Studies on the non-mevalonate isoprenoid biosynthetic pathway. Simple methods for preparation of isotope-labeled (E)-1-hydroxy-2-methylbut-2-enyl 4-diphosphate

The Active-Site [4Fe-4S] Cluster in the Isoprenoid Biosynthesis Enzyme IspH Adopts Unexpected Redox States during Ligand Binding and Catalysis

(E)-4-Hydroxy-3-methylbut-2-enyl diphosphate reductase, or IspH (formerly known as LytB), catalyzes the terminal step of the bacterial methylerythritol phosphate (MEP) pathway for isoprene synthesis. This step converts (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP) into one of two possible isomeric products, either isopentenyl diphosphate (IPP) or dimethylallyl diphosphate (DMAPP). This reaction involves the removal of the C4 hydroxyl group of HMBPP and addition of two electrons. IspH contains a [4Fe-4S] cluster in its active site, and multiple cluster-based paramagnetic species of uncertain redox and ligation states can be detected after incubation with reductant, addition of a ligand, or during catalysis. To characterize the clusters in these species, 57Fe-labeled samples of IspH were prepared and studied by electron paramagnetic resonance (EPR), 57Fe electron-nuclear double resonance (ENDOR), and Mössbauer spectroscopies. Notably, this ENDOR study provides a rarely reported, complete determination of the 57Fe hyperfine tensors for all four Fe ions in a [4Fe-4S] cluster. The resting state of the enzyme (Ox) has a diamagnetic [4Fe-4S]2+ cluster. Reduction generates [4Fe-4S]+ (Red) with both S = 1/2 and S = 3/2 spin ground states. When the reduced enzyme is incubated with substrate, a transient paramagnetic reaction intermediate is detected (Int) which is thought to contain a cluster-bound substrate-derived species. The EPR properties of Int are indicative of a 3+ iron-sulfur cluster oxidation state, and the Mössbauer spectra presented here confirm this. Incubation of reduced enzyme with the product IPP induced yet another paramagnetic [4Fe-4S]+ species (Red+P) with S = 1/2. However, the g-tensor of this state is commonly associated with a 3+ oxidation state, while Mössbauer parameters show features typical for 2+ clusters. Implications of these complicated results are discussed.

Phosphoantigens glue butyrophilin 3A1 and 2A1 to activate Vγ9Vδ2 T cells

In both cancer and infections, diseased cells are presented to human Vγ9Vδ2 T cells through an 'inside out' signalling process whereby structurally diverse phosphoantigen (pAg) molecules are sensed by the intracellular domain of butyrophilin BTN3A11-4. Here we show how-in both humans and alpaca-multiple pAgs function as 'molecular glues' to promote heteromeric association between the intracellular domains of BTN3A1 and the structurally similar butyrophilin BTN2A1. X-ray crystallography studies visualized that engagement of BTN3A1 with pAgs forms a composite interface for direct binding to BTN2A1, with various pAg molecules each positioned at the centre of the interface and gluing the butyrophilins with distinct affinities. Our structural insights guided mutagenesis experiments that led to disruption of the intracellular BTN3A1-BTN2A1 association, abolishing pAg-mediated Vγ9Vδ2 T cell activation. Analyses using structure-based molecular-dynamics simulations, 19F-NMR investigations, chimeric receptor engineering and direct measurement of intercellular binding force revealed how pAg-mediated BTN2A1 association drives BTN3A1 intracellular fluctuations outwards in a thermodynamically favourable manner, thereby enabling BTN3A1 to push off from the BTN2A1 ectodomain to initiate T cell receptor-mediated γδ T cell activation. Practically, we harnessed the molecular-glue model for immunotherapeutics design, demonstrating chemical principles for developing both small-molecule activators and inhibitors of human γδ T cell function.

Late-Stage Chemoenzymatic Installation of Hydroxy-Bearing Allyl Moiety on the Indole Ring of Tryptophan-Containing Peptides

The late‐stage functionalization of indole‐ and tryptophan‐containing compounds with reactive moieties facilitates downstream diversification and leads to changes in their biological properties. Here, the synthesis of two hydroxy‐bearing allyl pyrophosphates is described. A chemoenzymatic method is demonstrated which uses a promiscuous indole prenyltransferase enzyme to install a dual reactive hydroxy‐bearing allyl moiety directly on the indole ring of tryptophan‐containing peptides. This is the first report of late‐stage indole modifications with this reactive group.

