Biosynthesis of Nonessential Amino Acids
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Questions and Answers

What is the carbon skeleton provider for the biosynthesis of selenocysteine?

  • Selenophosphate
  • ATP
  • Selenite
  • Serine (correct)

    • Which amino acids do humans need to obtain from the diet because they cannot be synthesized by the body?

  • Transaminated amino acids
  • Proteogenic amino acids
  • Essential amino acids (correct)
  • Nonessential amino acids

    • Which compound serves as the selenium donor in the biosynthesis of selenocysteine?

  • Serine
  • ATP
  • Selenophosphate (correct)
  • Selenite

    • What catalyzes the formation of selenophosphate from ATP and selenite?

    <p>Selenophosphate synthetase</p> Signup and view all the answers

    What is the main difference between the number of enzymes required for synthesizing essential amino acids and nonessential amino acids?

    <p>Nonessential amino acids require fewer enzymes</p> Signup and view all the answers

    Which general mechanism is primarily involved in the biosynthesis of nonessential amino acids?

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

    Which tRNA is first charged with serine in the biosynthesis of selenocysteine?

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

    How is glutamate synthesized in the context of nonessential amino acid biosynthesis?

    <p>From transamination of α -keto acid</p> Signup and view all the answers

    How is selenium incorporated into selenocysteine from serine in the biosynthesis process?

    <p>Substitution of serine oxygen with selenium from selenophosphate</p> Signup and view all the answers

    Which amino acid is NOT listed as a possible amino donor for the synthesis of glutamate through transamination?

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

    What does the large number of enzymes required for synthesizing essential amino acids compared to nonessential amino acids suggest?

    <p>It implies a positive survival advantage for retaining essential amino acid synthesis ability</p> Signup and view all the answers

    How is alanine formed through transamination?

    <p>From pyruvate using glutamate as the amino donor</p> Signup and view all the answers

    What is the amino donor in the formation of serine through transamination?

    <p>3-phosphoglycerate</p> Signup and view all the answers

    Which vitamin acts as an intermediate carrier of the amino group in transamination reactions?

    <p>Vitamin B6 (pyridoxal phosphate)</p> Signup and view all the answers

    How is aspartate synthesized through transamination?

    <p>From oxaloacetate using glutamate as the amino donor</p> Signup and view all the answers

    How is glutamate formed from free ammonia?

    <p>Catalyzed by glutamate dehydrogenase with α-ketoglutarate</p> Signup and view all the answers

    What serves as the source of reducing equivalents in the formation of glutamate from free ammonia?

    <p>NADPH and NADH</p> Signup and view all the answers

    Which enzyme catalyzes the interconversion of serine and glycine?

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

    What is the source of glycine in the biosynthesis of nonessential amino acids?

    <p>Glyoxylate and glutamate</p> Signup and view all the answers

    Which enzyme is involved in the catalysis of glycine synthesis from glyoxylate and glutamate?

    <p>Glycine aminotransferase</p> Signup and view all the answers

    What is the precursor for tyrosine biosynthesis?

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

    Why is tyrosine considered nutritionally nonessential?

    <p>The conversion to phenylalanine is irreversible.</p> Signup and view all the answers

    Which amino acid can potentially replace tyrosine in the diet?

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

    How is proline formed from glutamate?

    <p>Reduced and cyclized</p> Signup and view all the answers

    What is the enzyme responsible for synthesizing asparagine from aspartate?

    <p>Asparagine synthetase</p> Signup and view all the answers

    Which process ensures that the reaction for synthesizing asparagine is strongly favored?

    <p>Coupled hydrolysis of pyrophosphate (PPi) to Pi</p> Signup and view all the answers

    What mechanism is involved in the biosynthesis of hydroxyproline and hydroxylysine?

    <p>Hydroxylation of proline and lysine</p> Signup and view all the answers

    What is required for the hydroxylation of peptidyl prolyl and peptidyl lysyl residues to occur?

