrogenase?
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What role does glutamine play in the transport of ammonia?
What role does glutamine play in the transport of ammonia?
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Which process collects amino groups from many amino acids in the liver?
Which process collects amino groups from many amino acids in the liver?
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What role does pyridoxal phosphate play in aminotransferases?
What role does pyridoxal phosphate play in aminotransferases?
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What enzyme converts glutamine into glutamate and ammonia?
What enzyme converts glutamine into glutamate and ammonia?
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During the glucose-alanine cycle, what happens to alanine in the liver?
During the glucose-alanine cycle, what happens to alanine in the liver?
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Which of the following best describes the function of α-ketoglutarate in low energy scenarios?
Which of the following best describes the function of α-ketoglutarate in low energy scenarios?
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What forms the major disposition of amino groups derived from amino acids?
What forms the major disposition of amino groups derived from amino acids?
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Which two sources supply nitrogen in the urea cycle?
Which two sources supply nitrogen in the urea cycle?
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What is the primary disposal route for ammonia in the body?
What is the primary disposal route for ammonia in the body?
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Which branched-chain amino acid is not classified as exclusively ketogenic?
Which branched-chain amino acid is not classified as exclusively ketogenic?
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What happens during the oxidative decarboxylation of branched-chain amino acids?
What happens during the oxidative decarboxylation of branched-chain amino acids?
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Which product is NOT formed from the catabolism of Isoleucine?
Which product is NOT formed from the catabolism of Isoleucine?
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What compound is generated as a non-toxic storage form of ammonia?
What compound is generated as a non-toxic storage form of ammonia?
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Which enzyme is responsible for the metabolism of branched-chain amino acids?
Which enzyme is responsible for the metabolism of branched-chain amino acids?
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Which of the following is considered a glucogenic amino acid?
Which of the following is considered a glucogenic amino acid?
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How much of the body's energy is typically derived from proteins?
How much of the body's energy is typically derived from proteins?
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What is the primary product formed during Phase I of amino acid catabolism?
What is the primary product formed during Phase I of amino acid catabolism?
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Which of the following describes the process of transamination?
Which of the following describes the process of transamination?
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What role does glutamate play in nitrogen metabolism?
What role does glutamate play in nitrogen metabolism?
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Which statement correctly describes the disposal of nitrogen from the body?
Which statement correctly describes the disposal of nitrogen from the body?
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What is the role of active transporters in the metabolism of amino acids?
What is the role of active transporters in the metabolism of amino acids?
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During oxidative deamination, what product is released?
During oxidative deamination, what product is released?
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Which of the following best describes the metabolic fate of the carbon skeleton of α-keto acids?
Which of the following best describes the metabolic fate of the carbon skeleton of α-keto acids?
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What role does N-acetylglutamate play in the urea cycle?
What role does N-acetylglutamate play in the urea cycle?
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Which of the following statements about carbamoyl phosphate synthetase II is true?
Which of the following statements about carbamoyl phosphate synthetase II is true?
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Which factor determines the specificity of an aminotransferase enzyme?
Which factor determines the specificity of an aminotransferase enzyme?
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What is a direct product of the overall stoichiometry of the urea cycle?
What is a direct product of the overall stoichiometry of the urea cycle?
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Where does the formation of carbamoyl phosphate occur?
Where does the formation of carbamoyl phosphate occur?
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What occurs to urea once it diffuses from the liver?
What occurs to urea once it diffuses from the liver?
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What can happen if there is kidney failure regarding ammonia levels?
What can happen if there is kidney failure regarding ammonia levels?
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Which enzyme is involved in producing ammonia from glutamate?
Which enzyme is involved in producing ammonia from glutamate?
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After a protein-rich meal, what happens to N-acetylglutamate levels?
After a protein-rich meal, what happens to N-acetylglutamate levels?
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Study Notes
Nitrogen Metabolism
- Nitrogen disposal is a crucial aspect of metabolism
- The urea cycle is a key part of nitrogen metabolism
- Catabolism of carbon skeletons and nitrogen-containing substances are important elements.