Synthesis of Aryl and Heterocyclic Polyenes and Their Activity in Free Radical Scavenging

Aryl polyenes and heterocyclic polyenes with the same central C20 unit of β,β-carotene but different terminal groups were synthesised. Their free radical scavenging activity was measured by a 1,1-diphenyl-2-picrylhydrazinyl spectrophotometric method. The results indicated that all the new compounds have free radical scavenging activity.

Exceptionally high percentage of IPP synthesis by Ginkgo biloba IspH is mainly due to Phe residue in the active site

(E)-4-Hydroxy-3-methylbut-2-enyl diphosphate (HMBPP) reductase (IspH, HDR or LytB) is an Fe/S enzyme catalyzing the reductive dehydroxylation of HMBPP to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) in the last step of methylerythritol phosphate (MEP) pathway. The MEP pathway is known to produce 4-6:1 ratio of IPP and DMAPP mixture by the last enzyme, IspH. Plant IspH in plastids follows same catalytic mechanism as others, but GbIspH (Ginkgo biloba IspH) was reported to produce a mixture of IPP and DMAPP in a ratio of 16:1. Present catalytic mechanisms of IspH involve a common allyl anion intermediate, and the intramolecular proton transfer to the allyl moiety is considered as the key reaction step determining the product between IPP and DMAPP. The F212 residue in plant IspH was found as a potential amino acid residue that could mediate the proton transfer to the allyl anion intermediate before the product release. In this report, catalytic function of GbIspH F212 residue (H74 in E. coli), especially during the product formation in the active site, was studied by means of site-directed mutation. The product ratio of IPP/DMAPP was measured as 6.5 ± 0.1 for F212H GbIspH, and the value was close to the reported bacterial IspH having His residue on that specific position. Along with the other F212Y mutant, of which ratio was determined as 10.9 ± 0.1, the results strongly support that the Phe residue in plant IspH is the key amino acid residue that allows exclusive production of IPP in plant chloroplast.

N-Terminal Region of GbIspH1, Ginkgo biloba IspH Type 1, May Be Involved in the pH-Dependent Regulation of Enzyme Activity

GbIspH1, IspH type 1 in Ginkgo biloba chloroplast, is the Fe/S enzyme catalyzing the reductive dehydroxylation of HMBPP to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) at the final step of methylerythritol phosphate pathway in chloroplast. Compared to the bacterial IspH, plant IspH, including GbIspH1, has an additional polypeptide chain at the N-terminus. Here, biochemical function of the N-terminal region of GbIspH1 was investigated with the N-terminal truncated GbIspH1 (GbIspH1-truncated). Both wild type GbIspH1 (GbIspH1-full) and GbIspH1-truncated were catalytically active and produced IPP and DMAPP in a ratio of 15 : 1. Kinetic parameters of K_M (17.3 ± 1.9 and 14.9 ± 2.3 µM) and k_cat (369 ± 10 and 347 ± 12 min⁻¹) at pH 8.0 were obtained for GbIspH1-full and GbIspH1-truncated, respectively. Interestingly, GbIspH1-full and GbIspH1-truncated showed significantly different pH-dependent activities, and the maximum enzyme activities were obtained at pH 8.0 and 7.5, respectively. However, catalytic activation energies (E_a) of GbIspH1-full and GbIspH1-truncated were almost the same with 36.5 ± 1.6 and 35.0 ± 1.9 kJ/mol, respectively. It was suggested that the N-terminal region of GbIspH1 is involved in the pH-dependent regulation of enzyme activity during photosynthesis.

endo/exo stereoselectivity in Diels-Alder reactions of α,β-dialkylated conjugated enals to cyclic 1,3-dienes: intermediates in the synthesis of (-)-β-santalol and its analogs

