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Biogenesis of Organic Nitrogen
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Biogenesis of Organic Nitrogen

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Questions and Answers

Which amino acid plays a key role in removing toxic ammonia from muscles?

  • Glutamine
  • Folate
  • Pyridoxine
  • Alanine (correct)

    • What is the pathway that operates when NH4+ levels are high?

  • Carbamoyl phosphate synthetase
  • Glutamate dehydrogenase/glutamine synthetase (correct)
  • Glutamate synthase/glutamine synthetase
  • Asparagine synthetase

    • What is the name of the coenzyme form of Vitamin B6?

  • Folic acid
  • Pyridoxamine
  • Pyridoxal
  • Pyridoxal phosphate (correct)

    • What is the role of glutamine synthetase and glutaminase in most tissues?

    <p>Removing toxic ammonia</p> Signup and view all the answers

    What is the primary function of asparagine synthetase?

    <p>To convert aspartate to asparagine</p> Signup and view all the answers

    What is the characteristic of folate coenzymes that helps them be retained within cells?

    <p>Multiple glutamate residues</p> Signup and view all the answers

    What is the first step in the final route for assimilating ammonia?

    <p>The formation of carbamoyl phosphate</p> Signup and view all the answers

    What is the target of a number of useful anticancer, antibacterial, and antiparasitic drugs?

    <p>Dihydrofolate reductase</p> Signup and view all the answers

    What is the role of pyridoxal phosphate in transamination?

    <p>It acts as a coenzyme</p> Signup and view all the answers

    What is the initial step in all pyridoxal phosphate reactions?

    <p>Schiff base formation</p> Signup and view all the answers

    What is the functional group attached to the pyridine ring in pyridoxal phosphate?

    <p>An aldehyde group</p> Signup and view all the answers

    What is the role of the glucose-alanine cycle in muscles?

    <p>Removing toxic ammonia</p> Signup and view all the answers

    What is the intermediate formed during the catalytic cycle of aminotransferases?

    <p>A Schiff base</p> Signup and view all the answers

    What is the intermediate form of pyridoxal phosphate in transamination reactions?

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

    What is the vitamin from which pyridoxal phosphate is derived?

    <p>Vitamin B6</p> Signup and view all the answers

    When do levels of NH4+ trigger the glutamate synthase/glutamine synthetase pathway?

    <p>When NH4+ levels are low</p> Signup and view all the answers

    What is a possible consequence of a genetic defect in methionine synthase?

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

    Which of the following vitamins is required for the prevention of homocysteinemia?

    <p>Folic acid</p> Signup and view all the answers

    What is the role of polyamines in cell proliferation?

    <p>Stabilizing duplex DNA structures</p> Signup and view all the answers

    What is the neurotransmitter synthesized from glutamate through decarboxylation?

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

    What is the intermediate formed in the synthesis of nitric oxide from arginine?

    <p>Nw-hydroxy-L-arginine</p> Signup and view all the answers

    What is the result of iodine deficiency in the biosynthesis of thyroid hormones?

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

    What is the source of thyroid hormones in the body?

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

    What is the fate of the iodinated forms of tyrosine in the biosynthesis of thyroid hormones?

    <p>They are released from thyroglobulin by proteolytic degradation</p> Signup and view all the answers

    What is the result of the breakdown of heme proteins in animals?

    <p>Production of bilirubin and reuse of amino acids and iron</p> Signup and view all the answers

    Which of the following amino acids serves as a precursor to neurotransmitters?

    <p>All of the above</p> Signup and view all the answers

    What is the enzyme involved in the hydroxylation of tryptophan to produce serotonin?

    <p>Tetrahydrobiopterin-dependent aromatic amino acid hydroxylase</p> Signup and view all the answers

    What is the mechanism involved in the reaction of aromatic amino acid hydroxylases?

    <p>NIH shift</p> Signup and view all the answers

    What is the result of the abnormal accumulation of heme precursors in porphyrias?

    <p>Overproduction of unnatural type I porphyrins or abnormally high flux through d-ALA synthetase</p> Signup and view all the answers

    What is the source of most of the heme that is catabolized?

    <p>Aged erythrocytes</p> Signup and view all the answers

    What is the enzyme involved in the decarboxylation of tryptophan to produce serotonin?

    <p>PLP-dependent decarboxylase</p> Signup and view all the answers

    What is the structural relationship between tryptophan hydroxylase and phenylalanine hydroxylase?

    <p>They are both mechanistically and structurally related</p> Signup and view all the answers

    What is the consequence of a deficiency of vitamin B12 on folate coenzymes?

    <p>Accumulation of 5-methyltetrahydrofolate</p> Signup and view all the answers

    What is the relationship between folate and B12 metabolism?

    <p>B12 deficiency leads to folate deficiency</p> Signup and view all the answers

    What is the product of the hydrolytic cleavage of asparagine by asparaginase?

