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What is the process of complete degradation of proteins into individual amino acids called?
Which of the following describes the role of ubiquitin in protein degradation?
Which process is involved in tagging proteins for degradation in the proteasome?
What type of enzymes are involved in the degradation of proteins?
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How are amino acids primarily maintained in the body?
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What happens to amino acids after they are used in the body?
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What is the primary function of the urea cycle in the context of amino acid metabolism?
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Which of the following statements about amino acids is false?
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What is the daily range of protein degradation and synthesis in the body?
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What happens to excess amino acids when dietary protein intake exceeds the body's needs?
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Which of the following is a significant catabolic pathway for amino acids?
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Which process can provide energy when carbohydrates are in short supply?
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What is the role of proteins tagged by ubiquitin?
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What is the main end product of protein metabolism in amino acids?
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Which of the following substances is NOT produced as a result of amino acid catabolism?
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Which group of amino acids is primarily used for energy production through conversion to glucose?
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Which amino acid is primarily responsible for oxidative deamination to release free NH3 for urea synthesis?
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Which process is described as the transfer of an amino group from one amino acid to an alpha-keto acid?
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Which nitrogenous compounds are produced during the biosynthesis from amino acids?
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Which amino acids do not undergo transamination?
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What is the role of pyridoxal phosphate (PLP) in transamination?
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What are the possible fates of the carbon skeleton of amino acids after transamination?
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What does deamination specifically involve?
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Why is serum transaminase important in medical diagnostics?
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What is the primary purpose of oxidative deamination?
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Which enzyme is primarily involved in oxidative deamination of glutamate?
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What happens when the concentration of ammonia is high in the body?
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Which of the following amino acids undergoes non-oxidative deamination?
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What is the role of alanine in ammonia transport?
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Where does oxidative deamination of glutamate primarily occur in mammals?
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Which statement about the metabolism of ammonia is incorrect?
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What is the role of glutamate in oxidative deamination?
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What is the waste product of nitrogen metabolism that is toxic to the body?
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Which type of animals primarily excrete nitrogen in the form of uric acid?
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During the urea cycle, which compound is formed by the cleavage of arginosuccinate?
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Which enzyme catalyzes the synthesis of carbamoyl phosphate in the urea cycle?
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What is the initial substrate required for the formation of citrulline in the urea cycle?
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Which molecule serves as a precursor for the synthesis of urea in the urea cycle?
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What is the end product of protein metabolism that is excreted in urine?
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What is the relationship between the urea cycle and the citric acid cycle?
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Study Notes
Protein Digestion and Degradation
- Proteins are the most abundant organic compounds in the body, making up around 10-12 kg of body dry weight.
- They perform various functions.
- They are nitrogen-containing macromolecules.
- Proteins are broken down into individual amino acids.
- Proteolysis refers to the complete degradation of proteins into free amino acids.
- Proteases and peptidases (proteolytic enzymes) are involved in this process.
- Some proteins are degraded by the ubiquitin-proteasome complex.
- Ubiquitin, a small protein, is present in all eukaryotic cells.
- Proteins destined for degradation in proteasomes are tagged with ubiquitin.
- This tagging reaction is catalyzed by enzymes called ubiquitin ligases.
- Once tagged, the protein is recognized by ligases, leading to its degradation.
Metabolic Pool of Amino Acids
- Amino acid pool is maintained through input and output sources.
- The body does not store amino acids.
- Sources for the amino acid pool:
- Turnover of body proteins: 300-400g of body protein is degraded and synthesized daily.
- Dietary protein intake: regularly supplies amino acids.
- Synthesis of non-essential amino acids: 11 out of 20 amino acids can be synthesized by the body.
- Utilization of amino acids from the body pool:
- Degradation of proteins, including enzymes, hormones, and immunoglobulins.
- Production of important nitrogenous compounds like porphyrins, purines, and pyrimidines.
- Providing around 10-15% of body energy.