Highly exo-selective [4+2] cycloadditions of cyclopenta-1,3-diene 2a to α,β-dialkyl conjugated enals 5 are compared with the analogous endo-favored Diels-Alder reaction of cyclohexa-1,3-diene 7. The exo-stereoselectivity is lower in the homologous case of methylcyclopenta-1,3-diene 9. This diastereoselectivity is discussed either in terms of a retro-homo-Diels-Alder reaction, associated with thermodynamic control, or with respect to either a competing hetero-Diels-Alder/Claisen or Cope domino pathway, or retro-Claisen/retro-hetero-Diels-Alder of the endo-homo-cycloadducts. These hypothetical mechanisms have been examined by DFT calculations at the MPW1K(CH2 Cl2 )/6-31+G** level of theory for the AlCl3 -mediated cycloadditions of 5d to 2a and 7. Application of Corey's methodology to the γ-halogeno-α-methyl-substituted dienophiles 5a and 5b allowed an enantioselective preparation of known and useful intermediates for the synthesis of either the naturally occurring (-)-β-santalol or its potentially olfactive structural analogs.

Immunogenic and antioxidant effects of a pathogen-associated prenyl pyrophosphate in Anopheles gambiae

Despite efficient vector transmission, Plasmodium parasites suffer great bottlenecks during their developmental stages within Anopheles mosquitoes. The outcome depends on a complex three-way interaction between host, parasite and gut bacteria. Although considerable progress has been made recently in deciphering Anopheles effector responses, little is currently known regarding the underlying microbial immune elicitors. An interesting candidate in this sense is the pathogen-derived prenyl pyrophosphate and designated phosphoantigen (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), found in Plasmodium and most eubacteria but not in higher eukaryotes. HMBPP is the most potent stimulant known of human Vγ9Vδ2 T cells, a unique lymphocyte subset that expands during several infections including malaria. In this study, we show that Vγ9Vδ2 T cells proliferate when stimulated with supernatants from intraerythrocytic stages of Plasmodium falciparum cultures, suggesting that biologically relevant doses of phosphoantigens are excreted by the parasite. Next, we used Anopheles gambiae to investigate the immune- and redox- stimulating effects of HMBPP. We demonstrate a potent activation in vitro of all but one of the signaling pathways earlier implicated in the human Vγ9Vδ2 T cell response, as p38, JNK and PI3K/Akt but not ERK were activated in the A. gambiae 4a3B cell line. Additionally, both HMBPP and the downstream endogenous metabolite isopentenyl pyrophosphate displayed antioxidant effects by promoting cellular tolerance to hydrogen peroxide challenge. When provided in the mosquito blood meal, HMBPP induced temporal changes in the expression of several immune genes. In contrast to meso-diaminopimelic acid containing peptidoglycan, HMBPP induced expression of dual oxidase and nitric oxide synthase, two key determinants of Plasmodium infection. Furthermore, temporal fluctuations in midgut bacterial numbers were observed. The multifaceted effects observed in this study indicates that HMBPP is an important elicitor in common for both Plasmodium and gut bacteria in the mosquito.

Primary MHC-class II(+) cells are necessary to promote resting Vδ2 cell expansion in response to (E)-4-hydroxy-3-methyl-but-2-enyl-pyrophosphate and isopentenyl pyrophosphate

Human Vγ9δ2 (Vδ2) T cells represent a unique effector T cell population in humans and primates detecting nonpeptid phosphoantigens, playing an important role in antimicrobial and antitumor immunity. Currently, it is believed that various leukocyte subsets can promote phosphoantigen-driven Vδ2 cell expansion, but the essential cell type required remains elusive. We have used high purity cell sorting to analyze the cellular requirements for (E)-4-hydroxy-3-methyl-but-2-enyl-pyrophosphate (HMBPP)-driven Vδ2 cell expansion. To our knowledge, we show for the first time that primary human MHC-class II(+) cells are indispensable for HMBPP- and isopentenylpyrophosphate-driven Vδ2 cell expansion. In contrast, MHC-class II(-) cells are unable to promote Vδ2 cell expansion. Moreover, purified primary human TCRαβ(+) T cells, CD4(+), or CD8(+) T cells also failed to promote HMBPP-mediated Vδ2 expansion. Depletion of CD4(+)CD25(+) T cells demonstrated that inability of TCRαβ(+) cells to expand Vδ2 cells was not related to the presence of regulatory T cells. Separation of MHC-class II(+) cells into dendritic cells, monocytes, and B cells revealed that dendritic cells were the most potent Vδ2 expanders. Pulsing experiments demonstrated that HMBPP transforms MHC-class II(+) but not negative cells into Vδ2 expanders. MHC-class II-blocking experiments with mAbs and secondary MHC-class II induction on CD4(+) T cells after CD3/CD28 costimulation indicated that MHC-class II is necessary, but not sufficient to promote Vδ2 expansion. Our results provide novel insight into the primary cell-specific requirements for human Vδ2 expansion.

