Sulfur AA Flashcards by nysa .

Which amino acids are sulfur-containing?

Methionine, Cysteine, Homocysteine, Taurine

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Which enzyme catalyzes the conversion of methionine to S-adenosylmethionine (SAM)?

Methionine adenosyltransferase

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What is the primary function of S-adenosylmethionine (SAM)?

Methyl group donor in transmethylation reactions

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Which enzyme converts S-adenosylhomocysteine (SAH) to homocysteine?

S-adenosylhomocysteine hydrolase

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What are the two metabolic fates of homocysteine?

Remethylation (to methionine) or transsulfuration (to cysteine)

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Which enzyme catalyzes homocysteine remethylation to methionine?

Methionine synthase (requires Vitamin B12 and 5-MTHF)

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Which cofactor is required for methionine synthase?

Methylcobalamin (Vitamin B12)

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Which enzyme catalyzes the conversion of homocysteine to cystathionine in the transsulfuration pathway?

Cystathionine beta-synthase (CBS)

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Which cofactor does CBS require?

Pyridoxine (Vitamin B6)

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What is the fate of cystathionine in the transsulfuration pathway?

Converted to cysteine by cystathionase (requires B6)

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Which condition is caused by a deficiency of cystathionine beta-synthase?

Homocystinuria

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Which enzyme converts homocysteine to methionine using betaine?

Betain-homocysteine methyltransferase

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Which vitamin is needed for betaine-dependent homocysteine remethylation?

None (Betaine acts as a methyl donor directly)

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What happens if homocysteine levels are elevated?

Increased risk of thrombosis, atherosclerosis, and endothelial damage

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What is the role of 5,10-methylene-THF in homocysteine metabolism?

Converted to 5-MTHF, donating a methyl group for methionine synthesis

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Which enzyme converts 5,10-methylene-THF to 5-MTHF?

Methylene tetrahydrofolate reductase (MTHFR, requires FAD)

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What happens in MTHFR deficiency?

Impaired remethylation of homocysteine, leading to homocystinuria

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What is the function of cysteine in the body?

Precursor for glutathione, taurine, and coenzyme A

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Which enzyme converts cysteine to taurine?

Cysteine dioxygenase

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What is the role of taurine in the body?

Membrane stabilization, bile salt conjugation, antioxidant function

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What is the primary defect in cystathionuria?

Deficiency of cystathionase, leading to cystathionine accumulation

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What is the treatment for B6-responsive homocystinuria?

High-dose pyridoxine (Vitamin B6), low methionine diet, and cysteine supplementation

What is the treatment for non-B6-responsive homocystinuria?

Low methionine diet, betaine, folate, and B12 supplementation

Which lab test confirms cystinuria?

Positive cyanide-nitroprusside test (detects excess cystine in urine)

What is the major consequence of cystinuria?

Recurrent hexagonal kidney stones (cystine calculi)

How is cystinuria managed?

Hydration, alkalinization of urine (potassium citrate), and tiopronin or penicillamine

Which enzyme deficiency leads to cystinosis?

CTNS gene mutation causing lysosomal cystine accumulation

Which vitamin is essential for both remethylation and transsulfuration pathways?

Vitamin B6 (for CBS) and Vitamin B12 (for methionine synthase)

What is the relationship between folate and homocysteine metabolism?

Folate (5-MTHF) donates a methyl group for methionine synthesis, reducing homocysteine levels

Which metabolic disorder can be treated with cysteamine?

Cystinosis (reduces cystine accumulation in lysosomes)

A 12-year-old boy presents with marfanoid habitus, lens dislocation (downward), developmental delay, and frequent episodes of thrombosis. Lab tests reveal increased plasma homocysteine and low cysteine levels. What is the most likely diagnosis?

Homocystinuria (CBS deficiency)

What is the best initial treatment for homocystinuria?

Pyridoxine (B6) supplementation if responsive, plus methionine restriction and cysteine supplementation.

A 25-year-old woman has a history of recurrent deep vein thrombosis (DVT) and stroke at a young age. Lab tests show elevated homocysteine levels, but normal methionine levels. She is heterozygous for an MTHFR mutation. What is the treatment?

Folic acid (5-MTHF), vitamin B6, and vitamin B12 to lower homocysteine levels.

A 9-year-old girl has a positive cyanide-nitroprusside test, revealing excess cystine in urine. She has a history of recurrent kidney stones with a hexagonal shape seen on urinalysis. What is the most likely diagnosis?

Cystinuria (defect in renal cystine transport)

How is cystinuria managed?

Hydration, urinary alkalinization (potassium citrate), and thiol-containing drugs like tiopronin or penicillamine.

A newborn with failure to thrive, photophobia, and Fanconi syndrome (proximal tubule dysfunction) is found to have cystine crystal accumulation in lysosomes. What is the likely diagnosis?

Cystinosis (CTNS gene mutation)

What is the definitive treatment for cystinosis?

Cysteamine therapy (reduces cystine accumulation in lysosomes).

A 10-year-old child presents with developmental delay and excretion of high cystathionine levels in urine, but no major clinical symptoms. What is the most likely diagnosis?

Cystathionuria (deficiency of cystathionase)

What is the treatment for cystathionuria?

No specific treatment required; generally benign condition.

What is the difference between cystinuria and cystinosis?

Cystinuria is a renal transport defect, leading to kidney stones. Cystinosis is a lysosomal storage disease, causing multisystem involvement.

Why does homocystinuria cause thrombosis?

Homocysteine damages endothelial cells, leading to increased clot formation.

How does vitamin B6 help in homocystinuria?

B6 is a cofactor for CBS, helping convert homocysteine to cystathionine, reducing toxic homocysteine levels.

A patient with homocystinuria is unresponsive to B6 therapy. What is the next step?

Restrict methionine intake and supplement with betaine, folate, and B12.

What is a key ocular feature differentiating homocystinuria from Marfan syndrome?

Homocystinuria causes downward lens dislocation, while Marfan syndrome causes upward lens dislocation.

A 7-year-old boy presents with mental retardation, osteoporosis, and vascular complications. Labs show elevated homocysteine and methionine, with decreased cysteine. What is the most likely enzyme deficiency?

Cystathionine beta-synthase (CBS) deficiency (Homocystinuria).

Why do patients with homocystinuria have osteoporosis?

Homocysteine interferes with collagen cross-linking, leading to weak bones.

Which metabolic disorder leads to increased risk of myocardial infarction and stroke at a young age?

Homocystinuria (due to hypercoagulability from elevated homocysteine).

Why does cystinosis cause Fanconi syndrome?

Cystine accumulation in lysosomes damages proximal tubule cells, leading to Fanconi syndrome (loss of glucose, phosphate, amino acids, bicarbonate).

What is the genetic inheritance pattern of homocystinuria, cystinuria, and cystinosis?

All three are autosomal recessive.

Why does cystinuria lead to kidney stones?

Cystine is poorly soluble in urine and precipitates into hexagonal stones in acidic pH.

Why do patients with cystinosis need corneal transplants?

Cystine crystals deposit in the cornea, leading to visual impairment.

Which metabolic disorder is diagnosed using the cyanide-nitroprusside test?

Cystinuria (detects excess cystine in urine).

Which two disorders are associated with lens dislocation?

Homocystinuria (downward) and Marfan syndrome (upward).