The key enzyme of cholesterol synthesis is?

UPUMS Nsg officer-2023

Biochemistry & Nutrition

Easy

Appeared in: UPUMS Nsg officer-2023

Explanation

HMG-CoA reductase is the rate-limiting enzyme in the cholesterol synthesis pathway, meaning it controls the overall speed of production.
It catalyzes the irreversible conversion of HMG-CoA to mevalonate, which is the first committed step unique to cholesterol synthesis.
The activity of this enzyme is the primary site of regulation for the entire pathway, including feedback inhibition by cholesterol and hormonal control.
Clinically, it is the target of statin drugs (e.g., atorvastatin, simvastatin), which are widely prescribed to lower high cholesterol levels.

Option B: HMG-CoA dehydrogenase is not a recognized enzyme in the primary metabolic pathways of cholesterol or ketone body synthesis.
Option C: HMG-CoA synthase creates the precursor molecule HMG-CoA. However, this molecule is a common substrate for both cholesterol synthesis (in the cytosol) and ketone body formation (in the mitochondria). Therefore, this enzyme is not the committed or rate-limiting step specifically for cholesterol.
Option D: HMG-CoA lyase is an enzyme exclusively involved in ketogenesis (the formation of ketone bodies). It acts in the mitochondria to cleave HMG-CoA into acetoacetate, a ketone body.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Rate-limiting enzyme in cholesterol biosynthesis as background academic context rather than a clinical decision trigger.
Understanding the role of HMG-CoA reductase is crucial for managing patients with hypercholesterolemia, a major risk factor for atherosclerosis and cardiovascular disease.
Nurses administer HMG-CoA reductase inhibitors (statins) and must educate patients about their mechanism of action, the importance of adherence, and potential side effects like myopathy (muscle pain).
What if? If a patient has a genetic defect causing overactivity of HMG-CoA reductase, they would likely have familial hypercholesterolemia. This condition requires aggressive treatment with high-intensity statins to prevent premature heart attacks and strokes.

How to Approach the Question

First, identify the core of the question: it asks for the 'key' enzyme in cholesterol synthesis. In biochemistry, 'key' almost always refers to the rate-limiting or major regulatory step.
Recall the major metabolic pathways involving lipids. Cholesterol synthesis and ketogenesis are two related pathways that share an intermediate, HMG-CoA.
Analyze the options, which are all enzymes acting on HMG-CoA. This is a strong clue that the answer lies in differentiating their specific roles.
Differentiate the functions: HMG-CoA synthase makes HMG-CoA. HMG-CoA lyase breaks HMG-CoA for ketone bodies. HMG-CoA reductase reduces HMG-CoA for cholesterol.
Conclude that the reductase step is the first irreversible reaction committed solely to cholesterol synthesis, making it the 'key' regulatory enzyme.

Concept Tested & Keywords

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