Are free radicals involved in IspH catalysis? An EPR and crystallographic investigation

The [4Fe-4S] protein IspH in the methylerythritol phosphate isoprenoid biosynthesis pathway is an important anti-infective drug target, but its mechanism of action is still the subject of debate. Here, by using electron paramagnetic resonance (EPR) spectroscopy and (2)H, (17)O, and (57)Fe isotopic labeling, we have characterized and assigned two key reaction intermediates in IspH catalysis. The results are consistent with the bioorganometallic mechanism proposed earlier, and the mechanism is proposed to have similarities to that of ferredoxin, thioredoxin reductase, in that one electron is transferred to the [4Fe-4S](2+) cluster, which then performs a formal two-electron reduction of its substrate, generating an oxidized high potential iron-sulfur protein (HiPIP)-like intermediate. The two paramagnetic reaction intermediates observed correspond to the two intermediates proposed in the bioorganometallic mechanism: the early π-complex in which the substrate's 3-CH(2)OH group has rotated away from the reduced iron-sulfur cluster, and the next, η(3)-allyl complex formed after dehydroxylation. No free radical intermediates are observed, and the two paramagnetic intermediates observed do not fit in a Birch reduction-like or ferraoxetane mechanism. Additionally, we show by using EPR spectroscopy and X-ray crystallography that two substrate analogues (4 and 5) follow the same reaction mechanism.

Antigen-specific Vgamma2Vdelta2 T effector cells confer homeostatic protection against pneumonic plaque lesions

The possibility that Vgamma2Vdelta2 T effector cells can confer protection against pulmonary infectious diseases has not been tested. We have recently demonstrated that single-dose (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) plus IL-2 treatment can induce prolonged accumulation of Vgamma2Vdelta2 T effector cells in lungs. Here, we show that a delayed HMBPP/IL-2 administration after inhalational Yersinia pestis infection induced marked expansion of Vgamma2Vdelta2 T cells but failed to control extracellular plague bacterial replication/infection. Surprisingly, despite the absence of infection control, expansion of Vgamma2Vdelta2 T cells after HMBPP/IL-2 treatment led to the attenuation of inhalation plague lesions in lungs. Consistently, HMBPP-activated Vgamma2Vdelta2 T cells accumulated and localized in pulmonary interstitials surrounding small blood vessels and airway mucosa in the lung tissues with no or mild plague lesions. These infiltrating Vgamma2Vdelta2 T cells produced FGF-7, a homeostatic mediator against tissue damages. In contrast, control macaques treated with glucose plus IL-2 or glucose alone exhibited severe hemorrhages and necrosis in most lung lobes, with no or very few Vgamma2Vdelta2 T cells detectable in lung tissues. The findings are consist with the paradigm that circulating Vgamma2Vdelta2 T cells can traffic to lungs for homeostatic protection against tissue damages in infection.

Synthesis and biological activity of phosphonate analogues and geometric isomers of the highly potent phosphoantigen (E)-1-hydroxy-2-methylbut-2-enyl 4-diphosphate

Gammadelta-T-lymphocytes contribute to innate immunity and are selectively activated by nonpeptide phosphorylated molecules (so-called phosphoantigens) produced by organisms responsible for causing a broad range of infectious diseases. gammadelta-T-cells are also activated by synthetic phosphoantigens and are cytotoxic to tumor cells. Here we report the synthesis, NMR characterization, and comparative biological evaluation of new pyrophosphate, phosphonate, and pyrophosphonate monoesters whose structures correspond to isosteric analogues and stereoisomers of the highly potent isoprenoid metabolite ( E)-1-hydroxy-2-methylbut-2-enyl 4-diphosphate called HDMAPP (hydroxy-dimethyl-allyl pyrophosphate). Both pyrophosphate and pyrophosphonate series elicit promising gammadelta-T-cell stimulatory responses in vitro, the pyrophosphonate ester (C-HDMAPP) being by far more stable than its parent pyrophosphate ester (HDMAPP) with improved ADMET properties and a similar pharmacodynamic profile based on in vivo studies in nonhuman primate. In both series, we found that E-stereoisomers are the most active derivatives and that Z-stereoisomers show very marginal bioactivity levels. These results indicate that the use of bioisosteric analogues of HDMAPP may represent promising new leads for immunotherapy.

