Amino Acid Catabolism and Protein Digestion
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What is the primary fate of the amino group during amino acid catabolism?

  • It is incorporated into nucleotides.
  • It is transformed into fatty acids.
  • It is converted to glucose.
  • It is removed as NH4+. (correct)

    • Which of the following amino acids plays a key role in nitrogen metabolism by being easily converted into TCA intermediates?

  • Tryptophan
  • Glutamine (correct)
  • Phenylalanine
  • Methionine

    • How do many terrestrial vertebrates and sharks primarily excrete nitrogen?

  • As nitrites through the skin.
  • As uric acid to conserve water.
  • As ammonia directly into the environment.
  • As urea, which is less toxic than ammonia. (correct)

    • What is a characteristic of uric acid as a form of nitrogen excretion in animals?

    <p>It allows for water conservation.</p> Signup and view all the answers

    Which of the following statements about the catabolism of excess amino acids is true?

    <p>They are sent to the liver for catabolism.</p> Signup and view all the answers

    Which of the following amino acids is classified as essential?

    <p>Isoleucine</p> Signup and view all the answers

    What percentage of the body's energy production is accounted for by the catabolism of amino acid carbon skeletons?

    <p>10%</p> Signup and view all the answers

    Which amino acid is considered conditionally essential, especially during growth or illness?

    <p>Arginine</p> Signup and view all the answers

    Which amino acid belongs to the nonessential category?

    <p>Serine</p> Signup and view all the answers

    Which group of amino acids requires dietary sources to meet the body's needs?

    <p>Essential amino acids</p> Signup and view all the answers

    What are SGOT and SGPT commonly referred to in liver function tests?

    <p>Serum aspartate and alanine transaminase</p> Signup and view all the answers

    What does elevated levels of ALT and AST indicate?

    <p>Liver dysfunctions such as cirrhosis or hepatitis</p> Signup and view all the answers

    How is ammonia transported in the bloodstream?

    <p>As glutamine</p> Signup and view all the answers

    What happens to excess ammonia in the body?

    <p>It is transformed into urea in the liver</p> Signup and view all the answers

    Where does oxidative deamination occur within the cell?

    <p>Mitochondrial matrix</p> Signup and view all the answers

    Which enzyme removes ammonia collected in glutamate?

    <p>Glutamate dehydrogenase</p> Signup and view all the answers

    What process combines transamination and oxidative deamination for ammonia excretion?

    <p>Transdeamination</p> Signup and view all the answers

    What does BUN measure?

    <p>Total nitrogen under various forms</p> Signup and view all the answers

    What condition can arise from excessive levels of nitrogenous materials in the blood due to kidney failure?

    <p>Azotemia</p> Signup and view all the answers

    Which neurotransmitter is primarily increased due to excess ammonia in the brain?

    <p>Glutamate</p> Signup and view all the answers

    What is the primary treatment for asterixis associated with ammonia toxicity?

    <p>Administration of lactulose</p> Signup and view all the answers

    How does excess glutamate in the brain relate to liver failure?

    <p>It builds up due to the impaired conversion to glutamine.</p> Signup and view all the answers

    What process is inhibited in the presence of hyperammonemia in the brain?

    <p>Urea cycle activity</p> Signup and view all the answers

    What is a common clinical manifestation of ammonia toxicity in the brain?

    <p>Asterixis</p> Signup and view all the answers

    Which of the following best describes NH4+ in terms of its membrane permeability?

    <p>Not permeable but in equilibrium with NH3</p> Signup and view all the answers

    What occurs to excess glutamate when ammonia levels are high due to liver dysfunction?

    <p>It accumulates in neurons and can be converted to GABA.</p> Signup and view all the answers

    During vigorous exercise, muscles rely primarily on which metabolic pathway?

    <p>Glycolysis</p> Signup and view all the answers

    Which compound is predominantly formed when excess NH3 is metabolized in the brain?

    <p>Glutamine</p> Signup and view all the answers

    Which amino acids are classified as both glucogenic and ketogenic?

    <p>Isoleucine</p> Signup and view all the answers

    What effect do sulfonamide antibiotics have on bacterial folic acid synthesis?

    <p>They act as competitive inhibitors.</p> Signup and view all the answers

    What is a common treatment for Phenylketonuria (PKU)?

    <p>Synthetic BH4 supplementation</p> Signup and view all the answers

    Which enzyme's activity is decreased in patients with PKU due to mutations?

    <p>Phenylalanine Hydroxylase</p> Signup and view all the answers

    Which of the following statements about THF (Tetrahydrofolate) is true?

    <p>THF is involved in one-carbon unit transfers.</p> Signup and view all the answers

    What is the primary result of decreased activity of Phenylalanine Hydroxylase (PAH)?

    <p>Accumulation of phenylalanine.</p> Signup and view all the answers

    Which two antibiotics are commonly used together to target bacterial folate synthesis?

    <p>Sulfamethoxazole and Trimethoprim</p> Signup and view all the answers

    Phenylalanine catabolism is significantly altered in which condition?

