Cholesterol Biosynthesis (Mevalonate Pathway)

1. 1

   2 acetyl-CoA → acetoacetyl-CoA + CoA-SH

   Cytosolic acetoacetyl-CoA thiolase 2.3.1.9 ST-0091 Reversible CoA-SH
   In Acetyl-CoA

   Out Acetoacetyl-CoA CoA-SH
   › Notes

   Cytosolic acetoacetyl-CoA thiolase condenses two acetyl-CoA molecules. This reversible reaction supplies the C4 intermediate.

2. 2

   Acetoacetyl-CoA + acetyl-CoA + H2O → HMG-CoA + CoA-SH

   Cytosolic HMG-CoA synthase (HMGCS1) 2.3.3.10 ST-0092 Irreversible/directional CoA-SH H2O
   In Acetoacetyl-CoA Acetyl-CoA H2O

   Out HMG-CoA CoA-SH
   › Notes

   Cytosolic HMG-CoA synthase (HMGCS1) forms 3-hydroxy-3-methylglutaryl-CoA. It is distinct from mitochondrial HMGCS2 of ketogenesis.

3. 3

   HMG-CoA + 2 NADPH + 2 H+ → mevalonate + 2 NADP+ + CoA-SH

   HMG-CoA reductase (HMGCR) 1.1.1.34 ST-0093 Irreversible rate-limiting NADP+ NADPH CoA-SH
   In HMG-CoA NADPH H+

   Out Mevalonate NADP+ CoA-SH
   › Notes

   ER-associated HMG-CoA reductase (HMGCR) performs a two-stage reduction via mevaldehyde. This is the rate-limiting, committed, and essentially irreversible step of cholesterol biosynthesis and the target of statins.

4. 4

   Mevalonate + ATP → 5-phosphomevalonate + ADP

   Mevalonate kinase 2.7.1.36 ST-0094 Irreversible/directional ATP
   In Mevalonate ATP

   Out 5-Phosphomevalonate ADP
   › Notes

   Mevalonate kinase phosphorylates the C5 hydroxyl group. ATP is required.

5. 5

   5-Phosphomevalonate + ATP → 5-pyrophosphomevalonate + ADP

   Phosphomevalonate kinase 2.7.4.2 ST-0095 Irreversible/directional ATP
   In 5-Phosphomevalonate ATP

   Out 5-Pyrophosphomevalonate ADP
   › Notes

   Phosphomevalonate kinase adds a second phosphate using ATP.

6. 6

   5-Pyrophosphomevalonate + ATP → 3-phospho-5-pyrophosphomevalonate + ADP

   Mevalonate-5-pyrophosphate decarboxylase 4.1.1.33 ST-0096 Irreversible/directional ATP
   In 5-Pyrophosphomevalonate ATP

   Out 3-Phospho-5-pyrophosphomevalonate ADP
   › Notes

   Mevalonate-5-pyrophosphate decarboxylase phosphorylates the C3 hydroxyl. ATP is consumed; this preparatory phosphorylation enables the subsequent decarboxylation.

7. 7

   3-Phospho-5-pyrophosphomevalonate → isopentenyl pyrophosphate (IPP) + CO2 + Pi

   Mevalonate-5-pyrophosphate decarboxylase 4.1.1.33 ST-0097 Irreversible/directional Pi
   In 3-Phospho-5-pyrophosphomevalonate

   Out Isopentenyl pyrophosphate (IPP) CO2 Pi
   › Notes

   The same mevalonate-5-pyrophosphate decarboxylase catalyzes decarboxylation/elimination to form activated C5 isoprene unit IPP. Together, steps 4–7 consume three ATP per IPP.

8. 8

   IPP ⇌ dimethylallyl pyrophosphate (DMAPP)

   Isopentenyl pyrophosphate isomerase 5.3.3.2 ST-0098 Reversible
   In Isopentenyl pyrophosphate (IPP)

   Out Dimethylallyl pyrophosphate (DMAPP)
   › Notes

   Isopentenyl pyrophosphate isomerase rearranges IPP to the allylic isomer DMAPP. No net ATP or NADPH is required.

9. 9

   DMAPP + IPP → geranyl pyrophosphate (GPP) + PPi

   Geranyl pyrophosphate synthase 2.5.1.1 ST-0099 Irreversible/directional PPi
   In Dimethylallyl pyrophosphate (DMAPP) Isopentenyl pyrophosphate (IPP)

   Out Geranyl pyrophosphate (GPP) PPi
   › Notes

   Geranyl pyrophosphate synthase carries out a head-to-tail prenyl transfer to form the C10 isoprenoid. Pyrophosphate release helps drive the reaction.