Prolonged (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate-driven antimicrobial and cytotoxic responses of pulmonary and systemic Vgamma2Vdelta2 T cells in macaques

Although phosphoantigen-specific Vgamma2Vdelta2 T cells appear to play a role in antimicrobial and anticancer immunity, mucosal immune responses and effector functions of these gammadelta T cells during infection or phospholigand treatment remain poorly characterized. In this study, we demonstrate that the microbial phosphoantigen (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) plus IL-2 treatment of macaques induced a prolonged major expansion of circulating Vgamma2Vdelta2 T cells that expressed CD8 and produced cytotoxic perforin during their peak expansion. Interestingly, HMBPP-activated Vgamma2Vdelta2 T cells underwent an extraordinary pulmonary accumulation, which lasted for 3-4 mo, although circulating Vgamma2Vdelta2 T cells had returned to baseline levels weeks prior. The Vgamma2Vdelta2 T cells that accumulated in the lung following HMBPP/IL-2 cotreatment displayed an effector memory phenotype, as follows: CCR5+CCR7-CD45RA-CD27+ and were able to re-recognize phosphoantigen and produce copious amounts of IFN-gamma up to 15 wk after treatment. Furthermore, the capacity of massively expanded Vgamma2Vdelta2 T cells to produce cytokines in vivo coincided with an increase in numbers of CD4+ and CD8+ alphabeta T cells after HMBPP/IL-2 cotreatment as well as substantial perforin expression by CD3+Vgamma2- T cells. Thus, the prolonged HMBPP-driven antimicrobial and cytotoxic responses of pulmonary and systemic Vgamma2Vdelta2 T cells may confer immunotherapeutics against infectious diseases and cancers.

Nonmevalonate terpene biosynthesis enzymes as antiinfective drug targets: substrate synthesis and high-throughput screening methods

The nonmevalonate isoprenoid pathway is an established target for antiinfective drug development. This paper describes high-throughput methods for the screening of 2C-methyl-D-erythritol synthase (IspC protein), 4-diphosphocytidyl-2C-methyl-D-erythritol synthase (IspD protein), 4-diphosphocytidyl-2C-methyl-D-erythritol kinase (IspE protein), and 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (IspF protein) against large compound libraries. The assays use up to three auxiliary enzymes. They are all monitored photometrically at 340 nm and are robust as documented by Z-factors of >or=0.86. 13C NMR assays designed for hit verification via direct detection of the primary reaction product are also described. Enzyme-assisted methods for the preparation, on a multigram scale, of isoprenoid biosynthesis intermediates required as substrates for these assays are reported. Notably, these methods enable the introduction of single or multiple 13C labels as required for NMR-monitored assays. The preparation of 4-diphosphosphocytidyl-2C-methyl-D-erythritol 2-phosphate in multigram quantities is described for the first time.

Regio- and stereoselective reactions of gem-difluorinated vinyloxiranes with heteronucleophiles

[Reaction: see text]. Regio- and stereoselectivity in reactions of gem-difluorinated vinyloxiranes with heteronucleophiles were successfully controlled. Halogen atoms were introduced regioselectively at the allylic epoxide carbon with an inversion in stereochemistry using MgBr2*Et2O or Li2CuCl4 to produce anti-vic-halohydrine. The other diastereomers were obtained selectively using LiBr/AcOH or BCl3, and SN2' type products were formed selectively with excellent E preference by changing the reaction temperature. Moreover, a further investigation led us to find that a regio- and stereoselective SN2' addition of several Brnsted acids was dependent on the pKa values of the acids. Under strong acidic conditions, we exclusively obtained E allylic alcohols.