    <p>Aspartate and ammonia</p> Signup and view all the answers

    What is the outcome of the transamination of aspartate by aspartate transaminase?

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

    Which of the following amino acids is degraded to succinyl-CoA?

    <p>Glucogenic amino acid</p> Signup and view all the answers

    What is the role of hydrogen sulfide in the body?

    <p>Regulation of vascular blood flow and blood pressure</p> Signup and view all the answers

    What is the consequence of folic acid deficiency?

    <p>Increased risk of cardiovascular disease and birth defects</p> Signup and view all the answers

    What is the cause of pernicious anemia?

    <p>Deficiency of a glycoprotein needed for intestinal absorption of vitamin B12</p> Signup and view all the answers

    Study Notes

    Biogenesis of Organic Nitrogen

    • When NH4+ levels are high, the glutamate dehydrogenase/glutamine synthetase pathway operates.
    • When NH4+ levels are low, the glutamate synthase/glutamine synthetase pathway predominates.
    • Asparagine synthetase catalyzes a reaction comparable to that of glutamine synthetase, using ammonia or glutamine to convert aspartate to asparagine.
    • The final route for assimilating ammonia first forms carbamoyl phosphate, which is catalyzed by carbamoyl phosphate synthetase, using either ammonia or glutamine as the nitrogen donor.

    The Nitrogen Economy

    • Transamination is the reversible transfer of an amino group from an α-amino acid to an α-keto acid, with pyridoxal phosphate as a coenzyme.
    • Aminotransferases utilize a coenzyme, pyridoxal phosphate, derived from vitamin B6, which has an aldehyde functional group, CHO, attached to a pyridine ring.
    • The catalytic cycle begins with condensation of this aldehyde with the α-amino group of an amino acid, to give a Schiff base, or aldimine, intermediate.
    • The carrier is glutamine in most tissues, except for muscles, where this role is played by alanine.

    Amino Acid Degradation and Metabolism of Nitrogenous End Products

    • The glucose–alanine cycle removes toxic ammonia from muscle.
    • Glutamine synthetase and glutaminase do the same for most other tissues.

    Coenzymes Involved in Nitrogen Metabolism

    • Vitamin B6 is also called pyridoxine, and its active coenzyme form is pyridoxal phosphate (PLP), which is oxidized to an aldehyde and has a phosphorylated hydroxymethyl group at position 5.
    • Pyridoxamine phosphate (PMP) is an intermediate form in transamination reactions.
    • All pyridoxal phosphate reactions involve initial Schiff base formation, followed by bond labilization caused by electron withdrawal to the coenzyme’s pyridine ring.
    • Folate coenzymes contain multiple glutamate residues, which help them be retained within cells and bind more tightly to enzymes.
    • Dihydrofolate reductase is the target for a number of useful anticancer, antibacterial, and antiparasitic drugs.

    Pathways of Amino Acid Degradation

    • Glucogenic amino acids are degraded to one of five intermediates: pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate.
    • Ketogenic amino acids are degraded to acetyl-CoA or acetoacetate.
    • Some amino acids are both glucogenic and ketogenic.
    • Hydrogen sulfide (H2S), derived from cysteine, is a powerful gaseous signaling molecule involved in the regulation of vascular blood flow and blood pressure.

    Amino Acids as Biosynthetic Precursors

    • Homocysteinemia can result from genetic defects in cystathionine β-synthase, methionine synthase, or 5,10-methylenetetrahydrofolate reductase, or from dietary deficiency of folic acid, vitamin B6, or vitamin B12.
    • Polyamines are required for cell proliferation because of their roles in stabilizing duplex DNA structures.
    • Glutamate is a precursor for compounds that function in transmission of nerve impulses, and its decarboxylation yields γ-aminobutyric acid (GABA), which is a neurotransmitter.
    • Arginine is a precursor for nitric oxide and creatine phosphate.

    Porphyrin and Heme Metabolism

    • Heme protein degradation in animals releases amino acids and iron, which are reused, and bilirubin, which must be solubilized for excretion.
    • Porphyrias involve abnormal accumulations of heme precursors, either from overproduction of the unnatural type I porphyrins or from abnormally high flux through δ-ALA synthetase.

    Amino Acids and Their Metabolites as Neurotransmitters and Biological Regulators

    • Glutamate, tyrosine, glycine, and tryptophan serve as neurotransmitters or precursors to neurotransmitters.
    • The pathway to serotonin begins with hydroxylation of tryptophan by a tetrahydrobiopterin-dependent aromatic amino acid hydroxylase, similar to phenylalanine hydroxylase.
    • Tryptophan hydroxylase and tyrosine hydroxylase are both tetrahydrobiopterin-dependent monooxygenases and are mechanistically and structurally related to phenylalanine hydroxylase.

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