The mevalonate pathway, also known as the isoprenoid pathway or

HMG-CoA reductase HMG-CoA reductase (3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, official symbol HMGCR) is the rate-controlling enzyme (NADH-dependent, ; NADPH-dependent, ) of the mevalonate pathway, the metabolic pathway that produces cholesterol and ...

pathway is an essential

metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical ...

present in

eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...

, archaea, and some

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

. The pathway produces two five-carbon building blocks called

isopentenyl pyrophosphate Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an isoprenoid precursor. IPP is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway) and in the ''non-mevalonate'' MEP pathway of i ...

(IPP) and

dimethylallyl pyrophosphate Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the MEP pathway of isoprenoid precursor biosynt ...

(DMAPP), which are used to make isoprenoids, a diverse class of over 30,000 biomolecules such as

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membr ...

vitamin K Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-synthesis modification of certain proteins that are required for blood coagulat ...

coenzyme Q10 Coenzyme Q, also known as ubiquinone and marketed as CoQ10, is a coenzyme family that is ubiquitous in animals and most bacteria (hence the name ubiquinone). In humans, the most common form is coenzyme Q10 or ubiquinone-10. It is a 1,4-ben ...

, and all

steroid hormones A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typically made in the adrenal cortex, hence ''cortico-'') and sex steroids (typically made in the gonads or placenta). Wi ...

. The mevalonate pathway begins with

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized fo ...

and ends with the production of IPP and DMAPP. It is best known as the target of

statin Statins, also known as HMG-CoA reductase inhibitors, are a class of lipid-lowering medications that reduce illness and mortality in those who are at high risk of cardiovascular disease. They are the most common cholesterol-lowering drugs. L ...

s, a class of cholesterol lowering drugs. Statins inhibit

within the mevalonate pathway.

Upper 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

molecules to yield

acetoacetyl-CoA Acetoacetyl CoA is the precursor of HMG-CoA in the mevalonate pathway, which is essential for cholesterol biosynthesis. It also takes a similar role in the ketone bodies synthesis ( ketogenesis) pathway of the liver. In the ketone bodies digesti ...

. This is followed by a second condensation to form

HMG-CoA β-Hydroxy β-methylglutaryl-CoA (HMG-CoA), also known as 3-hydroxy-3-methylglutaryl coenzyme A, is an intermediate in the mevalonate and ketogenesis pathways. It is formed from acetyl CoA and acetoacetyl CoA by HMG-CoA synthase. The research o ...

(3-hydroxy-3- methyl-glutaryl-CoA). Reduction of HMG-CoA yields (R)-

mevalonate Mevalonic acid (MVA) is a key organic compound in biochemistry; the name is a contraction of dihydroxymethylvalerolactone. The carboxylate anion of mevalonic acid, which is the predominant form in biological environments, is known as ''mevalonat ...

. These first 3 enzymatic steps are called the upper mevalonate pathway.

Lower mevalonate pathway

The lower mevalonate pathway which converts (R)-

into IPP and DMAPP has 3 variants. In

, mevalonate is phosphorylated twice in the 5-OH position, then decarboxylated to yield IPP. In some archaea such as ''

Haloferax volcanii ''Haloferax volcanii'' is a species of organism in the genus '' Haloferax'' in the Archaea. Description and significance Microbiologist Benjamin Elazari Volcani first discovered ''Haloferax volcanii'', a self-named extremophile, in the 1930s. ...

'', 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). A third mevalonate pathway variant found in ''

Thermoplasma acidophilum ''Thermoplasma acidophilum'' is an archaeon, the type species of its genus. ''T. acidophilum'' was originally isolated from a self-heating coal refuse pile, at pH 2 and 59 °C. Its genome has been sequenced. It is highly flagellated and ...

'', 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).

Regulation and feedback

Several key enzymes can be activated through

DNA transcription Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules calle ...

al regulation on activation of

SREBP Sterol regulatory element-binding proteins (SREBPs) are transcription factors that bind to the sterol regulatory element DNA sequence TCACNCCAC. Mammalian SREBPs are encoded by the genes '' SREBF1'' and '' SREBF2''. SREBPs belong to the basic ...

(sterol regulatory element-binding protein-1 and -2). This intracellular sensor detects low

levels and stimulates endogenous production by the HMG-CoA reductase pathway, as well as increasing

lipoprotein A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...

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.

Pharmacology

A number of

drugs A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhalat ...

target the ''mevalonate pathway'': *

Statin Statins, also known as HMG-CoA reductase inhibitors, are a class of lipid-lowering medications that reduce illness and mortality in those who are at high risk of cardiovascular disease. They are the most common cholesterol-lowering drugs. L ...

s (used to decrease cholesterol levels); *

Bisphosphonate Bisphosphonates are a class of drugs that prevent the loss of bone density, used to treat osteoporosis and similar diseases. They are the most commonly prescribed drugs used to treat osteoporosis. They are called bisphosphonates because they ...

s (used to treat various bone-degenerative diseases)

Diseases

A number of

diseases A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that ...

affect the ''mevalonate pathway'': *

Mevalonate Kinase Deficiency Mevalonate kinase deficiency (MKD) is an autosomal recessive metabolic disorder that disrupts the biosynthesis of cholesterol and isoprenoids. It is a very rare genetic disease. It is characterized by an elevated level of immunoglobulin D in t ...

** Mevalonic Aciduria ** Hyperimmunoglobulinemia D Syndrome (HIDS).

Alternative pathway

Plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

, most

, and some

protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histor ...

such as

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or deat ...

parasites have the ability to produce isoprenoids using an alternative pathway called the methylerythritol phosphate (MEP) or

non-mevalonate pathway The non-mevalonate pathway—also appearing as the mevalonate-independent pathway and the 2-''C''-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate (MEP/DOXP) pathway—is an alternative metabolic pathway for the biosynthesis of the i ...

. 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 Isotopomers or isotopic isomers are isomers with isotopic atoms, having the same number of each isotope of each element but differing in their positions. The result is that the molecules are either constitutional isomers or stereoisomers solely ...

. In higher plants, the MEP pathway operates in

plastids The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cy ...

while the mevalonate pathway operates in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

. Examples of bacteria that contain the MEP pathway include ''

Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...

'' and pathogens such as '' Mycobacterium tuberculosis''.

Enzymatic reactions

References

External links

Rensselaer Polytechnic Institute
page on cholesterol synthesis (including regulation) {{MetabolismMap Metabolic pathways