    <p>$O_2$, ascorbate, Fe^2+$, and $\alpha$-ketoglutarate</p> Signup and view all the answers

    Why are dietary hydroxyproline and hydroxylysine not incorporated during protein synthesis?

    <p>There is no tRNA for these hydroxylated amino acids</p> Signup and view all the answers

    What is the primary source of hydroxyproline and hydroxylysine in collagen?

    <p>Hydroxylation of peptidyl prolyl and peptidyl lysyl residues catalyzed by skin enzymes</p> Signup and view all the answers

    What catalyzes the synthesis of asparagine from aspartate?

    <p>Asparagine synthase</p> Signup and view all the answers

    What conditions are necessary for the strong favoring of the coupled hydrolysis of pyrophosphate to phosphate in a reaction?

    <p>Coupled hydrolysis with another high-energy bond</p> Signup and view all the answers

    What is the role of prolyl hydroxylase and lysyl hydroxylase in collagen biosynthesis?

    <p>Catalyze the hydroxylation of peptidyl residues in collagen</p> Signup and view all the answers

    Why are dietary hydroxyproline and hydroxylysine not integrated into protein synthesis?

    <p>Lack of appropriate tRNA molecules for these amino acids</p> Signup and view all the answers

    What is the main function of asparagine synthetase in the biosynthesis of asparagine?

    <p>Synthesizing asparagine from aspartate and ammonia</p> Signup and view all the answers

    What is the role of Fe2+ in the hydroxylation process catalyzed by prolyl and lysyl hydroxylases?

    <p>Serving as a cofactor for the enzymes</p> Signup and view all the answers

    What are the key components required for the hydroxylation of peptidyl prolyl and peptidyl lysyl residues?

    <p>$O_2$ and NADH</p> Signup and view all the answers

    What deficiency can lead to scurvy, resulting in symptoms like bleeding gums and impaired wound healing?

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

    Which amino acid is hydroxylated cotranslationally and can be replaced by cysteine, potentially impairing catalytic activity?

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

    Which condition can arise due to impairments in human selenoproteins and is linked to selenium deficiency cardiomyopathy?

    <p>Keshan disease</p> Signup and view all the answers

    Which human enzyme contains selenocysteine at its active site and catalyzes the conversion of thyroxine to triiodothyronine?

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

    In the biosynthesis of nonessential amino acids, what is decarboxylated to succinate for every mole of proline or lysine hydroxylated?

    <p>$\alpha$ –ketoglutarate</p> Signup and view all the answers

    Which selenoprotein participates in redox reactions and contains selenocysteine at its active site?

    <p>Glutathione peroxidase</p> Signup and view all the answers

    What is the consequence of replacing selenocysteine with cysteine in human selenoproteins, as mentioned in the text?

    <p>Impaired catalytic activity</p> Signup and view all the answers

    Which vitamin deficiency could lead to symptoms like bleeding gums and swelling joints as mentioned in the text?

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

    What effect does replacing selenocysteine with cysteine have on catalytic activity, as discussed in the text?

    <p>Impaired catalytic activity</p> Signup and view all the answers

    Which disease has been linked to impairments in human selenoproteins according to the text?

    <p>Keshan disease</p> Signup and view all the answers

    Study Notes

    Biosynthesis of Nonessential Amino Acids

    • Humans cannot synthesize 10 proteogenic amino acids and must obtain them from the diet, known as nutritionally essential amino acids.
    • The number of enzymes required to synthesize nutritionally essential amino acids is large compared to nonessential amino acids, suggesting a positive survival advantage in retaining the ability to manufacture 'easy' amino acids while losing the ability to make 'difficult' amino acids.