10. 10

    GPP + IPP → farnesyl pyrophosphate (FPP) + PPi

    Farnesyl pyrophosphate synthase 2.5.1.10 ST-0100 Irreversible/directional PPi
    In Geranyl pyrophosphate (GPP) Isopentenyl pyrophosphate (IPP)

    Out Farnesyl pyrophosphate (FPP) PPi
    › Notes

    Farnesyl pyrophosphate synthase performs the next head-to-tail condensation, forming C15 FPP. FPP is also a branch-point precursor for ubiquinone, dolichol, and protein prenylation.

11. 11

    2 FPP + NADPH + H+ → squalene + 2 PPi + NADP+

    ER-associated squalene synthase 2.5.1.21 ST-0101 Irreversible/directional NADP+ NADPH PPi
    In Farnesyl pyrophosphate (FPP) NADPH H+

    Out Squalene PPi NADP+
    › Notes

    ER-associated squalene synthase makes the C30 hydrocarbon through presqualene pyrophosphate. This is the first committed step toward sterol formation after FPP; NADPH is required.

12. 12

    Squalene + O2 + NADPH + H+ → (S)-2,3-oxidosqualene + NADP+ + H2O

    Squalene monooxygenase (squalene epoxidase) 1.14.14.17 ST-0102 Irreversible/directional NADP+ NADPH FAD O2 H2O
    In Squalene O2 NADPH H+

    Out (S)-2,3-Oxidosqualene NADP+ H2O
    › Notes

    Squalene monooxygenase (squalene epoxidase), an FAD-dependent ER monooxygenase, uses NADPH and oxygen to form the epoxide. This is an important secondary regulatory point.

13. 13

    (S)-2,3-Oxidosqualene → lanosterol

    Lanosterol synthase (oxidosqualene cyclase) 5.4.99.7 ST-0103 Irreversible/directional
    In (S)-2,3-Oxidosqualene

    Out Lanosterol
    › Notes

    Lanosterol synthase (oxidosqualene cyclase) protonates and cyclizes the linear epoxide through a concerted carbocation cascade to generate the tetracyclic sterol nucleus. No ATP is directly consumed.

14. 14

    Lanosterol → zymosterol/intermediate sterols → cholesterol

    ↪ zymosterol/intermediate sterols → cholesterol

    ER sterol-conversion enzyme series (19-step lanosterol->cholesterol) ST-0104 Irreversible
    In Lanosterol
    › Notes

    A series of ER enzymes removes three methyl groups, reduces double bonds, and rearranges the sterol skeleton. Key human enzymes include lanosterol 14α-demethylase (CYP51A1), sterol C4-methyl oxidase complex, sterol Δ14-reductase (TM7SF2/LBR), sterol Δ8–Δ7 isomerase (EBP), sterol C5-desaturase (SC5D), and 24-dehydrocholesterol reductase (DHCR24) or 7-dehydrocholesterol reductase (DHCR7), depending on the terminal route. These oxygenase/reductase reactions require molecular oxygen and reducing equivalents, chiefly NADPH; the overall pathway is effectively irreversible.

Showing all 14 steps.

Overview

Cholesterol biosynthesis converts acetyl-CoA to cholesterol through the mevalonate pathway and provides precursors for steroid hormones, bile acids, vitamin D, and membrane sterols. Cholesterol is synthesized in many tissues, but liver is a quantitatively important site for whole-body cholesterol homeostasis. The pathway is highly energy- and NADPH-intensive and is controlled principally at HMG-CoA reductase.

Cellular location

Early reactions occur in the cytosol; HMG-CoA reductase is an integral protein of the smooth endoplasmic reticulum (ER) with a cytosolic catalytic domain. Squalene epoxidation, oxidosqualene cyclization, and later sterol remodeling occur on or in the smooth ER. Hepatocytes, enterocytes, adrenal cortex, gonads, and rapidly proliferating cells are active sites of synthesis.

Net energetics

Six isopentenyl units are needed for one cholesterol molecule; thus synthesis uses 18 acetyl-CoA and 18 ATP for conversion of six mevalonate molecules into six activated C5 units. The commonly quoted minimum reducing-power cost is 16 NADPH (12 at HMG-CoA reductase, one at squalene synthase, one at squalene monooxygenase, and additional reductive sterol-remodeling steps), with molecular oxygen required during sterol oxygenation. The exact net balance of NADPH and O2 across lanosterol-to-cholesterol remodeling depends on the detailed route and accounting convention, but cholesterol synthesis is unambiguously ATP- and NADPH-expensive.

Clinical significance

Cholesterol maintains membrane order and is the obligate precursor of steroid hormones, bile acids, and vitamin D. HMG-CoA reductase inhibition by statins lowers hepatic cholesterol, increases LDL-receptor expression, and thereby reduces circulating LDL cholesterol. Defects in late sterol synthesis cause severe developmental syndromes; for example, DHCR7 deficiency causes Smith–Lemli–Opitz syndrome with low cholesterol and elevated 7-dehydrocholesterol. Excess hepatic cholesterol and impaired LDL clearance contribute to atherosclerotic cardiovascular disease.