    <p>Phenylketonuria</p> Signup and view all the answers

    Which amino acid is exclusively ketogenic?

    <p>Leucine</p> Signup and view all the answers

    Which of the following is a characteristic symptom of Phenylketonuria (PKU)?

    <p>Intellectual disabilities</p> Signup and view all the answers

    What is the role of L-glutamate in amino acid metabolism?

    <p>It serves as a temporary storage of nitrogen.</p> Signup and view all the answers

    Which cofactor is essential for enzymatic transamination reactions?

    <p>Pyridoxal phosphate (PLP)</p> Signup and view all the answers

    Which enzyme is primarily responsible for the transfer of amino groups from alanine to α-ketoglutarate?

    <p>Alanine transaminase (ALT)</p> Signup and view all the answers

    What is a consequence of free ammonia in the body?

    <p>It is toxic and should not be released.</p> Signup and view all the answers

    What defines the ping-pong mechanism in enzymatic transamination?

    <p>Sequential binding of substrates and release of products.</p> Signup and view all the answers

    Which of the following is not a function of transaminases?

    <p>Directly synthesizing amino acids</p> Signup and view all the answers

    How do transamination reactions manage ammonia in the body?

    <p>By capturing and transferring amine groups.</p> Signup and view all the answers

    What is the product generated when α-ketoglutarate accepts amino groups during transamination?

    <p>L-glutamate</p> Signup and view all the answers

    What is the common substrate for transaminases when an amine is transferred?

    <p>α-ketoglutarate</p> Signup and view all the answers

    In what condition are alanine transaminase (ALT) and aspartate transaminase (AST) levels measured?

    <p>To assess liver function</p> Signup and view all the answers

    Which statement best describes the enzymatic transamination process?

    <p>It includes a transfer of an amino group to a keto acid.</p> Signup and view all the answers

    Why is pyridoxal phosphate (PLP) important for transaminases?

    <p>It serves as a cofactor for the reactions.</p> Signup and view all the answers

    How can L-glutamate participate in amino acid biosynthesis?

    <p>By donating its amino groups when necessary.</p> Signup and view all the answers

    Study Notes

    Amino Acid Catabolism and Urea Production

    • Amino acid catabolism occurs during normal protein turnover, when animals exclusively consume meat, when the diet has excess amino acids, and during starvation/uncontrolled diabetes.
    • Dietary protein is converted into intracellular protein which is then catabolized into amino acids, carbon skeletons, and ammonia.
    • Excess amino acids are sent to the liver for catabolism.
    • Ammonia is converted into urea for excretion.
    • The urea cycle involves several enzymatic steps utilizing various intermediates.

    Digestion of Dietary Proteins

    • Food intake stimulates gastric mucosa cells to secrete gastrin.
    • Gastrin induces HCl secretion by parietal cells and pepsinogen secretion by chief cells.
    • Pepsinogen converts to pepsin in the stomach (low pH).
    • Pepsin breaks down proteins into peptides.
    • Trypsin and chymotrypsin break down peptides into smaller peptides in the small intestine.
    • Peptides are further broken down into amino acids by aminopeptidase and carboxypeptidases A and B in the small intestine.
    • Amino acids are absorbed.

    Amino Acid Catabolism

    • Absorbed amino acids can be used for protein synthesis or energy production.
    • Amino acids are initially converted into ammonia and alpha-keto acids.
    • Excess amino acids are catabolized in the liver; this process also carries ammonia to the liver.
    • Inside the liver, excess ammonia is either reused or converted into urea or uric acid for excretion.
    • Glutamate, glutamine, alanine, and aspartate play key roles in nitrogen metabolism and conversion into intermediates of the citric acid cycle.

    Fates of Nitrogen in Organisms

    • Plants retain almost all nitrogen.
    • Aquatic vertebrates release ammonia into their environment.
    • Terrestrial vertebrates and sharks excrete nitrogen as urea.
    • Urea is less toxic and highly soluble in water.
    • Birds and reptiles excrete nitrogen as uric acid (insoluble, conserves water).
    • Humans excrete both urea and uric acid.

    Step 1: Removal of the Amino Group

    • Free ammonia is toxic, so it's captured via transamination.
    • Transaminations transfer amines to a common metabolite (e.g., alpha-ketoglutarate) to produce glutamate.
    • Glutamate acts as a temporary nitrogen storage.

    Enzymatic Transamination

    • Aminotransferases (transaminases) catalyze transamination, using the pyridoxal phosphate cofactor (PLP, vitamin B6).
    • Alpha-ketoglutarate commonly receives the amino group, producing glutamate.
    • Glutamate can donate the amino group for other biochemical processes.

    Enzymatic Transamination: Ping-Pong Mechanism

    • Illustrates the stepwise mechanism of transaminase reactions.
    • An example of the mechanism is provided for alanine catabolism involving the enzymes glutamate-pyruvate transaminase (GPT)

    Transaminases and Liver Function Tests

    • Alanine transaminase (ALT) and aspartate transaminase (AST) are liver enzymes measured in serum.
    • High levels suggest liver dysfunction.
    • Sometimes liver function is indicated by SGOT or SGPT (serum values).

