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Questions and Answers
Transamination is the initial step in the catabolism of branched chain amino acids.
True
The main site for the catabolism of branched chain amino acids is the liver.
False
Branched chain α-keto acid dehydrogenase complex is not influenced by covalent modification.
False
Maple Syrup Urine Disease is caused by a deficiency in α-keto acid dehydrogenase complex.
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The urine of individuals affected by Maple Syrup Urine Disease has an odor similar to burnt leaves.
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Covalent modification of the α-keto acid dehydrogenase complex involves phosphorylation and dephosphorylation.
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Acyl CoA dehydrogenase is one of the enzymes involved in the catabolism of branched chain amino acids.
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Mental retardation and death in infancy can occur due to complications from Maple Syrup Urine Disease.
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Branched chain amino acids are classified as non-essential amino acids.
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Intermittent Branched Chain Ketonuria is characterized by a severe deficiency of the α-keto acid dehydrogenase complex.
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The breath and urine of individuals with IsovalERIC Acidaemia is described as having a 'fruity' odour.
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Phenylketonuria can result from a deficiency in any factor associated with the hydroxylation of phenylalanine.
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The accumulation of isovalerate in urine is a result of a deficiency of Isovaleryl CoA dehydrogenase.
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Phenylalanine hydroxylase is primarily located in the kidneys.
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Large doses of thiamine are ineffective in managing intermittent branched chain ketonuria.
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The cofactor required for the conversion of phenylalanine to tyrosine is dihydrobiopterin.
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Restriction of branched chain amino acids is less effective for managing intermittent branched chain ketonuria than for other conditions.
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Symptoms of isovaleric acidaemia might include vomiting, acidosis, and coma when affected individuals consume protein.
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The symptoms of intermittent branched chain ketonuria typically appear early in life.
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Phenylketonuria is characterized by the accumulation of tyrosine in the blood.
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Affected individuals with phenylketonuria often have a 'mousy' odour in their urine due to phenylacetate.
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Management of phenylketonuria does not require dietary restrictions.
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The Guthrie test is used to detect phenylpyruvate in urine.
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Tyrosine can be catabolized to synthesize acetoacetate and fumarate.
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Individuals with phenylketonuria typically have high levels of skin pigmentation.
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Phenylketonuria can lead to a decline in IQ and severe mental retardation.
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High levels of tetrahydrobiopterin are necessary for prion diseases.
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Tyrosinaemia is characterized by the accumulation and excretion of phenylalanine.
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Transamination, dioxygenation, isomerization, and hydrolysis reactions are involved in tyrosine catabolism primarily occurring in the kidney.
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Elevated levels of tyrosine in neonates are often caused by a Vitamin C deficiency or immature liver enzymes.
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Tyrosinaemia affects males and females with a significant disparity in prevalence.
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Type I Tyrosinaemia is linked to the deficiency of tyrosine aminotransferase.
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Symptoms of acute Type I Tyrosinaemia can present within the first few months of life.
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Type II Tyrosinaemia is referred to as hepato-renal tyrosinaemia.
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The chronic form of Type I Tyrosinaemia typically presents around one year of age.
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Type III Tyrosinaemia is caused by a deficiency of fumarylacetoacetase.
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Patients with Type II Tyrosinaemia may experience painful skin lesions, particularly on the palms and soles.
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Homogentisic aciduria is due to a deficiency of homogentisate-1,2-dioxygenase.
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Intellectual disability and seizures are characteristic features of Type I Tyrosinaemia.
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Alkapton is primarily caused by a deficiency of homogentisate oxidase.
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Albinism results from a deficiency of the enzyme that converts phenylalanine to tyrosine.
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The residual effects of alkapton in tissues can lead to conditions like arthritis later in life.
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Urocanic acid is produced directly from the conversion of glutamate during histidine catabolism.
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Individuals with folate deficiency may experience an increase in FIGlu in their urine following a histidine load test.
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The synthesis of melanin exclusively occurs in the liver.
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Study Notes
Fate of the Carbon Skeleton of Amino Acids
- Amino acids are broken down (catabolized) to generate energy or produce other molecules.
- The fate of the branched-chain amino acid (BCAA) skeleton specifically, involves a 3-step process.
Catabolism of Branched Chain Amino Acids
- Occurs primarily in skeletal muscle.
- Additional sites include adipose, kidney, and brain tissues.
- The process begins with a specific aminotransferase.
- This enzyme converts the amino acids to α-keto acids in the cytosol.
- α-ketoglutarate accepts the amino group.
Three Common Steps in BCAA Catabolism
- 1. Transamination: Amino acids are converted to α-keto acids.
- 2. Oxidative Decarboxylation: α-keto acids are converted to acyl-CoA derivatives.
- 3. Dehydrogenation: Acyl-CoA derivatives are further metabolized.
Leucine, Isoleucine, and Valine
- Primarily catabolized in skeletal muscle.
- Requires a branched-chain α-keto acid dehydrogenase complex.