Reconstitution of an apicoplast-localised electron transfer pathway involved in the isoprenoid biosynthesis of Plasmodium falciparum

In the malaria parasite Plasmodium falciparum isoprenoid precursors are synthesised inside a plastid-like organelle (apicoplast) by the mevalonate independent 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway. The last reaction step of the DOXP pathway is catalysed by the LytB enzyme which contains a [4Fe-4S] cluster. In this study, LytB of P. falciparum was shown to be catalytically active in the presence of an NADPH dependent electron transfer system comprising ferredoxin and ferredoxin-NADP(+) reductase. LytB and ferredoxin were found to form a stable protein complex. These data suggest that the ferredoxin/ferredoxin-NADP(+) reductase redox system serves as the physiological electron donor for LytB in the apicoplast of P. falciparum.

IspH Protein of Escherichia coli: Studies on Iron−Sulfur Cluster Implementation and Catalysis

The ispH gene of Escherichia coli specifies an enzyme catalyzing the conversion of 1-hydroxy-2-methyl-2-(E)-butenyl diphosphate into a mixture of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) in the nonmevalonate isoprenoid biosynthesis pathway. The implementation of a gene cassette directing the overexpression of the isc operon involved in the assembly of iron-sulfur clusters into an Escherichia coli strain engineered for ispH gene expression increased the catalytic activity of IspH protein anaerobically purified from this strain by a factor of at least 200. For maximum catalytic activity, flavodoxin and flavodoxin reductase were required in molar concentrations of 40 and 12 microM, respectively. EPR experiments as well as optical absorbance indicate the presence of a [3Fe-4S](+) cluster in IspH protein. Among 4 cysteines in total, the 36 kDa protein carries 3 absolutely conserved cysteine residues at the amino acid positions 12, 96, and 197. Replacement of any of the conserved cysteine residues reduced the catalytic activity by a factor of more than 70 000.

Synthesis of enantiopure 2-C-methyl-D-erythritol 4-phosphate and 2,4-cyclodiphosphate from D-arabitol

[reaction: see text] Two key intermediates of the newly discovered mevalonate-independent pathway for isoprenoid biosynthesis were prepared. Optically pure 2-C-methyl-D-erythritol 4-phosphate and 2,4-cyclodiphosphate were chemically synthesized from D-arabitol using a convenient benzylidene and tert-butyldimethylsilyl protection of polyhydroxylated intermediates. The new scheme offers a straightforward route to analogues and labeled forms.

A direct synthesis of hyperolactone C

Hyperolactone C is prepared from furan 4. The key transformation is a tandem Claisen rearrangement/lactonization.

The deoxyxylulose phosphate pathway of isoprenoid biosynthesis: studies on the mechanisms of the reactions catalyzed by IspG and IspH protein

Earlier in vivo studies have shown that the sequential action of the IspG and IspH proteins is essential for the reductive transformation of 2C-methyl-d-erythritol 2,4-cyclodiphosphate into dimethylallyl diphosphate and isopentenyl diphosphate via 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate. A recombinant fusion protein comprising maltose binding protein and IspG protein domains was purified from a recombinant Escherichia coli strain. The purified protein failed to transform 2C-methyl-d-erythritol 2,4-cyclodiphosphate into 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate, but catalytic activity could be restored by the addition of crude cell extract from an ispG-deficient E. coli mutant. This indicates that auxiliary proteins are required, probably as shuttles for redox equivalents. On activation by photoreduced 10-methyl-5-deaza-isoalloxazine, the recombinant protein catalyzed the formation of 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate from 2C-methyl-d-erythritol 2,4-cyclodiphosphate at a rate of 1 nmol x min(-1) x mg(-1). Similarly, activation by photoreduced 10-methyl-5-deaza-isoalloxazine enabled purified IspH protein to catalyze the conversion of 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate into a 6:1 mixture of isopentenyl diphosphate and dimethylallyl diphosphate at a rate of 0.4 micromol x min(-1) x mg(-1). IspH protein could also be activated by a mixture of flavodoxin, flavodoxin reductase, and NADPH at a rate of 3 nmol x min(-1) x mg(-1). The striking similarities of IspG and IspH protein are discussed, and plausible mechanistic schemes are proposed for the two reactions.