Week 3

Chapter 9: Transamination and Deamination Reactions Overview

  • Amino Acid Metabolism and Hydrocarbons

    • Hydrocarbons from amino acids can appear in carbohydrates or lipids.

    • Almost all of the α-amino nitrogen from amino acid catabolism funnels through glutamate.

  • Deaminases

    • Glutaminase, asparaginase, and glutamate dehydrogenase are classified as deaminases.

  • ALT and AST in Hepatocytes

    • Alanine aminotransferase (ALT) concentrations are elevated in the cytosol of hepatocytes from small animal species.

    • Aspartate aminotransferase (AST) is present in both the cytosol and mitochondria of liver cells and plays a role in the malate shuttle.

  • Coenzymes in Amino Acid Catabolism

    • Essential coenzymes for amino acid catabolism include:

    • Pyridoxal phosphate (derived from vitamin B6): Required for transaminations and amino acid decarboxylations.

    • Tetrahydrofolate: Links amino acid and nucleotide biosynthesis; carries one-carbon (C1) fragments.

    • Biotin: A CO2 shuttler essential for certain carboxylation reactions in glucogenic amino acids.

    • Vitamin B12 (Cobalamin): Required for rearranging methylmalonyl-CoA to succinyl-CoA, important in gluconeogenesis.

  • Transamination Reactions

    • Transaminases (aminotransferases) catalyze the transfer of amino groups from amino acids to keto acid acceptors, resulting in their conversion to α-keto acids:

    • Pyruvate → Ala (Alanine)

    • Oxaloacetate (OAA) → Asp (Aspartate)

    • α-ketoglutarate (α-KG) → Glu (Glutamate)

    • Transamination reactions are reversible and occur in various tissues, making them essential for life.

  • Exception to Transamination

    • Exceptions include threonine, lysine, proline, and hydroxyproline, which do not follow typical degradation pathways.

  • NH(_4^+) Release

    • Most NH(_4^+) released from amino acids results from the action of transaminases and glutamate dehydrogenase.

    • Liver and kidney tissues also utilize amino acid oxidases, which oxidize amino acids to α-imino acids, subsequently adding water to release NH(_4^+), forming α-keto acids.

    • This process produces reduced flavin (FADH(2)), reoxidized by O(2) to form hydrogen peroxide (H(2)O(2)), which is then decomposed by catalase.

  • Complexity of Metabolism

    • The metabolism of the α-keto acid hydrocarbons is complex, integrating various amino acid types with carbohydrate and lipid metabolism via the tricarboxylic acid (TCA) cycle.

    • Hydrocarbons can yield CO(2) and H(2)O or serve in synthesizing carbohydrates (glucogenic) or lipids (ketogenic).

  • Deamination Reactions

    • The oxidative deamination of glutamate (Glu), catalyzed by mitochondrial glutamate dehydrogenase (GLDH), releases ammonium ion (NH(_4^+)), crucial for urea synthesis in the liver.

    • Urea is the primary end product of nitrogen metabolism in mammals, being non-toxic compared to ammonia (NH(_3)), which is excreted by ammonotelic species such as many fish.

  • GLDH Characteristics

    • Glutamate plays a significant role in deamination and is the primary amino acid undergoing this reaction at an appreciable rate in mammals.

    • GLDH uses NAD(^+) or NADP(^+) as oxidizing agents, producing NADH or NADPH.

    • It can catalyze both oxidative deamination and the reverse reaction, allowing NH(_4^+) scavenging in the presence of α-KG.

  • Glutaminase and Asparaginase

    • Glutaminase and asparaginase catalyze hydrolysis of the amide side chains of glutamine (Gln) and asparagine (Asn), respectively, leading to NH(_4^+) formation.

    • Glutaminase is active in GABA and Glu-secreting neurons and proximal renal tubular epithelial cells, with variability based on metabolic states (acidosis vs. alkalosis).

    • Certain tumors show elevated needs for Gln and Asn, thus these enzymes are investigated as potential antitumor agents.

  • Other Notable Deamination Processes

    • Threonine, glycine, histidine, and serine also undergo nonoxidative deamination, and they can additionally be transaminated.

