In Menke’s disease, which of the following enzymes would be affected?

Correct Answer: D. Lysyl oxidase

Menkes disease is an X-linked recessive disorder of copper metabolism caused by mutations in the ATP7A gene, which encodes a copper-transporting ATPase. This defect impairs copper delivery to cuproenzymes throughout the body. Lysyl oxidase is a copper-dependent enzyme essential for cross-linking collagen and elastin by oxidizing lysine and hydroxylysine residues to aldehydes (allysine and hydroxyallysine), which then spontaneously condense to form Schiff bases and aldol products. Without adequate copper, lysyl oxidase activity is severely reduced, leading to defective collagen and elastin cross-linking. This explains the clinical hallmarks of Menkes disease: connective tissue fragility (kinky hair, skin laxity, vascular rupture), bone abnormalities (osteoporosis, metaphyseal spurs), and neurological deterioration (due to defective dopamine β-hydroxylase and cytochrome c oxidase, also copper-dependent). The diagnosis is confirmed by low serum copper and ceruloplasmin levels. Early copper supplementation (copper histidinate or copper chloride) may slow progression if initiated before neurological damage becomes irreversible.

Why the other options are wrong

A. Prolyl oxidase — Prolyl oxidase is not a recognized enzyme in collagen metabolism. The correct enzyme involved in proline modification is prolyl hydroxylase (which requires vitamin C and iron, not copper), which hydroxylates proline residues after collagen synthesis. This is a distractor that confuses students unfamiliar with the distinction between hydroxylation (vitamin C-dependent) and oxidative cross-linking (copper-dependent) steps in collagen maturation. B. Prolyl hydroxylase — Prolyl hydroxylase is a vitamin C and iron-dependent enzyme, not copper-dependent. It hydroxylates proline to hydroxyproline in the collagen triple helix, which is essential for stability but occurs before cross-linking. Menkes disease does not affect this enzyme; patients with vitamin C deficiency (scurvy) show prolyl hydroxylase deficiency. This option tests whether students confuse the two distinct enzymatic steps in collagen maturation. C. Lysyl hydroxylase — Lysyl hydroxylase is also vitamin C and iron-dependent (not copper-dependent) and hydroxylates lysine residues to hydroxylysine in collagen. While hydroxylysine is the substrate for lysyl oxidase cross-linking, the hydroxylase itself is not affected in Menkes disease. This is a classic NBE trap: both lysyl hydroxylase and lysyl oxidase involve lysine, but only the latter is copper-dependent and affected in Menkes disease.

High-Yield Facts

Mnemonics

Cu-dependent vs. Vit C-dependent in Collagen Cu-LOX (Lysyl Oxidase = copper) vs. VitC-ProHy/LysHy (Prolyl/Lysyl Hydroxylase = vitamin C). Cross-linking needs copper; hydroxylation needs vitamin C. Menkes = Kinky Hair + Copper Menkes → Metal (copper) defect → Messed-up collagen cross-links → Manifests as kinky hair, vascular fragility, neurodegeneration. Use when you see 'kinky hair disease.'

NBE Trap

NBE pairs "lysine" with both lysyl hydroxylase and lysyl oxidase to trap students who recognize lysine involvement but don't distinguish between vitamin C-dependent hydroxylation and copper-dependent oxidative cross-linking. The presence of "lysyl" in three options (B, C, D) further obscures the copper-dependency distinction.

Clinical Pearl

In Indian pediatric practice, Menkes disease typically presents in the first 2–3 months of life with poor feeding, developmental delay, and characteristic "steely" or kinky hair. Early recognition and copper supplementation (copper histidinate 50–100 µg/kg/day IV) can prevent catastrophic neurological deterioration; delayed diagnosis often results in severe seizures and death by age 3. Genetic counseling for X-linked inheritance is critical in affected families.

_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 3 (Genetic Disorders); Harper's Biochemistry, Ch. 49 (Collagen); KD Tripathi Pharmacology, Ch. 57 (Trace Elements)_