Laser Light Strips Bacterial Membranes of Key Electron Carrier, Enabling In-Place Study

Bacteria run their energy metabolism through a tangled web of membrane-bound enzymes, and at the heart of that web sit the lipoquinones: small, fat-soluble molecules that shuttle protons and electrons between the enzymes that generate them and the enzymes that consume them. A team reporting in the journal Discover Bacteria has now devised a way to remove these carriers from mycobacterial membranes on demand, using nothing more exotic than a violet-blue laser diode, and then to add them back in chemically defined forms. The method opens a route to studying the electron transport system of Mycobacterium smegmatis in its native membrane environment rather than in reconstituted fragments.

The motivation comes from a long-standing gap in microbiology. Lipoquinones have been used for decades as taxonomic markers because their structures, particularly the length and saturation of their isoprenyl side chains, are highly conserved within bacterial taxa. Mycobacteria, for example, carry predominantly menaquinone MK-9(II-H2), a naphthoquinone headgroup bearing nine isoprene units with the second unit fully saturated. Yet while the structures are well catalogued, remarkably little is known about how those structural variations affect function, or about the relative importance of the enzymes that use each lipoquinone as electron acceptor or donor. Because the electron transport system is modular, with interchangeable dehydrogenases feeding a shared membrane pool, the authors argue that meaningful characterization demands working in situ, inside intact inverted membrane vesicles where all components retain their native stoichiometry.

Inverted membrane vesicles, or IMVs, are prepared by probe sonication of bacterial cells, which shears the envelope into sealed fragments with the enzymatic faces of the membrane accessible to the assay medium. The team characterized their preparations carefully. NADH dehydrogenase activity was followed spectrophotometrically by the decrease in absorbance at 340 nanometers as NADH is oxidized, and independently by the reduction of ferricyanide at 420 nanometers, with potassium cyanide blocking downstream electron flow to the cytochromes. Comparing rates with and without the detergent Tween-20, which solubilizes the membrane so that substrate reaches enzymes regardless of vesicle orientation, indicated roughly 60 percent inversion of the vesicles. Notably, Triton X-100, the detergent used in the classic protocol this work adapted, inhibited M. smegmatis NADH dehydrogenase activity, so the researchers screened several alternatives and settled on Tween-20 at an optimized pH of 7.5 in a mixed buffer of constant ionic strength.

A potential confounding activity was also ruled out. Cyanide-resistant alternative oxidases, first identified in bacteria in 2003, could in principle siphon electrons away from the cytochrome pathway. Genome searches across thousands of Actinobacteriota genomes had found no such genes in the Mycobacteriaceae, and the team verified this experimentally: potassium cyanide nearly eliminated oxygen consumption in their vesicles, with a half-maximal inhibitory concentration of 0.40 millimolar measured by high-resolution respirometry.

With the assay platform validated, the researchers asked which substrates actually drive respiration in these membranes. Testing glycerol-3-phosphate, lactate, NADH, malate, proline, and succinate in an Oxygraph-2k respirometer, they found that only NADH and malate supported robust oxygen consumption; every other substrate delivered less than ten percent of the rate seen with those two. That focused attention on the two classes of NADH dehydrogenase, the proton-pumping type 1 Ndh I encoded by fourteen nuo genes and the non-proton-pumping type 2 Ndh II, and on malate:quinone oxidoreductase, or Mqo, which in M. smegmatis is the sole route to regenerating oxaloacetate for the TCA cycle because the genome lacks a malate dehydrogenase gene.

Inhibitor experiments indicated that Ndh II carries most of the NADH-oxidizing burden under the aerobic, nutrient-abundant growth conditions used. Chlorpromazine, previously reported as a selective Ndh II inhibitor, suppressed roughly 88 percent of total NADH dehydrogenase activity at 400 micromolar, with an IC50 of 97.6 micromolar, while rotenone, a canonical Ndh I inhibitor, achieved only about 32 percent maximal inhibition. The authors caution that chlorpromazine is evidently not specific for Ndh II, since it eliminated around 90 percent of combined activity, and they suggest on the basis of prior flavoenzyme work that phenothiazines likely interact with both dehydrogenases at or near their flavin-binding sites. The dominance of Ndh II is consistent with earlier western blot data showing that Ndh I is scarce under standard growth conditions but rises when carbohydrates are absent.