- This complex uses 5 coenzymes (TPP, CoASH, Lipoamide, FAD, NAD+).
- The complex involves 3 enzymes (decarboxylase, a transacylase, a dihydrolipoyl dehydrogenase).
Regulation of BCAA Catabolism
- The branched-chain α-keto acid dehydrogenase complex is regulated by covalent modification.
- It responds to the presence of branched chain amino acids in the diet.
- Phosphorylation inactivates the complex; dephosphorylation activates the complex.
Maple Syrup Urine Disease (MSUD)
- Deficiency in the branched-chain α-keto acid dehydrogenase complex.
- Accumulation of α-keto acids in the blood and urine.
- Urine and sweat of affected individuals have a maple syrup odor.
- This condition leads to mental retardation and death in infancy.
- Dietary treatment involves restricting intake of branched-chain amino acids.
- Some cases respond to thiamine administration
Intermittent Branched Chain Ketonuria
- A variant of MSUD.
- Symptoms are less severe and appear later in life.
- Symptoms manifest intermittently.
- Dietary restriction of branched-chain amino acids is more effective.
Isovaleric Aciduria
- Caused by a deficiency of isovaleryl-CoA dehydrogenase.
- Leads to the accumulation and excretion of isovalerate in the urine.
- Body fluids exhibit a "cheesy" odor.
- Mild mental retardation may be present
- Treated with dietary restriction of leucine.
Catabolism of Phenylalanine
- Phenylalanine is hydroxylated to tyrosine by phenylalanine hydroxylase.
- Tetrahydrobiopterin is a cofactor for this reaction.
- Defects in phenylalanine hydroxylase can lead to phenylketonuria (PKU).
Phenylketonuria (PKU)
- Characterized by the accumulation of phenylalanine and phenylketones.
- Phenylalanine and phenylketones are excreted in the urine.
- Causes a "mousy" odor in the urine.
- Leads to severe mental retardation, a low IQ, failure to grow, and dilution of hair and skin pigmentation.
- Dietary management includes restriction of phenylalanine and increasing tyrosine intake.
- Diagnosis of PKU involves the Guthrie test and ferric chloride test.
Metabolism of Tyrosine
- Tyrosine is catabolized to fumarate and acetoacetate.
- Reactions include transamination, dioxygenation, isomerization, and hydrolysis.
- The primary site for catabolism is the liver.
Tyrosinemia
- Accumulation and excretion of tyrosine and its metabolites.
- Elevated tyrosine levels can be temporary, due to vitamin C deficiency or immature liver enzymes (common in premature babies).
- Untreated, this condition leads to significant medical problems.
- Three distinct types: Type I, Type II, and Type III.
Type I Tyrosinaemia
- Caused by fumarylacetoacetate deficiency.
- Accumulation of maleyl acetoacetate and fumaryl acetoacetate.
- Can lead to oxidative damage and cell death, especially in the liver and kidneys.
- Two forms: Acute (manifests in first few months) and Chronic.
- Acute form symptoms: poor appetite, stunted growth, vomiting, cabbage-like odor, liver inflammation, jaundice, irritability, and lethargy.
- Chronic form symptoms: Cirrhosis of the liver, polyneuropathy, and kidney problems.
- Type I tyrosinaemia is also known as hepato-renal tyrosinaemia.
Type II Tyrosinaemia
- Caused by a tyrosine aminotransferase deficiency.
- Features include eye and skin lesions, and mental retardation.
Type III Tyrosinaemia
- Caused by para-hydroxyphenylpyruvate dioxygenase deficiency.
- Characteristic features include intellectual disability, seizures, and intermittent ataxia (loss of coordination).
Homogentisic Aciduria (Alkaptonuria)
- Deficiency of homogentisate 1,2-dioxygenase.
- Affected individuals excrete large amounts of homogentisic acid in the urine.
- Urine often darkens upon exposure to air.
- Leads to ochronosis (darkening of tissues).
- Symptoms often present later in life with arthritis.
Melanin Synthesis
- Melanin is a pigment in skin, hair, and eyes.
- Synthesized in melanosomes within melanocytes (pigment-producing cells).
Albinism
- Deficiency of tyrosinase leads to impaired melanin synthesis.
- Characterized by a lack of pigment in skin, hair, and eyes.
- High sensitivity to sunlight. Elevated risk of sunburn and skin cancer.
Catabolism of Histidine
- Histidine catabolism begins with conversion to urocanic acid.
- Urocanic acid is converted to formiminoglutamate.
- Deficiency of enzymes in this pathway can result in histidinaemia and impact folate metabolism.
- Diagnosis often relies on blood and/or urine tests.
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Description
Test your knowledge on the breakdown and catabolism of branched-chain amino acids (BCAAs) such as leucine, isoleucine, and valine. This quiz covers the processes involved, including transamination, oxidative decarboxylation, and dehydrogenation, as well as the specific roles of enzymes and tissue sites. Dive into the molecular mechanisms and understand how the carbon skeleton of amino acids is utilized in metabolism.