Amino Acid Metabolism: Disposal of Nitrogen
- Amino acids cannot be stored; excess amino acids are catabolized
- Amino acid catabolism has two phases:
- Phase 1: Removal of the α-amino group, forming NH4+ and α-keto acid through transamination and oxidative deamination
- The amino group is recycled for amino acid biosynthesis or excreted in urine
- Urea formation in the liver for excretion in urine is the major nitrogen removal route
- Phase 2: The carbon skeleton of the α-ketoacids is converted into common metabolic intermediates (e.g., CO2, H2O, glucose, fatty acids), fueling energy production pathways
- Phase 1: Removal of the α-amino group, forming NH4+ and α-keto acid through transamination and oxidative deamination
The fate of Nitrogen (N) in Different Organisms
- Ammonotelic animals: Excrete ammonia (e.g., bony fish, amphibian larvae)
- Ureotelic animals: Excrete urea (e.g., many terrestrial vertebrates, sharks)
- Uricotelic animals: Excrete uric acid (e.g., birds, reptiles).
Digestion of Dietary Proteins
- Dietary proteins are broken down into amino acids through a series of enzymatic processes in the gastrointestinal tract.
- Enzymes like pepsin (stomach), trypsin, chymotrypsin, carboxypeptidases (pancreas), and aminopeptidases (small intestine) are involved in protein digestion
- Free amino acids and dipeptides are absorbed by epithelial cells in the small intestine and transported into the portal circulation.
Transport of Amino Acids into Cells
- Amino acids are transported into cells via active transporters.
Overall Nitrogen Metabolism
- Amino acids are the building blocks of nitrogen-containing compounds
- Amino acid catabolism is a critical process in nitrogen metabolism
- Nitrogen enters the body in various forms (from food), converted into amino acids, and then exits as urea.
Metabolic Fates of Amino Groups: Transamination and Oxidative Deamination
- The transfer of the α-amino group to α-ketoglutarate to form a-keto acids and glutamate is the first step in amino acid catabolism, known as transamination
- This reaction is catalyzed by aminotransferases.
- The process occurs in the cytosol of hepatocytes (liver cells) during transamination.
- Alternatively, oxidative deamination involves the removal of ammonia (NH4+)
- Glutamate is crucial from which the funneling of amino groups happen to, because it delivers those groups for the subsequent metabolic steps.
Substrate Specificity of Aminotransferase
- Each aminotransferase is specific to one or a few amino acids.
- α-ketoglutarate is the primary acceptor of amino groups.
Mechanism of Action of Aminotransferases
- All aminotransferases require pyridoxal phosphate (a derivative of vitamin B6), where the enzyme's catalytic activity relies on cofactor's activity.
- All amino acids except threonine and serine take part in transamination reactions.
Threonine and Serine
- These amino acids undergo dehydratase enzyme-mediated deamination.
Two Important Transferases
- Alanine aminotransferase (ALT): transfers the amino group of alanine to α-ketoglutarate to form pyruvate to glutamate, crucial for glucose-alanine cycle.
- Aspartate aminotransferase (AST): transfers the amino group of aspartate to α-ketoglutarate to form oxaloacetate and glutamate, important for urea cycle.
Oxidative Deamination
- During oxidative deamination, glutamate is deaminated in mitochondria by L-glutamate dehydrogenase to generate NH4+ and α-ketoglutarate
- The process is essential for the disposal of excess nitrogen.
- Enzymes that are involved in this process are: -L-Glutamate dehydrogenase
- Oxidative deamination is regulated by factors such as the relative concentrations of glutamate, α-ketoglutarate, and ammonia.
Transdeamination
- The combined action of aminotransferase and glutamate dehydrogenase is the process called transdeamination
- The overall reaction of amino acid catabolism involves these two reaction pathways.
Regulation of Oxidative Deamination
- The reaction's direction depends on the relative concentrations of glutamate, α-ketoglutarate, and ammonia.
- High protein intake increases glutamate and ammonia levels.
- Allosteric regulation of glutamate dehydrogenase is through ATP and GTP (inhibitors), and ADP and GDP (activators).
