Mevalonate

Mevalonate is the carboxylate form of mevalonic acid and occupies the first committed small-molecule position in the classical isoprenoid-biosynthesis route. Mevalonic acid has the molecular formula C6H12O4; loss of the acidic proton gives the mevalonate anion, whose salts and biochemical forms are commonly discussed under the same name.¹ In cells, the compound is produced when HMG-CoA reductase reduces 3-hydroxy-3-methylglutaryl-CoA using NADPH. This reaction follows the condensation of two molecules of acetyl-CoA with a third acetyl-CoA-derived unit and is the pathway's principal regulated step.²

Mevalonate is then activated rather than incorporated directly into a final product. Mevalonate kinase adds phosphate at carbon 5, phosphomevalonate kinase adds a second phosphate, and mevalonate diphosphate decarboxylase produces isopentenyl diphosphate, releasing carbon dioxide. Isopentenyl diphosphate can isomerize to dimethylallyl diphosphate, providing the two interchangeable C5 building blocks used in larger isoprenoids.

The mevalonate pathway supplies the carbon skeletons for sterols and a wide range of essential isoprenoids. In animals, its products include cholesterol, steroid hormones, bile acids, ubiquinone, dolichol, and the lipid groups attached to certain signaling proteins by protein prenylation.³ Condensation of the C5 units generates geranyl, farnesyl, and geranylgeranyl diphosphates, which serve as branch-point intermediates rather than merely as end products. Plants use the pathway chiefly in the cytosol for sterols, brassinosteroids, sesquiterpenes, and triterpenes, while a related methylerythritol phosphate pathway operates in plastids.

In mammals, pathway flux is controlled through feedback and transcriptional regulation. Low sterol availability activates sterol regulatory element-binding protein 2, increasing expression of HMG-CoA reductase and other biosynthetic components; sterol abundance promotes reductase degradation and suppresses transcription.⁴ This arrangement couples mevalonate production to membrane needs and to the availability of downstream sterols.

Mevalonate-pathway activity is a major pharmacological target because HMG-CoA reductase controls the supply of cholesterol and nonsterol isoprenoids. Statins inhibit this enzyme by occupying its catalytic site, lowering mevalonate formation and subsequently reducing hepatic cholesterol synthesis; cells can also experience changes in protein prenylation and other nonsterol outputs when pathway flux falls.³ Experimental addition of mevalonate is therefore used as a pathway-rescue test, although rescue depends on cellular uptake, concentration, and the experimental system.

Inherited defects later in the pathway illustrate why mevalonate is not simply a cholesterol precursor. Loss-of-function variants in mevalonate kinase cause a spectrum that includes mevalonic aciduria and hyperimmunoglobulinemia D syndrome, now commonly termed mevalonate kinase deficiency. The clinical phenotypes include recurrent inflammation, fever, developmental abnormalities in severe cases, and elevated mevalonate-derived metabolites; the molecular basis involves impaired isoprenoid metabolism rather than one isolated sterol defect.⁵

Mevalonate has distinct biochemical roles depending on the organism, compartment, and protonation state. In eukaryotic cells, pathway enzymes can be distributed between cytosolic and organelle-associated pools, and substrate channeling or local metabolite availability may make a measured bulk concentration an imperfect guide to pathway flux. Plants divide isoprenoid production between the cytosolic mevalonate pathway and the plastidial methylerythritol phosphate pathway, with metabolic exchange between compartments adding further complexity.

A less obvious feature is the energetic investment required after mevalonate is formed: two kinase reactions consume ATP before decarboxylation yields the reactive C5 diphosphate pool. That investment creates chemically activated intermediates suitable for carbon-carbon bond formation. The pathway also has ancient alternatives: many bacteria lack the classical mevalonate route and use the methylerythritol phosphate pathway, whereas some organisms retain mevalonate-pathway enzymes with lineage-specific organization and regulation.² Thus, “mevalonate” identifies both a molecule and a metabolic control point shared unevenly across life.

HMG-CoA reductase

The NADPH-dependent enzyme that reduces HMG-CoA to mevalonate; it is the principal target of statin drugs.

Mevalonate kinase

The ATP-dependent enzyme that phosphorylates mevalonate to mevalonate 5-phosphate.

Isopentenyl diphosphate

A C5 activated isoprenoid produced downstream of mevalonate and interconverted with dimethylallyl diphosphate.

Protein prenylation

Covalent attachment of isoprenoid groups to proteins, often influencing their membrane association and signaling.