    3 General Mechanisms of Nonessential Amino Acids

    1. Transamination

    • Formation of nonessential amino acids through transamination reactions.
    • Glutamate: Can be synthesized by transamination of α-ketoglutarate, with the amino donor being any amino acid (except lysine, threonine, proline, hydroxylysine, and hydroxyproline).
    • Alanine: Transamination of pyruvate forms alanine, with the amino donor being glutamate or aspartate.
    • Aspartate: Can be synthesized by transamination of oxaloacetate, with the amino donor being glutamate.
    • Serine: Synthesized by the oxidation, transamination, and subsequent dephosphorylation of 3-phosphoglycerate, an intermediate of glycolysis.

    2. Assimilation of Free Ammonia

    • Glutamine: Formation of glutamine from free ammonia and glutamate, catalyzed by glutamine synthetase, which is an ATP-dependent reaction.

    3. Modification of the Carbon Skeletons of Existing Amino Acids

    • Cysteine: Contains atoms donated by both methionine and serine, formed from homocysteine and serine through cystathionine, and hydrolyzed by cystathionine lyase.
    • Glycine: Three mammalian routes for glycine formation:
      • From serine, which is converted to glycine by the removal of its hydroxymethyl group, catalyzed by serine hydroxymethyl transferase.
      • From glyoxylate and glutamate or alanine, catalyzed by glycine aminotransferase.
      • From choline.
    • Tyrosine: Phenylalanine is hydroxylated to form tyrosine, catalyzed by phenylalanine hydroxylase.
    • Proline: Glutamate is reduced and cyclized to form proline.
    • Asparagine: Synthesized from aspartate, catalyzed by asparagine synthetase, with coupled hydrolysis of pyrophosphate to Pi by pyrophosphatase.

    Hydroxyproline and Hydroxylysine

    • Occur principally in collagen.
    • Neither dietary hydroxyproline nor dietary hydroxylysine is incorporated during protein synthesis.
    • Peptidyl hydroxyproline and hydroxylysine arise from proline and lysine, but only after these amino acids have been incorporated into peptides.
    • Hydroxylation of peptidyl prolyl and peptidyl lysyl residues, catalyzed by prolyl hydroxylase and lysyl hydroxylase, requires molecular O2, ascorbate, Fe2+, and α-ketoglutarate.

    Selenocysteine, the 21st Amino Acid

    • Biosynthesis requires serine, selenite, ATP, a specific tRNA, and several enzymes.

    • Serine provides the carbon skeleton of selenocysteine.

    • Selenophosphate, formed from ATP and selenite, serves as the selenium donor.

    • The specific tRNASec is first charged with serine by the ligase that charges tRNASer, and then the serine oxygen is replaced by selenium from selenophosphate.### Biosynthesis of Hydroxyproline and Hydroxylysine

    • For every mole of proline or lysine hydroxylated, one mole of α-ketoglutarate is decarboxylated to succinate.

    • Ascorbate is required for hydroxyproline and hydroxylysine hydroxylases.

    • Deficiency of ascorbate results in scurvy, characterized by: • Bleeding gums • Swelling joints • Impaired wound healing

    • Scurvy is due to impaired stability of collagen.

    Biosynthesis of Selenocysteine

    • Selenocysteine is the 21st amino acid.
    • At least 25 human selenoproteins are known.
    • Selenocysteine is present at the active site of several human enzymes that catalyze redox reactions.
    • Examples of enzymes that use selenocysteine include: • Thioredoxin reductase • Glutathione peroxidase • Deiodinase (converts thyroxine to triiodothyronine)
    • Selenocysteine participates in the catalytic mechanism of these enzymes.
    • Replacement of selenocysteine by cysteine impairs catalytic activity.
    • Impairments in human selenoproteins are implicated in: • Tumorigenesis • Atherosclerosis • Selenium deficiency cardiomyopathy (Keshan disease)
    • Selenocysteine arises cotranslationally during its incorporation into peptides, unlike hydroxyproline and hydroxylysine.

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    Biosynthesis Of Nonessential Amino Acids PDF

    Description

    Explore the biosynthesis of nonessential amino acids with a focus on glycine's interconversion with serine. Learn about the role of serine hydroxymethyltransferase and the importance of H4 folate in this process.

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