A more conceptually significant result concerned the organization of the lipoquinone pool itself. Whether respiratory quinones move freely as a single shared pool or are partitioned into private channels dedicated to individual enzyme complexes has been debated extensively in the mitochondrial literature. When the team supplied saturating malate alone, ferricyanide reduction reached 9.2 picomoles per second per microgram of protein, versus 3.2 with saturating NADH alone. Supplying both substrates together, in either order, yielded intermediate rates of 6.7 and 7.3, not an additive sum. Control experiments confirmed that NADH does not directly inhibit Mqo. The intermediate values imply that both dehydrogenase classes draw on a single, universally accessible menaquinone pool, and that menaquinone availability becomes rate-limiting when electron donors are saturating.

The depletion method itself grew out of an unexpected obstacle. Early reports suggested ultraviolet light at 360 nanometers could strip lipoquinones from membranes, but available UV sources proved ineffective. Turning to the violet-blue range of 380 to 480 nanometers, which has attracted interest for microbial inactivation, the team built an optical setup around a 405 nanometer laser diode, circularizing the beam to a two-centimeter spot delivering 1.0 milliwatt per square centimeter. First they showed that menaquinone-3 and menaquinone-4, irradiated for four hours in hexane or in aqueous detergent dispersion, underwent genuine chemical modification: infrared spectra showed a drastic loss of the carbonyl stretching peak near 1659 wavenumbers and the appearance of a broad hydroxyl band, consistent with photoredox chemistry of isoprenyl-naphthoquinones, though the exact products remain to be fully elucidated.

Applied to intact vesicles, the laser worked strikingly well. Oxygen consumption driven by NADH fell by 97.6 percent after 90 minutes of irradiation, with a depletion half-time of 17 minutes; NADH dehydrogenase and Mqo activities dropped by 94.9 and 69.2 percent respectively after 120 minutes. Crucially, the damage was reversible. Adding detergent-solubilized MK-4 to irradiated vesicles restored NADH dehydrogenase activity to non-irradiated levels at 200 micromolar, and MK-3 achieved the same at concentrations as low as 100 micromolar, following Michaelis-Menten kinetics with an apparent Km of 7.1 micromolar. Because activity could be fully rescued, the authors conclude that Ndh I, Ndh II, and Mqo emerge from irradiation essentially unharmed, and that the activity loss reflects lipoquinone depletion rather than protein damage. The rescue concentrations also imply that the effective endogenous menaquinone concentration accessible in these detergent-containing assays lies in the 100 to 200 micromolar range, two orders of magnitude below estimates of 10 to 15 millimolar for mycobacterial membranes.

One subtle observation hints at physiological regulation: Mqo activity in non-irradiated vesicles rose 2.3-fold when extra MK-4 was supplied, whereas NADH dehydrogenase activity rose only 1.4-fold, suggesting that Mqo is not saturated by native membrane lipoquinone levels and may be more sensitive to quinone availability in living cells. For now, the practical payoff is methodological. The work establishes that bacterial membranes can be photodepleted of their electron carriers and reconstituted with structurally defined synthetic analogs, laying the foundation for systematic structure-activity studies of how conserved lipoquinone variations shape electron transport, in mycobacteria and beyond. Given that oxidative phosphorylation is a validated drug target in pathogenic mycobacteria, anchored by bedaquiline and the cytochrome bc1:aa3 inhibitor Q203, a tool that dissects respiratory flexibility in its native membrane context could prove broadly useful, even as the authors frame the present study as groundwork for future characterization rather than a therapeutic advance in itself.

Subject of Research: Selective photodepletion of mycobacterial membrane lipoquinone to study the electron transport system in situ

Article Title: Selective laser ablation of mycobacterial membrane lipoquinone facilitates study of the electron transport system in situ

Article References: Russell Serrano, A., Markham, S. A., Dolan, C. C., Arhouma, Z., Whitcomb, L. A., Schlink, D. M., Rose, L., Kostenkova, K., Jankowska, E., Davenport, A., Menoni, C. S., Chicco, A. J., Crans, D. C., & Crick, D. C. (2026). Selective laser ablation of mycobacterial membrane lipoquinone facilitates study of the electron transport system in situ. Discover Bacteria, 3(1), Article 9. https://doi.org/10.1007/s44351-026-00045-x

Image Credits: AI Generated

DOI: 10.1007/s44351-026-00045-x

Keywords: Mycobacterium smegmatis, lipoquinone, menaquinone, electron transport system, inverted membrane vesicles, laser irradiation, NADH dehydrogenase, malate:quinone oxidoreductase, oxidative phosphorylation, respirometry, enzyme assays, Selective