Organic Chemistry · Biomolecules: Lipids
Biosynthesis of Steroids
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In 30 seconds
The body builds cholesterol and every steroid hormone from a surprisingly simple starting material: acetyl coenzyme A (Acetyl-CoA Two-carbon thioester that feeds the pathway), a two-carbon thioester. The pathway proceeds in three phases. First, three acetyl-CoA units condense to form Mevalonate Six-carbon intermediate from HMG-CoA reduction Full entry → (six carbons). Second, mevalonate is converted into isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP) — the activated five-carbon isoprene units that also feed terpenoid biosynthesis (Topic 27.5). Third, six isoprene units join to give farnesyl pyrophosphate (FPP); two FPP molecules couple tail-to-tail to make Squalene 30-carbon acyclic hydrocarbon with six double bonds Full entry → (\(C{30}H{50}\)), an acyclic hydrocarbon with six double bonds. Squalene is oxidized to Squalene 2,3-epoxide Epoxidized squalene Full entry →, and an enzyme-catalyzed cyclization folds the chain into the four fused rings of Lanosterol First tetracyclic sterol of the pathway (30 C) Full entry →. Demethylations and double-bond migrations then convert lanosterol to cholesterol (\(C{27}H{46}O\)).
Every step is an ordinary reaction type — Claisen condensation, aldol addition, decarboxylation, phosphorylation, epoxidation, and carbocation-driven cyclization — a showcase of how simple mechanistic motifs assemble a complex polycyclic natural product.
Why this matters
- The statin mechanism: HMG-CoA reductase Enzyme that reduces HMG-CoA to mevalonate Full entry →, which makes mevalonate, is the rate-determining step of cholesterol synthesis. Statins (atorvastatin, simvastatin) competitively inhibit it — one of the most prescribed drug mechanisms in medicine.
- Cholesterol balance: The body synthesizes most of its cholesterol (much of it in the liver). Synthesis explains why diet alone rarely controls cholesterol and why enzyme inhibition works.
- Hormone precursor: All steroid hormones are downstream products of cholesterol, so blocking synthesis can affect hormone production.
- Isoprene connection: The same activated C5 units build terpenoids, vitamin A, vitamin E, coenzyme Q; steroid synthesis is one branch of a shared isoprenoid tree.
- Exams: Pathway ordering, carbon bookkeeping (6 isoprene units = 30 C, 3 methyls lost), and the rate-limiting enzyme are standard questions.
The college version
Core Concepts
Phase 1: Acetyl-CoA to mevalonate
Two acetyl-CoA molecules condense in a Claisen-like reaction to give acetoacetyl-CoA; a third adds in an aldol-like reaction to give 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). HMG-CoA reductase reduces the thioester to a primary alcohol using two NADPH, producing mevalonate (\(C6H{12}O_3\)). This slow, irreversible reduction is the committed, rate-determining step — the statin target. High cholesterol down-regulates the enzyme.
Phase 2: Mevalonate to activated isoprene units
Mevalonate is phosphorylated twice (two ATPs) to mevalonate 5-pyrophosphate; a third phosphorylation sets up a decarboxylative elimination — loss of \(CO_2\) and water gives isopentenyl pyrophosphate (IPP), the activated five-carbon unit. An isomerase gives dimethylallyl pyrophosphate (DMAPP). The pyrophosphate leaving group (\(PP_i\)) drives every later C–C bond-forming step.
Phase 3: Isoprene coupling and squalene formation
Isoprene units join head-to-tail: pyrophosphate leaves DMAPP, generating an allylic carbocation attacked by the double bond of IPP. The product, geranyl pyrophosphate (GPP) (\(C{10}\)), adds another IPP to give farnesyl pyrophosphate (FPP) (\(C{15}\)). Two FPP molecules join tail-to-tail (NADPH reductive coupling) to give squalene.
Phase 4: Epoxidation and the cyclization cascade
A monooxygenase uses \(O2\) and NADPH to epoxidize the terminal double bond of squalene, giving squalene 2,3-epoxide. Protonation opens the epoxide to a tertiary carbocation, and the enzyme folds the chain so a cascade of electrophilic cyclizations builds rings A, B, C, then D, with hydride and methyl shifts — carbocation chemistry converting one acyclic chain into the four fused rings of lanosterol (\(C{30}H_{50}O\)).
Phase 5: Lanosterol to cholesterol
Lanosterol is a 30-carbon sterol with extra methyls at C-4 (two) and C-14 (one). About nineteen enzyme-catalyzed steps remove those methyls as \(CO2\), move the C-8 double bond to C-5, and reduce the C-24 side-chain double bond. The result is cholesterol, \(C{27}H_{46}O\) — 30 carbons minus three.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| HMG-CoA | Mevalonate | HMG-CoA is the substrate; mevalonate is the product; the reductase step, not the molecule, is the drug target |
| Squalene | Cholesterol | Squalene is acyclic (all 6 DBE are double bonds); cholesterol is tetracyclic (4 rings + 1 C=C); cyclization converts one into the other |
| Isoprene unit (5 C) | Mevalonate (6 C) | IPP/DMAPP are the coupling units; mevalonate loses \(CO_2\) to become them |
| IPP | DMAPP | Isomers: IPP's double bond sits one carbon further from the pyrophosphate; DMAPP is the electrophilic partner that starts each coupling |
| Lanosterol (30 C) | Cholesterol (27 C) | Lanosterol carries three extra methyls (C-4 ×2, C-14); their removal gives cholesterol |
| "Synthesis blocked" | "Enzyme blocked" | Drugs block enzymes, not "cholesterol" — name the enzyme (HMG-CoA reductase) in mechanism questions |
| Statin mechanism | Bile-acid sequestrant mechanism | Statins inhibit liver synthesis; sequestrants bind bile acids in the gut — different targets, same goal |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Making cholesterol is like building a long train from little two-seat cars (acetyl-CoA), then folding that train up into a flat four-room house (the steroid skeleton). One special worker (HMG-CoA reductase) controls how fast the whole thing runs, and some medicines put a brake on that worker so the body builds fewer houses.
