A 12-year-old boy is brought to the emergency department because of acute onset abdominal pain. On arrival, he also complains of nausea and shortness of breath in addition to epigastric pain. He has previously been admitted to the hospital several times for respiratory infections with Pseudomonas species and uses a nebulizer and a chest wall oscillation vest at home. The patient's acute condition is found to be due to premature activation of an enzyme that normally interacts with the brush border. Which of the following describes the activity of this enzyme?

Activates pancreatic enzyme precursors

An 18-month-old boy of Ashkenazi-Jewish descent presents with loss of developmental milestones. On ocular exam, a cherry-red macular spot is observed. No hepatomegaly is observed on physical exam. Microscopic exam shows lysosomes with onion-skin appearance. What is the most likely underlying biochemical abnormality?

Accumulation of GM2 ganglioside

Accumulation of ceramide trihexoside

Accumulation of glucocerebroside

Accumulation of galactocerebroside

A 6-month-old boy is referred to a geneticist after he is found to have persistent hypotonia and failure to thrive. He has also had episodes of what appears to be respiratory distress and has an enlarged heart on physical exam. There is a family history of childhood onset hypertrophic cardiomyopathy, so a biopsy is performed showing electron dense granules within the lysosomes. Genetic testing is performed showing a defect in glycogen processing. A deficiency in which of the following enzymes is most likely to be responsible for this patient's symptoms?

Lysosomal alpha 1,4-glucosidase

A patient in a phase 1 trial for a novel epoxide reductase inhibitor, being studied for stroke prophylaxis, develops pain and erythema on the right thigh two days after starting the trial. This rapidly progresses to a purpuric rash with necrotic bullae within 24 hours. Lab results show a PTT of 29 seconds, PT of 28 seconds, and INR of 2.15. What is the most likely pathogenesis of this condition?

Decreased antithrombin III activity

Activator protein deficiencies for USMLE Step 3: High-Yield Biochemistry Lessons

Activator Proteins - The Lipid Chaperones

Sphingolipid Activator Proteins (SAPs) are small, non-enzymatic glycoproteins essential for the lysosomal degradation of sphingolipids. They act as "lipid chaperones."
Function: Solubilize lipid substrates, extract them from membranes, and present them to specific lysosomal hydrolases.
Two main types:
- Saposins (A, B, C, D): Derived from a single precursor, prosaposin.
- GM2 Activator Protein (GM2AP): Required for the degradation of GM2 ganglioside by Hexosaminidase A.

⭐ A single PSAP gene codes for a large precursor protein that is cleaved into the four essential saposins (A, B, C, and D).

Prosaposin structure and saposin A, B, C, D domains

Mechanism:

Clinical Correlation: Deficiency of an activator protein leads to a sphingolipidosis, presenting similarly to a deficiency of the corresponding enzyme.

Clinical Syndromes - When Chaperones Fail

Activator proteins (Saposins A, B, C, and GM2A) are crucial for lipid catabolism by presenting lipids to their respective hydrolases. Deficiencies mimic the corresponding enzyme-deficient lysosomal storage disease but with normal in vitro enzyme activity.

⭐ The AB variant of Tay-Sachs disease is clinically identical to classic Tay-Sachs, but enzyme assays for Hexosaminidase A are deceptively normal because the enzyme itself is functional-only its activator is missing.

Disease	Deficient Activator	Enzyme (Still Active)	Accumulated Lipid	Key Features
Tay-Sachs (AB variant)	GM2A	Hexosaminidase A	GM2 ganglioside	Neurodegeneration, cherry-red spot on macula, hyperacusis.
Gaucher Disease	Saposin C	Glucocerebrosidase	Glucocerebroside	Hepatosplenomegaly, pancytopenia, bone crises. Milder phenotype.
Metachromatic Leukodystrophy	Saposin B	Arylsulfatase A	Sulfatide	Central & peripheral demyelination, ataxia, dementia.
Krabbe Disease	Saposin A	Galactosylceramidase	Galactosylceramide	Peripheral neuropathy, optic atrophy, globoid cells.
Farber Disease	Saposin B	Acid Ceramidase	Ceramide	Painful joints, subcutaneous nodules, hoarse cry.
GM1 Gangliosidosis	Saposin B	β-Galactosidase	GM1 Ganglioside	Neurodegeneration, hepatosplenomegaly, skeletal dysplasia.

High‑Yield Points - ⚡ Biggest Takeaways

Activator protein deficiencies are a rare type of lysosomal storage disease where the hydrolase enzyme is intact but non-functional due to a faulty cofactor.

These disorders clinically mimic their enzyme-deficiency counterparts; for instance, GM2-AP deficiency presents like Tay-Sachs disease.

Key function: Activator proteins (e.g., saponins) solubilize lipid substrates, making them accessible to enzymes.

Diagnosis is tricky: Enzyme activity in lab assays is normal or even ↑, a major clue.

Genetic analysis is required for definitive diagnosis.

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