Transaminases: Key Enzymes

  • Alanine Aminotransferase (ALT)

    • ALT is crucial during starvation for transferring amino groups from skeletal muscle to the liver, facilitating urea or Gln incorporation.

    • Mostly found in the cytosol of hepatocytes, with presence in other tissues (neural, pancreatic, renal, etc.).

    • Comparative enzyme concentrations vary across species (higher in primates, dogs, cats; lower in pigs and horses).

    • Short serum half-life (2-4 hours); activity should be determined soon after sample collection.

  • Aspartate Aminotransferase (AST)

    • AST is found in both cytosol and mitochondria of liver cells, larger quantities also in muscle tissues and erythrocytes.

    • Used as a marker for hepatic necrosis, remains elevated in circulation for 2-3 days.

    • Not tissue-specific but serves in transaminating Asp to OAA and forming Glu from α-KG.

  • Other Transaminases

    • Other transaminases are present in organisms (e.g., cysteine transaminase, ornithine transaminase).

    • Essential for life, as no significant metabolic defects are associated with their reactions.

Summary of Transamination and Deamination

  • Transamination reactions are typically the first step in amino acid degradation and the last step in biosynthesis.

  • These reversible reactions occur primarily in hepatocytes, indicating the central role of glutamate in nitrogen metabolism for both biosynthesis and excretion.

  • While less frequent than transamination, oxidative deamination primarily involves glutamate, with only minimal other amino acids undergoing significant oxidative deamination.

  • Enzymatic processes transforming amino acids to α-keto acids are crucial for the integration of nitrogen metabolism with energy generation processes.

Objectives of This Chapter

  • Identify reversible reactions catalyzed by aminotransferases (AST, ALT) along with their major locations, substrates, and products.

  • Discuss the derivation of α-keto acids from amino acids.

  • Identify and explain four coenzymes involved in amino acid catabolism.

  • Describe the reaction catalyzed by GLDH, outline its regulation, and the connection of cofactors in this reaction to energy generation.

  • Explain the interconversion of Glu and Gln along with the locations of the respective enzymes.

  • Understand the significance of transamination as a primary event in amino acid degradation and biosynthesis.

  • Acknowledge the necessity of AST being present in both mitochondrial and cytoplasmic pools in liver cells.

Questions

  1. Most ammonia generated in the body normally comes from the oxidative deamination of:
    a. Ala
    b. Glu
    c. Asn
    d. Asp
    e. Gln

  2. Which coenzyme carries one-carbon fragments from amino acids to intermediates of purine and pyrimidine nucleotides?
    a. Glutamine
    b. α-Ketoglutarate
    c. Tetrahydrofolate
    d. Aspartate
    e. Vitamin B12

  3. Which enzyme is involved in the conversion of aspartate and α-ketoglutarate to oxaloacetate and glutamate, respectively?
    a. ALT
    b. GLDH
    c. SGPT
    d. SGOT
    e. Asparagine synthetase

  4. Glutamate dehydrogenase is present in all of the following, EXCEPT:
    a. Erythrocytes.
    b. Liver.
    c. Kidney.
    d. Brain.
    e. Muscle.

  5. There are no known metabolic defects associated with the conversion of:
    a. α-KG to Glu
    b. Ala to pyruvate
    c. Asp to OAA
    d. Gln to Glu
    e. These reactions are essential for life.

  6. Essentially all of the α-amino nitrogen from amino acid catabolism is funneled through:
    a. Acetyl-CoA
    b. α-Keto acids
    c. Catalase
    d. Asparagine
    e. Glutamate

  7. The oxidative deamination of glutamate yields NH(_4^+) and:
    a. NAD(^+)
    b. Oxaloacetate
    c. Gln
    d. Pyruvate
    e. α-KG

  8. The primary end product of nitrogen metabolism in mammals is:
    a. NH(3) b. NH(4^+)
    c. Urea
    d. NADH
    e. Uric acid

Answers

  1. b

  2. c

  3. d

  4. a

  5. b

  6. e

  7. e

  8. c