Q44. The metabolic disorders, Alkaptonuria and Phenylketonuria are caused by defects in the enzymes
P. Glucose-6-phosphatase
Q. Phenylalanine hydroxylase
R. Homogentisate 1,2-dioxygenase
S. Tyrosinase
Alkaptonuria and Phenylketonuria (PKU) are inborn errors of metabolism linked to specific enzyme deficiencies in tyrosine catabolism. The correct answer is (A) Q, R.
Enzyme-Deficiency Matching
Phenylketonuria results from defects in phenylalanine hydroxylase (Q), which converts phenylalanine to tyrosine, leading to phenylalanine buildup and neurotoxicity if untreated. Alkaptonuria stems from homogentisate 1,2-dioxygenase (R) deficiency, causing homogentisic acid accumulation, dark urine, and ochronosis [prior context]. Glucose-6-phosphatase (P) relates to glycogen storage disease, and tyrosinase (S) to albinism.
Option Analysis
-
(A) Q, R: Correct—Q for PKU (phenylalanine hydroxylase blocks Phe → Tyr); R for Alkaptonuria (homogentisate dioxygenase halts tyrosine breakdown).
-
(B) P, R: Incorrect—P (glucose-6-phosphatase) causes von Gierke disease, not these disorders.
-
(C) P, Q: Incorrect—P mismatches both; Q fits PKU but pairs wrongly.
-
(D) Q, S: Incorrect—S (tyrosinase) causes oculocutaneous albinism via melanin synthesis defect, not PKU or Alkaptonuria.
Introduction to Alkaptonuria Phenylketonuria Enzyme Defects
Alkaptonuria Phenylketonuria enzyme defects highlight tyrosine metabolism disorders key for GATE Life Sciences. Alkaptonuria involves homogentisate 1,2-dioxygenase failure, while PKU stems from phenylalanine hydroxylase deficiency, causing metabolite accumulation.
Phenylketonuria: Phenylalanine Hydroxylase Defect
PKU arises from mutations in the PAH gene, impairing phenylalanine-to-tyrosine conversion. Untreated, it leads to intellectual disability via phenylalanine toxicity. Newborn screening and low-Phe diets manage it effectively.
Alkaptonuria: Homogentisate Dioxygenase Deficiency
This rare disorder blocks homogentisic acid breakdown, resulting in urine darkening on oxidation and joint pigmentation (ochronosis). Enzyme replacement is under research [prior context].
GATE Q44: Matching Metabolic Disorders to Enzymes
Options test precise enzyme recall in amino acid pathways.
| Option | Enzymes | Disorder Match | Correct? | Reason |
|---|---|---|---|---|
| (A) Q, R | Phenylalanine hydroxylase, Homogentisate dioxygenase | PKU, Alkaptonuria | Yes | Exact defects |
| (B) P, R | Glucose-6-phosphatase, Homogentisate dioxygenase | No | No | P is glycogenolysis [prior] |
| (C) P, Q | Glucose-6-phosphatase, Phenylalanine hydroxylase | No | No | P irrelevant |
| (D) Q, S | Phenylalanine hydroxylase, Tyrosinase | No | No | S for albinism |
GATE Prep Tips for Biochemistry
For Jaipur students targeting GATE, link Alkaptonuria Phenylketonuria enzyme defects to pathways: Phe → Tyr (Q), Tyr → homogentisate → maleylacetoacetate (R). Practice PYQs on inborn errors.
