Amino Acid Metabolism and Synthesis

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Questions and Answers

Which statement accurately describes the nature of phenylalanine and tyrosine?

  • Both are exclusively glucogenic.
  • Tyrosine is essential and phenylalanine is non-essential.
  • Both are essential amino acids.
  • Phenylalanine is essential while tyrosine is non-essential. (correct)

What is one of the anabolic fates of tyrosine?

  • Synthesis of catecholamines. (correct)
  • Formation of glucose.
  • Production of urea.
  • Conversion to fatty acids.

Which condition is a metabolic disorder associated with phenylalanine?

  • Maple syrup urine disease.
  • Glucose-6-phosphate dehydrogenase deficiency.
  • Phenylketonuria. (correct)
  • Lactose intolerance.

What is a clinical manifestation of phenylketonuria?

<p>Mossy odor of urine. (A)</p> Signup and view all the answers

How is tyrosine synthesized in the body?

<p>From phenylalanine. (A)</p> Signup and view all the answers

What role does melanin play in the body?

<p>Protection from UV rays. (D)</p> Signup and view all the answers

What is a consequence of mental retardation in phenylketonuria?

<p>Inhibition of pyruvate transport to mitochondria. (B)</p> Signup and view all the answers

What is required for the diagnosis of phenylketonuria in newborns?

<p>Ferric chloride test. (B)</p> Signup and view all the answers

Flashcards

Phenylalanine

Phenylalanine is an essential amino acid that cannot be synthesized by the body and must be obtained from the diet. It is a precursor to tyrosine, a non-essential amino acid that can be synthesized from phenylalanine.

Tyrosine

Tyrosine is a non-essential amino acid that can be synthesized from phenylalanine in the liver. It plays a role in the synthesis of various important molecules, including catecholamines, melanin, and thyroid hormones.

Catecholamines

Catecholamines are a group of neurotransmitters that regulate various physiological functions, including mood, alertness, and stress response. They are synthesized from tyrosine.

Phenylketonuria (PKU)

A genetic disorder caused by a deficiency in phenylalanine hydroxylase, an enzyme that converts phenylalanine to tyrosine. This results in the accumulation of phenylalanine in the body, leading to various health problems.

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Melanin

Melanin is a pigment that gives color to skin, hair, and eyes. It protects the skin from harmful UV rays. Melanin is synthesized from tyrosine.

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Thyroid hormones

Thyroid hormones are essential for regulating metabolism, growth, and development. They are synthesized from tyrosine and iodine.

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Tyrosinosis

A genetic disorder characterized by the inability to metabolize tyrosine properly, leading to its accumulation in the body. This can cause liver damage and other health problems.

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Albinism

A genetic disorder characterized by the lack of melanin production, resulting in pale skin, hair, and eyes. This is due to a deficiency in tyrosinase, an enzyme involved in melanin synthesis.

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Study Notes

Amino Acid Metabolism

  • Phenylalanine and tyrosine are aromatic amino acids
  • Both are mixed glucogenic and ketogenic
  • Phenylalanine is essential, while tyrosine is non-essential
  • Tyrosine is synthesized from phenylalanine in the liver

Tyrosine Synthesis

  • The process involves phenylalanine hydroxylase, oxygen, water, tetrahydrobiopterin, and dihydrobiopterin reductase
  • The reaction converts phenylalanine to tyrosine

Anabolic Fates of Tyrosine

  • Tyrosine is used to synthesize proteins and phosphoproteins
  • It is used to synthesize catecholamines.

Catecholamines Synthesis

  • Catecholamines include adrenaline (epinephrine) and noradrenaline (norepinephrine)
  • Adrenaline is primarily synthesized in the adrenal medulla
  • Noradrenaline is synthesized in the adrenal medulla and adrenergic nerve endings

Melanin Synthesis

  • Melanin protects skin from UV rays, acting as a natural antioxidant
  • Stored in melanocytes
  • Synthesized from tyrosine via tyrosinase (tyrosine hydroxylase)

Thyroid Hormone Biosynthesis

  • Iodine is transported and concentrated by thyroid follicles
  • Iodide is oxidized to iodine
  • Tyrosine residues on thyroglobulin are iodinated (MIT and DIT)
  • MIT and DIT combine to form T3 and T4

Metabolic Disorders of Phenylalanine

  • Phenylketonuria (PKU) - caused by a deficiency of phenylalanine hydroxylase or dihydrobiopterine reductase
  • Leads to phenylalanine accumulation in the blood and urine
  • An alternative pathway creates phenylpyruvate, phenylacetate, and phenyllactate, leading to a characteristic "mossy" odor in urine

Clinical Manifestations of PKU

  • Mental retardation due to impaired pyruvate transport in the brain
  • Failure to thrive and seizures
  • Skin lesions (eczema), light colored hair and skin (due to competitive inhibition of tyrosinase)

Diagnosis of PKU

  • Screening test using ferric chloride (FeCl3) performed within 4 days of birth
  • Amino acid profile (aminogram) can be used

Albinism

  • Inherited genetic deficiency of tyrosinase (tyrosine hydroxylase)
  • Leads to absence of melanin pigment
  • Types include ocular, cutaneous, and oculocutaneous albinism

Alkaptonuria

  • Inherited autosomal recessive deficiency of homogentisic acid oxidase
  • Leads to arthritis and generalized pigmentation (ochronosis)
  • Urine turns brownish-black upon exposure to air due to oxidation of homogentisic acid
  • Diagnosed by Fehling's test

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