Q.6 Which of the following enzymes is defective in galactosemia- a fatal genetic disorder in infants?
1. Glucokinase
2. Galactokinase
3, UDP-galactose 4- epimerase
4. Galactose-I •phosphate uridyltransferase
Answer: Option 4 (Galactose-1-phosphate uridyltransferase, ID: 68019147824).
Classic galactosemia, the fatal infant disorder, results from deficiency in galactose-1-phosphate uridyltransferase (GALT), causing toxic galactose-1-phosphate buildup leading to liver failure, cataracts, and sepsis.
Option Analysis
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1. Glucokinase: Glucose phosphorylating enzyme in glycolysis; unrelated to galactose metabolism pathway.
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2. Galactokinase: Converts galactose → galactose-1-phosphate (type II galactosemia); causes cataracts but milder, non-fatal effects.
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3. UDP-galactose 4-epimerase: Interconverts UDP-galactose ↔ UDP-glucose (type III); rare, variable symptoms, not typically fatal.
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4. Galactose-1-phosphate uridyltransferase (GALT): Critical step: galactose-1-P + UDP-glucose → glucose-1-P + UDP-galactose. Deficiency = classic (type I) galactosemia with rapid mortality without lactose-free diet.
Enzyme Pathway Comparison
| Enzyme | Step in Galactose Metabolism | Disorder Type | Severity |
|---|---|---|---|
| Glucokinase | None (glucose only) | N/A | N/A |
| Galactokinase | Gal → Gal-1-P | Type II | Mild |
| UDP-Gal 4-epimerase | UDP-Gal ↔ UDP-Glc | Type III | Variable |
| GALT | Gal-1-P + UDP-Glc → G1P + UDP-Gal | Type I (Classic) | Fatal |
Enzyme defective galactosemia fatal genetic disorder infants refers to galactose-1-phosphate uridyltransferase (GALT) deficiency causing classic galactosemia—the most severe form appearing days after milk feeding.
Galactose Metabolism Pathway Breakdown
Lactose → glucose + galactose → (galactokinase) galactose-1-P → (GALT) glucose-1-P + UDP-galactose → (epimerase) UDP-glucose recycling. GALT block causes galactose-1-P toxicity damaging liver, brain, eyes.
Disorder Types Comparison
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Classic (Type I, GALT): Vomiting, jaundice, E. coli sepsis, cataracts within week 1; 75% mortality untreated
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Type II (Galactokinase): Cataracts only
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Type III (Epimerase): Mild or asymptomatic
Clinical Diagnosis Table
| Finding | Classic GALT Deficiency | Treatment |
|---|---|---|
| Onset | 3-14 days post-milk | Lifelong lactose-free diet |
| Labs | ↑ Galactose-1-P, ↑ gal-1-P uridine | Newborn screening |
| Prognosis | Good if treated early | Neurocognitive issues persist |
GATE Life Sciences tip: “Fatal genetic disorder infants” = classic galactosemia = GALT deficiency. Master enzyme defective galactosemia for biochemistry PYQs.
