Question 39:
Which one of the following is a disease caused by defects in enzymes necessary for heme biosynthesis?
Answer: (B) Porphyria. This group of disorders arises from enzyme deficiencies in the heme biosynthesis pathway.
Option Explanations
Heme synthesis involves 8 enzymes; defects cause porphyrin accumulation. Each option tests metabolic disease knowledge.
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(A) Lesch Nyhan syndrome: Purine metabolism defect (HGPRT deficiency); leads to hyperuricemia/self-mutilation, not heme-related.
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(B) Porphyria: Correct—genetic deficiencies in heme enzymes (e.g., uroporphyrinogen synthase in acute intermittent porphyria) cause porphyrin buildup, neurovisceral/skin symptoms.
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(C) Huntington’s disease: Neurodegenerative; CAG repeat expansion in HTT gene affects neuronal function, unrelated to heme.
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(D) Marfan Syndrome: Connective tissue disorder from fibrillin-1 mutation; aortic/skeletal issues, not enzymatic heme defect.
Introduction
Disease caused by defects enzymes heme biosynthesis defines porphyrias, critical for GATE Life Sciences biochemistry. These arise from 8-step pathway blocks, unlike unrelated genetic disorders.
Heme Pathway Overview
Heme synthesis spans mitochondria/cytosol: ALA synthase → ferrochelatase. Defects accumulate toxic porphyrins.
Porphyrias (e.g., EPP, AIP) manifest as photosensitivity, abdominal pain.
Disease Comparison
| Disease | Enzyme Defect | Key Symptoms |
|---|---|---|
| Porphyria | Heme synthesis enzymes | Porphyrin accumulation, neurocutaneous |
| Lesch-Nyhan | HGPRT (purine) | Hyperuricemia, gout |
| Huntington’s | HTT gene repeat | Chorea, dementia |
| Marfan | Fibrillin-1 | Tall stature, lens dislocation |
GATE MCQ Solved
Question 39: Disease from heme biosynthesis enzyme defects?
(A) Lesch-Nyhan (B) Porphyria (C) Huntington’s (D) Marfan
(B)—Directly matches pathway enzymopathies.
