S13 - Urea Cycle: Amino Acid Catabolism in Mammals
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Questions and Answers

What is a source of amino acids for catabolism?

  • Excess sugars from diet
  • Fat stored in adipose tissue
  • DNA from dead cells
  • Leftover amino acids from normal protein turnover (correct)

    • All amino acids can undergo gluconeogenesis.

    False

    What is the purpose of the ubiquitin tagging system in protein degradation?

    To recognize and tag proteins that need to be degraded.

    Proteins are continuously synthesized and degraded, with half-lives ranging from ___________ to weeks.

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

    What is the primary function of proteasome in protein degradation?

    <p>To hydrolyze ubiquitinated proteins</p> Signup and view all the answers

    Match the following protein types with their characteristic half-lives:

    <p>Structural proteins (such as collagen) = Digestive enzymes Months or years = Minutes to weeks</p> Signup and view all the answers

    Lysosomal hydrolysis degrades all proteins in the body.

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

    What is the significance of protein turnover in tissues undergoing structural rearrangement?

    <p>High rates of protein degradation occur in tissues undergoing structural rearrangement, such as uterine tissue during pregnancy.</p> Signup and view all the answers

    What is the fate of the nitrogen atom after deamination of an amino acid?

    <p>It is incorporated into other compounds</p> Signup and view all the answers

    Amino acid degradation occurs mainly in the kidneys.

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

    What is the purpose of deamination in amino acid catabolism?

    <p>To remove the nitrogen atom from the amino acid, allowing the carbon skeleton to enter the metabolic mainstream.</p> Signup and view all the answers

    The α-keto acids resulting from deamination are further metabolized to enter the _____ mainstream.

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

    How do different animals excrete excess nitrogen?

    <p>Through ammonia, uric acid, or urea</p> Signup and view all the answers

    Match the following compounds with their role in nitrogen excretion:

    <p>Ammonia = Excreted by some aquatic animals Uric acid = Excreted by birds and reptiles Urea = Excreted by mammals</p> Signup and view all the answers

    What is the role of N-acetyl-glutamate in the urea cycle?

    <p>To activate carbamoyl phosphate synthetase I</p> Signup and view all the answers

    Amino acids are classified as either glucogenic or ketogenic, but not both.

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

    What is the difference between glucogenic and ketogenic amino acids?

    <p>Glucogenic amino acids yield pyruvate or intermediates of the citric acid cycle, while ketogenic amino acids yield acetoacetate or one of its precursors.</p> Signup and view all the answers

    Carbamoyl phosphate synthetase I is the main enzyme that ______________ the urea cycle.

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

    Match the following amino acids with their classification as glucogenic, ketogenic, or both:

    <p>Tyrosine = Both Threonine = Both Soleucine = Ketogenic Tryptophan = Both</p> Signup and view all the answers

    What is the result of the catabolism of ketogenic amino acids?

    <p>Ketone body production</p> Signup and view all the answers

    Essential amino acids can be synthesized by the human body.

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

    What is the role of arginine in the urea cycle?

    <p>Arginine activates carbamoyl phosphate synthetase I</p> Signup and view all the answers

    Amino acids are classified as glucogenic, ketogenic, or ______________ based on the intermediates produced during their catabolism.

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

    What is the result of the catabolism of glucogenic amino acids?

    <p>Glucose production</p> Signup and view all the answers

    Study Notes

    Amino Acid Catabolism in Mammals

    • Sources of amino acids for catabolism: leftover amino acids from normal protein turnover, dietary amino acids that exceed body's protein synthesis needs, and proteins broken down to supply amino acids for energy when sugars are scarce (starvation, malnutrition, diabetes mellitus).
    • Not all amino acids can undergo gluconeogenesis, and only ketogenic ones can do so.

    Protein Turnover

    • Proteins are continuously synthesized and degraded (turnover), including digestive enzymes and plasma proteins.
    • High rates of protein degradation occur in tissues undergoing structural rearrangement, such as uterine tissue during pregnancy.
    • Structural proteins, such as collagen, are metabolically stable and have half-lives measured in months or years.
    • Lysosomal hydrolysis degrades some proteins.

    Protein Degradation

    • Some proteins are targeted for degradation by a covalent attachment of ubiquitin (Ub) at the C-terminus.
    • Proteasome hydrolyzes ubiquitinated proteins.
    • Ubiquitin tagging system recognizes proteins that need to be degraded.

    Deamination

    • The α-amino group must be removed from amino acids, as there are no nitrogenous compounds in energy transduction pathways.
    • Deamination is an obligatory step in catabolism of all amino acids.
    • Once removed, nitrogen can be incorporated into other compounds or excreted.

    Urea Cycle

    • The major site of amino acid degradation in mammals is the liver.
    • Carbamoyl phosphate synthetase I is the main enzyme that starts the urea cycle.
    • This enzyme is activated by N-acetyl-glutamate, which is formed when glutamate and acetyl-CoA concentrations are high.

    Amino Acid Classification

    • Amino acids are classified as glucogenic, ketogenic, or both based on the intermediates produced during their catabolism.
    • Glucogenic amino acids yield pyruvate or citric acid cycle intermediates, while ketogenic amino acids yield acetoacetate or acetyl-CoA.

    Amino Acid Degradation

    • Different amino acids have different degradation pathways.
    • Some amino acids, such as phenylalanine, tyrosine, and tryptophan, are both glucogenic and ketogenic.

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    Description

    This quiz covers the sources of amino acids for catabolism, including leftover amino acids from normal protein turnover and dietary amino acids exceeding body's protein synthesis needs. It also explores the breakdown of proteins in the body.

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