Worked example
Example 1: Carbon bookkeeping — how many acetyl-CoA for one cholesterol?
Cholesterol has 27 carbons, and squalene has 30. Trace the carbon budget from acetyl-CoA (2 C each).
Write the relationship first. Six isoprene units (5 C each) give squalene: \(6 \times 5 = 30\) C. Each mevalonate (6 C) loses one carbon as \(CO_2\) to become IPP (5 C), and each mevalonate is built from three acetyl-CoA.
Compute the acetyl-CoA needed for squalene:
6 IPP × 3 acetyl-CoA1 IPP = 18 acetyl-CoA
Then account for the three methyls lost in lanosterol → cholesterol:
30 - 3 = 27 carbons in cholesterol
Answer: Eighteen acetyl-CoA supply squalene's 30 carbons (six lost as \(CO2\) during mevalonate decarboxylation); three more are removed later, giving cholesterol's \(C{27}\) formula — a favorite exam calculation.
Example 2: Degrees of unsaturation of squalene
Squalene is \(C{30}H{50}\). Use the DBE formula to predict rings + double bonds, and reconcile this with its role as the acyclic cyclization precursor.
Write the formula:
DBE = 2C + 2 + N - H - X2
Substitute \(C = 30\) and \(H = 50\):
DBE = 2(30) + 2 - 502 = 60 + 2 - 502 = 122 = 6
Answer: DBE = 6 — squalene is acyclic (0 rings), so all six unsaturations are C=C bonds, matching its six alkene units and its oily liquid nature. Lanosterol (\(C{30}H{50}O\), DBE = 6) shows the same count redistributed as four rings + two double bonds after cyclization.
Example 3: Predicting the committed step from regulation
A researcher finds that treating liver cells with a drug that blocks NADPH-dependent reduction of HMG-CoA lowers cholesterol production by 90% within hours. Which enzyme did the drug inhibit, and why is the effect so large?
Reason: The only NADPH-dependent reduction of HMG-CoA is catalyzed by HMG-CoA reductase. Because this step is rate-determining, inhibiting it throttles everything downstream.
Answer: The drug inhibits HMG-CoA reductase — the same logic by which statins work. A rate-determining step controls flux through the whole pathway, so downstream steps cannot compensate.
Key takeaways
- Pathway: acetyl-CoA → acetoacetyl-CoA → HMG-CoA → mevalonate → IPP/DMAPP → GPP → FPP → squalene → squalene 2,3-epoxide → lanosterol → cholesterol.
- HMG-CoA reductase is rate-determining; statins inhibit it; it uses NADPH and is feedback-regulated by cholesterol.
- Six activated isoprene units build squalene (\(C{30}H{50}\)); three acetyl-CoA (6 C) supply each mevalonate (6 C).
- Squalene epoxidation needs \(O_2\) + NADPH; cyclization to lanosterol is a carbocation cascade driven by epoxide protonation.
- Lanosterol → cholesterol removes three methyls (30 C → 27 C) and rearranges double bonds.
- Mevalonate → IPP involves phosphorylation + decarboxylation; pyrophosphate is the universal leaving group.
- In liver cells HMG-CoA reductase sits in the ER membrane; downstream enzymes act in the cytosol.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
List the pathway from acetyl-CoA to cholesterol, naming the C2, C5, C6, C15, and C30 intermediates.
Show answer
Acetyl-CoA (C2) → acetoacetyl-CoA (C4) → HMG-CoA (C6) → mevalonate (C6) → IPP/DMAPP (C5) → GPP (C10) → FPP (C15) → squalene (C30) → squalene 2,3-epoxide → lanosterol (C30) → cholesterol (C27).
Which enzyme is the rate-determining step, and what drug class inhibits it?
Show answer
HMG-CoA reductase, which reduces HMG-CoA to mevalonate; statins (atorvastatin, simvastatin, etc.) inhibit it competitively.
How many isoprene units (IPP/DMAPP) are incorporated into squalene? How many carbons does each contribute?
Show answer
Six IPP units (5 C each = 30 C), coupled as two FPP units (15 C each) tail-to-tail.
Where do the three missing carbons go when lanosterol (30 C) becomes cholesterol (27 C)?
Show answer
Removed as \(CO_2\) (oxidative decarboxylation of the C-4 methyls and C-14 methyl) during lanosterol → cholesterol conversions.
What role does the epoxide of squalene play in the cyclization?
Show answer
Epoxide protonation generates the carbocation that triggers the electrophilic cyclization cascade, folding the chain into rings A–D.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Acetyl-CoA
- Two-carbon thioester that feeds the pathway
- Mevalonate
- Six-carbon intermediate from HMG-CoA reduction
- IPP / DMAPP
- Activated five-carbon isoprene units (isomers)
- Squalene
- 30-carbon acyclic hydrocarbon with six double bonds
- Squalene 2,3-epoxide
- Epoxidized squalene
- Lanosterol
- First tetracyclic sterol of the pathway (30 C)
- HMG-CoA reductase
- Enzyme that reduces HMG-CoA to mevalonate
- Head-to-tail / tail-to-tail coupling
- How isoprene units join (C1 to C4, or C4 to C4)
- Acetyl CoA
- The two-carbon thioester \(CH_3C(=O)SCoA\)
Sources & references
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