- Excess amino acids are converted to glucose or fat for energy storage.
- Excess amino acids are lost as urea and excreted.
Major Functions of Amino Acids
- Derived from dietary protein.
- Substrates for biosynthesis of other nutrients and energy supply:
- Glycogenic amino acids can be converted into blood glucose for energy.
- Ketogenic amino acids can be converted into Acetyl CoA, which is used in the production of stored fat and energy.
- Biosynthesis of nitrogen-containing metabolites:
- Hemoglobin
- DNA and RNA (purines and pyrimidines)
- Neurotransmitters (GABA)
- Creatine.
- Other functions:
- Histamine
- Melanin/melatonin
- Hormone (tyrosine)
- Neurotransmitter (acetylcholine)
- Antibody
- Receptors
Overview of Amino Acid Metabolism
- Amino acid metabolism involves catabolism and anabolism.
Significant Degradation of Protein (Catabolism of Amino Acids)
- Amino acids are produced during catabolism under three conditions:
- Normal turnover of body proteins: amino acid residues are recycled to generate energy and molecular components.
- Dietary protein intake: if intake exceeds protein synthesis needs, amino acids are degraded.
- Breakdown of body proteins to supply amino acids when carbohydrate supplies are low (starvation, diabetes mellitus).
Metabolism of Amino Acids
-
Removal of amino group:
- This is the crucial step in amino acid catabolism.
- The nitrogen of amino groups cannot be used for energy production and needs to be removed from the body.
- About 95% of amino nitrogen is converted to urea and excreted in urine.
- The remaining 5% is released as NH3/NH4+ from glutamine in the tubular cells of the kidney.
- Amino acids undergo transamination followed by deamination to liberate ammonia.
- The amino group (NH2/NH3+) of amino acids is used for urea formation, the end product of protein metabolism.
- The carbon skeleton of amino acids is converted to keto acids, which can be used for:
- Energy utilization
- Glucose synthesis
- Formation of fat/ketone bodies
- Production of nonessential amino acids.
Transamination
- Defined as the transfer of an amino group (-NH2) from an amino acid to an α-keto acid/α-oxoacid.
- Involves the interconversion of a pair of amino acids and a pair of keto acids, catalyzed by a group of enzymes called transaminases (aminotransferases).
- Transaminases require pyridoxal phosphate (PLP), a coenzyme derived from vitamin B6.
- It is a reversible process.
- No free NH3 is liberated; only the transfer of amino groups occurs.
Transamination: Importance
- Important for redistribution of amino groups and production of non-essential amino acids.
- Involved in both catabolism (degradation) and anabolism (synthesis) of amino acids.
- Diverts excess amino acids towards energy production.
Glutamate
- Glutamate is the only amino acid that undergoes oxidative deamination to liberate free NH3 for urea synthesis.
- All amino acids undergo transamination except lysine, threonine, proline, and hydroxyproline.
- Serum transaminases are important for diagnostic purposes.
- α-ketoglutarate typically accepts amino groups.
- L-glutamate temporarily stores nitrogen.
- L-glutamate can donate its amino group when needed for amino acid biosynthesis.
- All aminotransferases rely on the pyridoxal phosphate cofactor.
Deamination
- Defined as the removal of the amino group from amino acids as NH3.
- Transamination only shuffles amino groups among amino acids.
- Deamination results in the liberation of ammonia for urea synthesis.
- Deamination can occur either oxidatively or non-oxidatively.
Oxidative Deamination
- Liberates free ammonia from the amino group of amino acids coupled with oxidation.
- It takes place in the liver and kidney.
- Purpose: To provide NH3 for urea synthesis and α-keto acids for various reactions, including energy production.
- Role of glutamate dehydrogenase: Most amino acids are transferred to α-keto acids to produce glutamate.
- Glutamate serves as a collection center for amino groups.
