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Questions and Answers
What is the primary product formed during the oxidative deamination of glutamate?
What is the primary product formed during the oxidative deamination of glutamate?
Which process directly removes an amide function group from amino acids?
Which process directly removes an amide function group from amino acids?
How does glutamate function in amino acid metabolism?
How does glutamate function in amino acid metabolism?
Which amino acids undergo non-oxidative deamination?
Which amino acids undergo non-oxidative deamination?
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Which compound serves as the first nitrogen contributor in the urea cycle?
Which compound serves as the first nitrogen contributor in the urea cycle?
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What is the function of pyridoxal phosphate (PLP) in amino acid metabolism?
What is the function of pyridoxal phosphate (PLP) in amino acid metabolism?
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Which of the following neurotransmitters is formed through the decarboxylation of amino acids?
Which of the following neurotransmitters is formed through the decarboxylation of amino acids?
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What is the role of N-acetyl glutamate in the urea cycle?
What is the role of N-acetyl glutamate in the urea cycle?
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Which intermediate of the urea cycle is also an intermediate of the citric acid cycle?
Which intermediate of the urea cycle is also an intermediate of the citric acid cycle?
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What forms can ammonia take when being transported to the liver?
What forms can ammonia take when being transported to the liver?
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Which amino acid is primarily responsible for the production of ammonia in mammalian tissues?
Which amino acid is primarily responsible for the production of ammonia in mammalian tissues?
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In the transamination reaction involving oxaloacetate, what amino acid is formed?
In the transamination reaction involving oxaloacetate, what amino acid is formed?
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The second nitrogen group required for urea synthesis is donated by which amino acid?
The second nitrogen group required for urea synthesis is donated by which amino acid?
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What is the primary function of carbamoyl phosphate synthetase I in the urea cycle?
What is the primary function of carbamoyl phosphate synthetase I in the urea cycle?
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What two cycles are collectively referred to as the 'Krebs Bicycle'?
What two cycles are collectively referred to as the 'Krebs Bicycle'?
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Which compound is primarily linked to ammonia detoxification in the liver?
Which compound is primarily linked to ammonia detoxification in the liver?
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What is the primary purpose of transamination in amino acid metabolism?
What is the primary purpose of transamination in amino acid metabolism?
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Which amino acids undergo transamination reactions?
Which amino acids undergo transamination reactions?
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What role does pyridoxal phosphate (PLP) play in transamination?
What role does pyridoxal phosphate (PLP) play in transamination?
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What is the main difference between oxidative and non-oxidative deamination?
What is the main difference between oxidative and non-oxidative deamination?
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What is the result of removing the alpha-amino group from an amino acid?
What is the result of removing the alpha-amino group from an amino acid?
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Which of the following amino acids cannot participate in transamination?
Which of the following amino acids cannot participate in transamination?
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What happens during oxidative deamination?
What happens during oxidative deamination?
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Which statement about the catabolism of amino acids is true?
Which statement about the catabolism of amino acids is true?
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Study Notes
Amino Acid Oxidation and the Urea Cycle
- Proteins are nitrogen-containing macromolecules made of L-α-amino acids.
- Dietary protein is the main source of amino acids.
- Amino acid catabolism is part of whole-body catabolism, contributing significantly to energy production.
- Carnivores rely heavily on amino acid oxidation for energy.
- Herbivores utilize amino acids to a lesser extent.
- Microorganisms also utilize amino acids as a source of fuel.
- Photosynthetic plants rarely utilize amino acids for energy.
- In animals, amino acid oxidation occurs in three different metabolic conditions: during normal protein synthesis, a protein-rich diet, and starvation/diabetes.
- Excess amino acids cannot be stored and are either excreted or used as fuel.
- The alpha-amino group of the amino acids is removed, and the carbon skeleton is converted into a major metabolic intermediate.
- Most amino groups from surplus amino acids are converted into urea.
- The major site of amino acid degradation in mammals is the liver.
Dietary Protein Degradation
- Gastrin stimulates HCl secretion to begin protein denaturation.
- Pepsinogen is converted to active pepsin.
- Pepsin cleaves proteins.
- Enzymes in the small intestine neutralize the acid and function in protein degradation.
- Secretin inhibits gastric acid secretion, signals the pancreas, and stimulates the gall bladder to release bile.
- Trypsinogen is converted to trypsin by enteropeptidase.
- Trypsin cleaves proteins using specific amino acid sites.
Amino Acid Pool
- Body proteins can be degraded, providing amino acids for the pool.
- Dietary proteins are digested, absorbed, and enter the pool.
- This pool is used for protein synthesis.
- Non-protein nitrogenous compounds can be synthesized from this pool.
Catabolism of Amino Acids
- Catabolism of amino acids leads to a net nitrogen loss that must be compensated by diet.