    Ammonia Transport

    • Excess ammonia is recycled into glutamine for transport to the liver.
    • Glutamine is transported to the liver, where it is converted back into ammonia.
    • The liver further transforms ammonia into urea or uric acid for excretion.

    Ammonia collected in glutamate

    • Oxidative deamination occurs in the mitochondrial matrix, using either NAD+ or NADP+ as electron acceptors.
    • Ammonia is processed into urea for excretion.

    Ammonia Toxicity: Asterixis

    • Ammonia is toxic to the brain, especially when liver function is compromised.
    • High ammonia levels lead to excess glutamate, excitotoxicity, and potentially hepatic encephalopathy and coma.
    • Asterixis (flapping tremor) is a symptom of hyperammonemia.
    • Treatment includes administering lactulose to bind ammonia and facilitate its excretion.

    The Glucose-Alanine Cycle

    • Vigorously active muscles perform glycolysis, generating pyruvate, some converted to lactate, and the rest to alanine.
    • Alanine transports amino groups and pyruvate to the liver.
    • The liver converts alanine back to pyruvate and uses it for gluconeogenesis, and ammonia is excreted as urea.
    • Glutamate in muscle can be converted to glutamine for ammonia transport to the liver.

    Ammonia is first converted into carbamoyl phosphate

    • Ammonia generated in the mitochondria enters the urea cycle.
    • The first reaction is the conversion of ammonia to carbamoyl phosphate via carbamoyl phosphate synthase 1.
    • This reaction requires ATP.

    The Urea Cycle

    • The urea cycle in the liver converts ammonia to urea for excretion.
    • The process utilizes various enzymes and intermediates such as citrulline, aspartate, and argininosuccinate.
    • The urea cycle is regulated, connected to the citric acid cycle, and significantly contributes to nitrogen metabolism.

    Essential vs. Nonessential and Conditionally Essential Amino Acids

    • Essential amino acids must be obtained from the diet.
    • Nonessential amino acids can be synthesized by the body.
    • Certain amino acids are conditionally essential, meaning their synthesis may be limited under specific circumstances or stages of life.

    Pathways of Amino Acid Degradation

    • Catabolism of amino acid carbon skeletons provides 10% of the body's energy.
    • The process converts various amino acids into intermediates of the citric acid cycle or keto acids for energy production.
    • Some amino acids are glucogenic (can form glucose) or ketogenic (can form ketone bodies), and some are both.

    Cofactors of Enzymes involved in Amino Acid Catabolism

    • Various cofactors, such as biotin and tetrahydrofolate (THF), are crucial for the metabolic pathways of amino acids involved in one-carbon transfer reactions.
    • Vitamins like B6, B9, and B12 play a significant role in the metabolism of various amino acids.
    • A variety of enzymes use various cofactors depending on the specific catabolic pathway of the amino acid in question.

    Fighting Bacterial Infections

    • Sulfonamides and trimethoprim drugs inhibit bacterial growth by affecting folic acid biosynthesis.

    Catabolism of phenylalanine and phenylketonuria (PKU)

    • Phenylalanine is normally converted into tyrosine through the enzyme phenylalanine hydroxylase, requiring the cofactor tetrahydrobiopterin.
    • In PKU, phenylalanine hydroxylase dysfunction inhibits this process, leading to phenylalanine accumulation and abnormal metabolites.
    • Treatment focuses on reducing phenylalanine intake in the diet and supplementing with synthetic tetrahydrobiopterin when appropriate.

    Methylation and S-adenosylmethionine (SAM)

    • Methylation reactions use SAM, which is derived from methionine.
    • SAM donates methyl groups in various biochemical processes (protein synthesis, DNA methylation).
    • Deficiencies in B vitamins and related disorders affect methionine and SAM's role in the liver, which can subsequently affect central nervous system function/health.

    Catabolism of Branched Chain Amino Acids (BCAAs)

    • Branched-chain amino acids (BCAAs) are catabolized primarily in extrahepatic tissues, involving a branched-chain aminotransferase and a branched-chain alpha-keto acid dehydrogenase complex.
    • The BCAAs are converted into intermediates that enter the citric acid cycle.
    • Genetic defects in the components of these pathways can result in disorders like maple syrup urine disease.

    Catabolism of methionine, isoleucine, threonine, and valine

    • The metabolism of these amino acids involves multiple steps involving different enzymes, resulting in the formation of various keto acids as intermediates that participate in catabolism.
    • Genetic defects impacting these pathways can cause disorders such as methylmalonic acidemia (MMA).

    Human Genetic Disorders Affecting Amino Acid Catabolism

    • Various genetic disorders impact different aspects of amino acid metabolism and related pathways, leading to different clinical presentations.

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    Amino Acid Catabolism and Urea Production PDF

    Description

    This quiz covers the processes of amino acid catabolism and the digestion of dietary proteins. It explores the role of dietary proteins, the urea cycle, and the enzymatic steps involved in breaking down proteins to amino acids. Test your knowledge on key concepts related to protein metabolism and digestion.

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