- Glutamate undergoes oxidative deamination catalyzed by GDH (glutamate dehydrogenase) to liberate ammonia, using NAD+ or NADP+ as a coenzyme.
- Glutamate dehydrogenase (GDH) is involved in both catabolic and anabolic reactions.
Non-Oxidative Deamination
- Some amino acids can be deaminated to liberate NH3 without undergoing oxidation.
- Amino acid dehydrases: Serine, threonine, and homoserine undergo non-oxidative deamination catalyzed by PLP-dependent dehydrases.
- Amino acid desulfhydrases: Cysteine and homocysteine undergo deamination coupled with desulfhydration to give keto acids.
Metabolism of Ammonia: Urea Cycle
- Ammonia is constantly being produced in the metabolism of amino acids.
- Exists as NH4+ ions.
- Production of NH3 occurs through the amino acids (transamination and deamination), biogenic amines, amino groups of purines and pyrimidines, and the action of intestinal bacteria on urea.
- Transport of ammonia between tissues and liver mostly occurs as glutamine or alanine.
- Alanine is important for NH3 transport from muscle to liver via the glucose-alanine cycle.
- Role of glutamine:
- Acts as a storehouse of NH3.
- Serves as a storage and transport form of NH3.
- Synthesized in liver, brain, and muscle.
- Freely diffusible in tissues.
- Synthesized from glutamate and ammonia.
- Enzyme: glutamine synthetase.
- Unneeded glutamine is processed in intestines, kidneys, and liver.
- Ammonia is transported safely in the bloodstream as glutamine.
Function of Ammonia
- A waste product of nitrogen metabolism.
- Used for synthesis of non-essential amino acids, purines, and pyrimidines.
- Maintains acid-base balance.
Disposal of Ammonia
- Three different types of nitrogen excretory forms:
- Ammoniotelic: Aquatic animals dispose of NH3 directly.
- Uricotelic: Reptiles and birds convert NH3 to uric acid.
- Ureotelic: Mammals, including humans, convert NH3 to urea.
Urea Cycle
- Urea is the end product of protein metabolism.
- The nitrogen of amino acids converted to toxic ammonia in the body is converted to urea and excreted in urine.
- Urea is synthesized in the liver and transported to the kidneys for excretion.
- Urea has two amino (-NH2) groups, one derived from NH3 and the other from aspartate.
- The carbon atom is supplied by CO2.
- There's an interrelation between the urea cycle and the citric acid cycle.
- Biosynthesis of urea involves five cyclic steps and five enzymes:
- Synthesis of carbamoyl phosphate
- Formation of citrulline
- Synthesis of arginosuccinate
- Cleavage of arginosuccinate
- Formation of urea
Steps in the Urea Cycle
- 1. Synthesis of carbamoyl phosphate: Carbamoyl phosphate synthase 1 catalyzes the condensation of NH4+ ion with CO2 to form carbamoyl phosphate. This step uses two ATP and requires N-acetylglutamate for its activity.
- 2. Formation of citrulline: Ornithine transcarbomylase synthesizes citrulline from carbamoyl phosphate and ornithine. Citrulline is then transported to the cytosol via a transporter system.
- 3. Synthesis of arginosuccinate: Arginosuccinate synthase condenses citrulline with aspartate to produce arginosuccinate. This step requires ATP, which is cleaved to AMP and pyrophosphate (PPi). Later, PPi is broken down to phosphate (Pi).
- 4. Cleavage of arginosuccinate: Arginosuccinase cleaves arginosuccinate to yield arginine and fumarate. Fumarate provides a link with the TCA cycle.
- 5. Formation of urea: Arginase cleaves arginine to produce urea and ornithine. Ornithine enters the mitochondria and is reused in the urea cycle.
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Description
Explore the intricate processes of protein digestion and degradation in this quiz. Learn about proteolysis, the role of proteases, and the ubiquitin-proteasome system. Test your understanding of amino acids and their metabolic pool in the human body.