- Amino acids contain an amino group that distinguishes their catabolism from other catabolic processes.
- Each degradative pathway involves a key step where the amino group is separated from the carbon skeleton and processed in specialized pathways.
- The catabolic pathways of amino acid degradation include transamination and deamination.
- Amino acids are converted to intermediates of the citric acid cycle or glucose.
Catabolism of the Amino Group
- Transamination is the transfer of an amino group from an amino acid to an α-keto acid.
- It's catalyzed by transaminases (aminotransferases).
- Pyridoxal phosphate (PLP) acts as a co-factor.
- The amino group is transferred to α-ketoglutarate, generating glutamate and the α-keto acid analog of the original amino acid.
Deamination
- Deamination removes the amino group as ammonia.
- Oxidative deamination involves the removal of ammonia from glutamate, catalyzed by glutamate dehydrogenase.
- This reaction requires NAD+ or NADP+ and generates α-ketoglutarate.
- Non-oxidative deamination is the direct removal of the amino group.
- This typically occurs in amino acids with hydroxyl groups (e.g., serine, threonine).
Direct Removal of an Amide Function Group
- Glutamine synthetase catalyzes the conjugation of ammonia and glutamate to produce glutamine.
- Glutamine acts as a carrier for ammonia, transported from peripheral tissues to the liver.
- Glutaminase, in the liver, hydrolyzes glutamine, releasing ammonia for urea synthesis.
Role of Glutamate
- Glutamate acts as a collector for amino groups from amino acids.
- It is the only amino acid that undergoes oxidative deamination at significant rates in mammalian tissues.
Non-Oxidative Deamination
- This process involves the direct removal of the amino group from hydroxyl-containing amino acids (serine, threonine, homoserine).
- The reaction is catalyzed by PLP-dependent dehydratases.
Decarboxylation
- Decarboxylation removes a carboxyl group, producing important biological amines (e.g., neurotransmitters).
- It involves the removal of CO2 from amino acids to yield amines.
Metabolism of Ammonia
- Ammonia exists as NH₄⁺ at physiological pH.
- It is produced from amino acids, biogenic amines, and other processes.
- Ammonia is very toxic and needs to be removed.
- The liver converts ammonia into urea via the urea cycle.
- Glutamine and alanine transport ammonia to the liver.
Transport of Ammonia
- Glutamine acts as a storage and transport form of ammonia.
- Glutaminase in the liver cleaves glutamine to release ammonia.
- Alanine transports ammonia to the liver in the glucose-alanine cycle.
The Glucose-Alanine Cycle
- Skeletal muscle produces pyruvate, lactate, and ammonia.
- Alanine in the cycle is used to transport ammonia to the liver.
- Pyruvate is used for gluconeogenesis in the liver.
- Glucose is returned to the muscle.
Nitrogen Excretion
- The three forms of nitrogen excretion in animals are ammonia, urea, and uric acid.
- Plants recycle nitrogen and typically excrete nitrogen only in unusual circumstances.
The Urea Cycle
- Ammonia is converted to urea in the liver mitochondria via the urea cycle.
- The urea cycle is the first metabolic pathway to be elucidated.
- The cycle has five enzymatic steps that interconvert various intermediates.
- Intermediates of the urea cycle are also used in the citric acid cycle.
Steps in the Urea Cycle
- Carbamoyl phosphate synthetase I captures ammonia, producing carbamoyl phosphate.
- Ornithine transcarbamoylase converts ornithine to citrulline.
- Argininosuccinate synthetase condenses an aspartate to citrulline to form argininosuccinate.
- Argininosuccinase cleaves argininosuccinate to yield arginine and fumarate.
- Arginase converts arginine to ornithine and urea.
Regulation of Urea Cycle
- Carbamoyl phosphate synthase I is the rate-limiting enzyme in the urea cycle.
- N-acetylglutamate activates the enzyme.
- The rate of urea synthesis correlates with N-acetylglutamate concentration.
End Products of Amino Acid Degradation
- Some amino acids have multiple degradation pathways.
- Ketogenic amino acids can produce ketone bodies.
- Glucogenic amino acids can lead to glucose formation.
- Different amino acids degrade into various intermediate products.
Summary of Amino Acid Catabolism
- Amino acids are catabolized into intermediates of the citric acid cycle or glucose. Some amino acids form ketone bodies, and some have multiple end products.
- Some genetic disorders can affect amino acid catabolism, leading to deficiencies in certain enzymes.
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Description
This quiz covers the processes of amino acid oxidation and the urea cycle, focusing on their roles in energy production and nitrogen excretion in animals. You will learn about the importance of dietary proteins and how different organisms utilize amino acids for energy. Test your knowledge on the metabolic conditions affecting amino acid catabolism.