Oxidative Deamination: Summary
- Glutamate dehydrogenase in the mitochondrial matrix uses NAD+ or NADP+ as oxidants.
- Key outcome: liberation of ammonia and conversion of glutamate to α-ketoglutarate.
- Oxidative deamination primarily occurs in the liver and kidneys.
Glutamine Transports Ammonia
- Many extrahepatic tissues produce ammonia from metabolic processes.
- Glutamine serves as a non-toxic carrier of ammonia in the bloodstream, transporting it to the liver from other tissues.
- The amide nitrogen of glutamine is released as ammonia. Glutamine acts as the non-toxic means of delivering ammonia to the liver for more processing.
"Glucose-Alanine Cycle"
- The glucose-alanine cycle links muscle metabolism to liver metabolism, providing a means for transporting nitrogen from muscles to liver for urea synthesis.
- The cycle involves these steps:
- Amino acid degradation in the muscles produces glutamate.
- Glutamate forms alanine by transferring its amino group to pyruvate (formed by glycolysis).
- Alanine travels to the liver.
- Alanine donates its amino group to α-ketoglutarate in the liver, reforming glutamate.
- Glutamate is deaminated to form ammonia (NH4+), which is incorporated into the urea cycle during urea synthesis.
- The cycle allows for the removal of nitrogen from muscles and the generation of glucose from pyruvate in the liver to fuel the muscles during energy demands.
The Citric Acid and Urea Cycles Are Linked
- Fumarate (a citric acid cycle intermediate) and urea cycle intermediates (e.g., aspartate) link these cycles, and are essential for each other. This allows for the disposal of ammonia while still using and/or supplying compounds from the citric acid cycle.
Urea Cycle
- Urea, the primary nitrogenous waste product of protein metabolism, is synthesized in the liver
- The urea cycle enzymes operate in both the cytosol and the mitochondria of liver cells.
- The process of urea synthesis from ammonia is referred to as the Urea Cycle.
Formation of Carbamoyl Phosphate
- Carbamoyl phosphate synthetase I, the rate limiting step, catalyzes the formation of carbamoyl phosphate.
- N-acetylglutamate plays a role as an allosteric activator.
CPS I vs. CPS II
- Carbamoyl phosphate synthetase I is essential to the urea cycle, whereas CPS II is key to the biosynthesis of pyrimidines.
Fate of Urea
- Urea diffuses from the liver and is transported to the kidneys.
- Some urea is cleaved into CO2 and NH3 in the intestine by bacterial urease.
- Remaining urea is excreted in the urine.
Metabolism of Ammonia
- Ammonia is a significant source and byproduct of nitrogen metabolism
- Several mechanisms exist for processing and removing excess ammonia from cells.
Transport of Ammonia in the Circulation
- Ammonia is continuously produced by tissues but rapidly removed to avoid toxicity
- Glutamine plays a crucial role as a non-toxic storage and transport form of ammonia, converting it to NH4+.
Catabolism of Branched-Chain Amino Acids
- The branched-chain amino acids (BCAAs) are primarily metabolized in peripheral tissues, particularly muscle.
- Degradation pathways involve transamination, oxidative decarboxylation, and subsequent degradation of the resulting α-keto acid derivatives. (e.g., products like succinyl CoA and branched chain keto acids)
Catabolism of Carbon Skeleton Amino Acids
- Amino acids can be grouped into ketogenic and glucogenic categories based on metabolic fates
- Ketogenic amino acids, such as leucine and lysine, produce ketone bodies like acetoacetate or Acetyl CoA.
- Glucogenic amino acids generate pyruvate or citric acid cycle intermediates and are used for glucose synthesis. Several amino acids have both glucogenic and ketogenic fates.
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Description
Explore the intricate processes involved in nitrogen metabolism, including the vital urea cycle and amino acid catabolism. This quiz delves into how different organisms dispose of nitrogen and the steps involved in amino acid breakdown. Test your knowledge on key metabolic pathways and nitrogen removal mechanisms.
