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 Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...

of the isoprenoid precursors

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 (DMAPP). The currently preferred name for this pathway is the MEP pathway, since MEP is the first committed metabolite on the route to IPP.

Isoprenoid precursor biosynthesis

The

mevalonate pathway 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. The pathway produces two five-carbon building blocks called isopentenyl ...

(MVA pathway or

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

pathway) and the MEP pathway are metabolic pathways for the biosynthesis of isoprenoid precursors: IPP and DMAPP. Whereas plants use both MVA and MEP pathway, most organisms only use one of the pathways for the biosynthesis of isoprenoid precursors. In plant cells IPP/DMAPP biosynthesis via the MEP pathway takes place in

plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...

organelles, while the biosynthesis via the MVA pathway takes place in the cytoplasm. Most gram-negative bacteria, the photosynthetic cyanobacteria and green algae use only the MEP pathway. Bacteria that use the MEP pathway include important pathogens such ''

Mycobacterium tuberculosis ''Mycobacterium tuberculosis'' (M. tb), also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, ''M. tuberculosis'' ha ...

''. IPP and DMAPP serve as precursors for the biosynthesis of isoprenoid (terpenoid) molecules used in processes as diverse as protein

prenylation Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to ...

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

maintenance, the synthesis of

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

s, protein anchoring and ''N''-glycosylation in all three domains of life. In photosynthetic organisms MEP-derived precursors are used for the biosynthesis of photosynthetic pigments, such as the carotenoids and the phytol chain of chlorophyll and light harvesting pigments. Bacteria such as ''

Escherichia coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...

'' have been engineered for co-expressing biosynthesis genes of both the MEP and the MVA pathway. Distribution of the metabolic fluxes between the MEP and the MVA pathway can be studied using ¹³C-glucose isotopomers.

Reactions

The reactions of the MEP pathway are as follows, taken primarily from Eisenreich and co-workers, except where the bold labels are additional local abbreviations to assist in connecting the table to the scheme above:

Inhibition and other pathway research

Dxs, the first enzyme of the pathway, is feedback inhibited by the products IPP and DMAPP. Dxs is active as a homo- dimer and the precise mechanism of enzyme inhibition has been debated in the field. It has been proposed that IPP/DMAPP are competing the co-factor TPP. A more recent study suggested that IPP/DMAPP trigger monomerisation and subsequent degradation of the enzyme, via interaction with a monomer interaction site that differs from the

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding s ...

of the

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

. DXP reductoisomerase (also known as: DXR, DOXP reductoisomerase, IspC, MEP synthase), is a key enzyme in the MEP pathway. It can be inhibited by the

natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...

fosmidomycin, which is under study as a starting point to develop a candidate antibacterial or antimalarial drug. The intermediate, HMB-PP, is a natural activator of human Vγ9/Vδ2 T cells, the major γδ T cell population in peripheral blood, and cells that "play a crucial role in the immune response to microbial pathogens". * IspH inhibitors: non-mevalonate Metabolic pathway that is essential for most bacteria but absent in humans, making it an ideal target for antibiotic development. This pathway, called methyl-D-erythritol phosphate (MEP) or non-mevalonate pathway, is responsible for biosynthesis of isoprenoids—molecules required for cell survival in most pathogenic bacteria and hence will be helpful in most usually antibacterial resistant bacteria.

Metabolic engineering of the MEP/Non-mevalonate pathway

The MEP pathway has been extensively studied and engineered ''

,'' a commonly used microbial species for laboratory research and application. IPP and DMAPP, the products of the MEP pathway can be used as substrates for the heterologous production of terpenoids with high value for application in the pharmaceutical and chemical industry. Upon expression of

heterologous The term heterologous has several meanings in biology. Gene expression In cell biology and protein biochemistry, heterologous expression means that a protein is experimentally put into a cell that does not normally make (i.e., express) that ...

genes from different organisms, production of terpenoids like

limonene Limonene () is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene, and is the major component in the essential oil of citrus fruit peels. The (+)-isomer, occurring more commonly in nature as the fragrance of oranges, ...

, bisabolene and

isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals (including humans) and its polymers ar ...

could be achieved in different microbial chassis. Studies overexpressing different biosynthesis genes of the pathway revealed that expression of Dxs and Idi, catalyzing the first step and last step of the MEP pathway could significantly increase the yield of MEP derived terpenoids. Dxs as the first enzyme of the pathway represents a bottleneck for the flux of carbon that enters the pathway. Idi which interconverts IPP to DMAPP and vice versa seems to be important for providing the respective substrate that is needed upon introduction of a heterologous carbon sink in engineered strains. A lot of metabolic engineering work on the MEP pathway has been done in

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

, photo-autotrophic microbes that can assimilate

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

from the atmosphere into various carbon containing metabolites, including terpenoids. For biotechnology,

are, thus, an attractive platform for the sustainable production of high-value compounds.

References

{{Cholesterol metabolism intermediates Metabolic pathways Cell biology