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This diagram shows the Mevalonate Pathway — specifically how it branches from a central intermediate (FPP/IPP) to produce not just cholesterol, but several other critical biological molecules including Coenzyme Q (ubiquinone). Here is a full breakdown:
The Mevalonate Pathway — Full Explanation
🔵 Entry Point: HMG-CoA → Mevalonate
HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) is the starting substrate.
The enzyme HMG-CoA reductase converts it to Mevalonate, using 2 NADPH molecules. This is the rate-limiting, key regulatory step of the entire pathway — and the target of statins (e.g., atorvastatin, rosuvastatin), which block this enzyme to lower cholesterol.
— Harper's Illustrated Biochemistry, 32nd Ed., p. 272; Lippincott's Biochemistry, 8th Ed., p. 620
🔵 Mevalonate → IPP (Isopentenyl Pyrophosphate)
Mevalonate undergoes three sequential phosphorylations (using ATP) followed by decarboxylation to form IPP, the universal 5-carbon isoprenoid building block. IPP can also be isomerized to dimethylallyl-PP (DPP).
IPP has two immediate fates shown in the diagram:
- Conversion to Isopentenyl adenosine → incorporated into t-RNA (important for protein synthesis — shown in red in the diagram)
- Condensation onward to FPP
🔵 IPP → FPP (Farnesyl Pyrophosphate) — The Central Branch Point
IPP + DPP → Geranyl-PP (GPP, C10) → + another IPP → FPP (Farnesyl Pyrophosphate, C15)
FPP is the critical branch point from which the pathway diverges into multiple products:
| Branch | Product | Biological Importance |
|---|---|---|
| FPP + FPP via squalene synthase | Squalene | Precursor to cholesterol |
| FPP + IPP via trans-prenyl transferase | Polyprenyl-PP → side chain of Ubiquinone (CoQ10) | Electron carrier in mitochondrial ETC |
| FPP + many IPP via cis-prenyl transferase | Polyprenyl-PP → Dolichol / Dolichyl-P | Carrier lipid for N-linked glycosylation of proteins (shown in red) |
| FPP farnesylated onto proteins via Farnesyl-protein transferase | Isoprenylated/prenylated proteins | Membrane anchoring, signal transduction (e.g., Ras oncogene) |
| FPP + IPP via all-trans-GGPP synthase → GGPP → farnesyl-protein transferase | Geranylgeranylated proteins | Also signal transduction & protein localization |
🔵 Squalene Branch → Cholesterol
Squalene (C30) undergoes:
- Oxidation → 2,3-Squalene epoxide (by squalene epoxidase)
- Ring cyclization by Oxidosqualene cyclase → Lanosterol
- Multiple enzymatic steps → Cholesterol
🔵 CoQ10 (Ubiquinone) Branch
From FPP, trans-prenyl transferase adds additional isoprene units to build the long hydrophobic side chain of Coenzyme Q10. The benzoquinone ring head comes from tyrosine (or phenylalanine) → 4-OH-benzoic acid → combined with polyprenyl-PP via Polyprenyl-4-hydroxy-benzoic acid transferase → Polyprenyl-4-OH-benzoic acid → eventually Coenzyme Q.
This is why statins, by blocking HMG-CoA reductase, also reduce CoQ10 synthesis — a proposed contributor to statin-associated myopathy.
Why This Diagram Matters Clinically
| Consequence | Mechanism |
|---|---|
| Statins lower cholesterol | Block HMG-CoA reductase, reducing all downstream products |
| Statin myopathy | Reduced CoQ10 synthesis impairs mitochondrial function in muscle |
| Bisphosphonates (e.g., alendronate) | Inhibit farnesyl-PP synthase → reduce prenylation of osteoclast proteins |
| Ras oncogene activity | Requires farnesylation (prenylation) for membrane anchoring and signaling |
| CDG syndromes | Defects in dolichol pathway impair protein glycosylation |
Summary Map
HMG-CoA
↓ [HMG-CoA reductase — statin target]
Mevalonate
↓
IPP ──→ Isopentenyl adenosine → t-RNA (protein synthesis)
↓
FPP ──→ Squalene → Lanosterol → Cholesterol
──→ GGPP → Geranylgeranylated proteins (signal transduction)
──→ Polyprenyl-PP + 4-OH-benzoate → CoQ10 (ubiquinone)
──→ Polyprenyl-PP → Dolichol/Dolichyl-P → protein glycosylation
──→ Farnesylated proteins (Ras, signal transduction)
— Harper's Illustrated Biochemistry, 32nd Ed.; Lippincott's Biochemistry, 8th Ed., pp. 619–622; Tietz Textbook of Laboratory Medicine, 7th Ed.
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