The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway is an essential metabolic pathway present in eukaryotes, archaea, and some bacteria.^[1] The pathway produces two five-carbon building blocks called isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), which are used to make isoprenoids, a diverse class of over 30,000 biomolecules such as cholesterol, vitamin K, coenzyme Q10, and all steroid hormones.^[2]

The mevalonate pathway begins with acetyl-CoA and ends with the production of IPP and DMAPP.^[3] It is best known as the target of statins, a class of cholesterol lowering drugs. Statins inhibit HMG-CoA reductase within the mevalonate pathway.

The mevalonate pathway of eukaryotes, archaea, and eubacteria all begin the same way. The sole carbon feed stock of the pathway is acetyl-CoA. The first step condenses two acetyl-CoA molecules to yield acetoacetyl-CoA. This is followed by a second condensation to form HMG-CoA (3-hydroxy-3- methyl-glutaryl-CoA). Reduction of HMG-CoA yields (R)-mevalonate. These first 3 enzymatic steps are called the upper mevalonate pathway.^[4]

The lower mevalonate pathway which converts (R)-mevalonate into IPP and DMAPP has 3 variants. In eukaryotes, mevalonate is phosphorylated twice in the 5-OH position, then decarboxylated to yield IPP.^[4] In some archaea such as Haloferax volcanii, mevalonate is phosphorylated once in the 5-OH position, decarboxylated to yield isopentenyl phosphate (IP), and finally phosphorylated again to yield IPP (Archaeal Mevalonate Pathway I).^[5] A third mevalonate pathway variant found in Thermoplasma acidophilum, phosphorylates mevalonate at the 3-OH position followed by phosphorylation at the 5-OH position. The resulting metabolite, mevalonate-3,5-bisphosphate, is decarboxylated to IP, and finally phosphorylated to yield IPP (Archaeal Mevalonate Pathway II).^[6][7]

Several key enzymes can be activated through DNA transcriptional regulation on activation of SREBP (sterol regulatory element-binding protein-1 and -2). This intracellular sensor detects low cholesterol levels and stimulates endogenous production by the HMG-CoA reductase pathway, as well as increasing lipoprotein uptake by up-regulating the LDL-receptor. Regulation of this pathway is also achieved by controlling the rate of translation of the mRNA, degradation of reductase and phosphorylation.^[1]

A number of drugs target the mevalonate pathway:

• Statins (used to decrease cholesterol levels);
• Bisphosphonates (used to treat various bone-degenerative diseases such as osteoporosis^[8])

A number of diseases affect the mevalonate pathway:

• Mevalonate Kinase Deficiency
  • Mevalonic Aciduria
  • Hyperimmunoglobulinemia D Syndrome (HIDS).

Plants, most bacteria, and some protozoa such as malaria parasites have the ability to produce isoprenoids using an alternative pathway called the methylerythritol phosphate (MEP) or non-mevalonate pathway.^[9] The output of both the mevalonate pathway and the MEP pathway are the same, IPP and DMAPP, however the enzymatic reactions to convert acetyl-CoA into IPP are entirely different. Interaction between the two metabolic pathways can be studied by using ¹³C-glucose isotopomers.^[10] In higher plants, the MEP pathway operates in plastids while the mevalonate pathway operates in the cytosol.^[9] Examples of bacteria that contain the MEP pathway include Escherichia coli and pathogens such as Mycobacterium tuberculosis.

Enzyme	Reaction	Description
Acetoacetyl-CoA thiolase		Acetyl-CoA (citric acid cycle) undergoes condensation with another acetyl-CoA molecule to form acetoacetyl-CoA
HMG-CoA synthase		Acetoacetyl-CoA condenses with another Acetyl-CoA molecule to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA).
HMG-CoA reductase		HMG-CoA is reduced to mevalonate by NADPH. This is the rate limiting step in cholesterol synthesis, which is why this enzyme is a good target for pharmaceuticals (statins).
mevalonate-5-kinase		Mevalonate is phosphorylated at the 5-OH position to yield mevalonate-5-phosphate (also called phosphomevalonic acid).
mevalonate-3-kinase		Mevalonate is phosphorylated at the 3-OH position to yield mevalonate-3-phosphate. 1 ATP is consumed.
mevalonate-3-phosphate-5-kinase		Mevalonate-3-phosphate is phosphorylated at the 5-OH position to yield mevalonate-5-phosphate (also called phosphomevalonic acid). 1 ATP is consumed.
phosphomevalonate kinase		mevalonate-5-phosphate is phosphorylated to yield mevalonate-5-pyrophosphate. 1 ATP is consumed.
mevalonate-5-pyrophosphate decarboxylase		Mevalonate-5-pyrophosphate is decarboxylated to yield isopentenyl pyrophosphate (IPP). 1 ATP is consumed.
isopentenyl pyrophosphate isomerase		isopentenyl pyrophosphate is isomerized to dimethylallyl pyrophosphate.

• Rensselaer Polytechnic Institute page on cholesterol synthesis (including regulation)