Methylerythritol phosphate pathway

The Methylerythritol phosphate pathway makes the universal C5 isoprenoid precursors IPP and DMAPP from central metabolic intermediates. It operates in many bacteria, in plant and algal plastids, and in the nonphotosynthetic apicoplast of malaria parasites. Unlike the mevalonate pathway, which begins with acetyl-CoA, it begins with pyruvate and glyceraldehyde 3-phosphate. The two pathways converge at IPP and DMAPP but are usually separated between cellular compartments in plants. This distribution gives the pathway importance in microbial physiology, plant metabolism, and antimicrobial drug research. Its name refers to methylerythritol phosphate, an intermediate formed midway through the sequence, while “non-mevalonate” distinguishes its carbon-building strategy from the better-known mevalonate route.

The pathway proceeds through seven main enzyme-catalyzed reactions. DXS condenses pyruvate with glyceraldehyde 3-phosphate to form 1-deoxy-D-xylulose 5-phosphate, and IspC, also called DXR, converts this compound into methylerythritol 4-phosphate. IspD, IspE, and IspF then produce CDP-methylerythritol, CDP-methylerythritol 2-phosphate, and methylerythritol 2,4-cyclodiphosphate. The terminal enzymes IspG and IspH generate hydroxymethylbutenyl diphosphate and then a mixture of IPP and DMAPP.¹

The reactions require several chemically distinctive cofactors and activated phosphate intermediates. IspC uses NADPH, whereas IspG and IspH contain iron-sulfur clusters that support difficult reductive transformations.² Isomerase activity then adjusts the IPP-to-DMAPP balance for downstream prenyltransferases.

IPP and DMAPP feed prenyltransferases that assemble longer isoprenoid chains and diverse terpenoid products. In bacteria, these products include components of the electron-transport quinones, cell-surface molecules, and membrane-associated compounds. In plants, plastidial products include carotenoids, chlorophyll side chains, tocopherols, plastoquinone, and many volatile or defensive terpenes.

Flux is controlled at several levels rather than by a single universal rate-limiting enzyme. DXS often responds strongly to substrate supply and feedback from downstream terpenoids, while transcription, protein stability, redox state, and the availability of NADPH or iron-sulfur assembly capacity also matter. Plants commonly coordinate the plastidial pathway with the cytosolic mevalonate pathway through exchange of some isoprenoid intermediates, although the degree of exchange varies among tissues and conditions.³

The pathway is a major antimicrobial target because humans lack it, while many pathogens depend on it. Fosmidomycin inhibits IspC and has been studied particularly against Plasmodium falciparum, whose apicoplast contains the pathway; clinical effectiveness is constrained by pharmacokinetics, uptake, and parasite-stage biology.

Several enzymes also illustrate unusual biochemical vulnerability. IspG and IspH are oxygen-sensitive iron-sulfur proteins, so their activity can be affected by oxidative stress and by defects in cluster assembly. The pathway has attracted metabolic-engineering interest because its intermediates can be redirected toward valuable terpenes, but balancing precursor toxicity, cofactors, competing sinks, and product export remains difficult. Isotope-labeling experiments were especially important historically: they established that bacterial and plant plastid isoprenoids could arise from a route chemically distinct from mevalonate-based synthesis.¹

IPP

Isopentenyl diphosphate, one of the two universal five-carbon isoprenoid precursors.

DMAPP

Dimethylallyl diphosphate, the allylic five-carbon precursor that combines with IPP in prenyltransferase reactions.

DXS

1-Deoxy-D-xylulose 5-phosphate synthase, the enzyme that initiates the pathway from pyruvate and glyceraldehyde 3-phosphate.

IspC

The common bacterial name for 1-deoxy-D-xylulose 5-phosphate reductoisomerase, also called DXR.

Apicoplast

A nonphotosynthetic plastid-like organelle in apicomplexan parasites, including Plasmodium species.