Biochemical characterization of protoporphyrinogen dehydrogenase and protoporphyrin ferrochelatase of Vibrio vulnificus and the critical complex formation between these enzymes

In Campylobacter jejuni, a new type of chaperone receives heme from ferrochelatase

Intracellular heme formation and trafficking are fundamental processes in living organisms. Bacteria and archaea utilize three biogenesis pathways to produce iron protoporphyrin IX (heme b) that diverge after the formation of the common intermediate uroporphyrinogen III (uro'gen III). In this study, we identify and provide a detailed characterization of the enzymes involved in the transformation of uro'gen III into heme in Campylobacter jejuni, demonstrating that this bacterium utilizes the protoporphyrin-dependent (PPD) pathway. In general, limited knowledge exists regarding the mechanisms by which heme b reaches its target proteins after this final step. Specifically, the chaperones necessary for trafficking heme to prevent the cytotoxic effects associated with free heme remain largely unidentified. In C. jejuni, we identified a protein named CgdH2 that binds heme with a dissociation constant of 4.9 ± 1.0 µM, and this binding is impaired upon mutation of residues histidine 45 and 133. We demonstrate that C. jejuni CgdH2 establishes protein-protein interactions with ferrochelatase, suggesting its role in facilitating heme transfer from ferrochelatase to CgdH2. Furthermore, phylogenetic analysis reveals that C. jejuni CgdH2 is evolutionarily distinct from the currently known chaperones. Therefore, CgdH2 is the first protein identified as an acceptor of intracellularly formed heme, expanding our knowledge of the mechanisms underlying heme trafficking within bacterial cells.

Chlorophyll a Synthesis in Rhodobacter sphaeroides by Chlorophyll Synthase of Nicotiana tabacum

The production of phytylated chlorophyll a (Chl a_P) in Rhodobacter sphaeroides, which uses phytylated bacteriochlorophyll a (BChl a_P), is the first step in expanding the light absorption spectra. Unlike the chlorophyll synthase (ChlG) of the Synechocystis sp. PCC6803, ChlGs of angiosperms, including Arabidopsis thaliana, Nicotiana tabacum, Avena sativa, and Oryza sativa, showed bacteriochlorophyll synthase activity and resistance to inhibition by bacteriochlorophyllide a (BChlide a), geranylgeranylated BChl a (BChl a_GG), and BChl a_P, collectively called bacteriochlorins. Among the angiosperm ChlGs, N. tabacum ChlG had the highest bacteriochlorophyll synthase activity and resistance to inhibition by bacteriochlorins. Expression of N. tabacum chlG in R. sphaeroides resulted in the formation of free Chl a_P in the presence of BChl a_P during photoheterotrophic growth, even though reactive oxygen species were generated.

The Photoactive Photosynthetic Reaction Center of a Rhodobacter sphaeroides Mutant Lacking 3-Vinyl (Bacterio)Chlorophyllide a Hydratase Contains 3-Vinyl Bacteriochlorophyll a

Rhodobacter sphaeroides mutant BF-lacking 3-vinyl (bacterio)chlorophyllide a hydratase (BchF)-accumulates chlorophyllide a (Chlide a) and 3-vinyl bacteriochlorophyllide a (3V-Bchlide a). BF synthesizes 3-vinyl bacteriochlorophyll a (3V-Bchl a) through prenylation of 3V-Bchlide a and assembles a novel reaction center (V-RC) using 3V-Bchl a and Mg-free 3-vinyl bacteriopheophytin a (3V-Bpheo a) at a molar ratio of 2:1. We aimed to verify whether a bchF-deleted R. sphaeroides mutant produces a photochemically active RC that facilitates photoheterotrophic growth. The mutant grew photoheterotrophically-implying a functional V-RC-as confirmed by the emergence of growth-competent suppressors of bchC-deleted mutant (BC) under irradiation. Suppressor mutations in BC were localized to bchF, which diminished BchF activity and caused 3V-Bchlide a accumulation. bchF expression carrying the suppressor mutations in trans resulted in the coproduction of V-RC and wild-type RC (WT-RC) in BF. The V-RC had a time constant (τ) for electron transfer from the primary electron donor P (a dimer of 3V-Bchl a) to the A-side containing 3V-Bpheo a (HA) similar to that of the WT-RC and a 60% higher τ for electron transfer from HA to quinone A (QA). Thus, the electron transfer from HA to QA in the V-RC should be slower than that in the WT-RC. Furthermore, the midpoint redox potential of P/P+ of the V-RC was 33 mV more positive than that of the WT-RC. R. sphaeroides, thus, synthesizes the V-RC when 3V-Bchlide a accumulates. The V-RC can support photoheterotrophic growth; however, its photochemical activity is inferior to that of the WT-RC. IMPORTANCE 3V-Bchlide a is an intermediate in the bacteriochlorophyll a (Bchl a)-specific biosynthetic branch and prenylated by bacteriochlorophyll synthase. R. sphaeroides synthesizes V-RC that absorbs light at short wavelengths. The V-RC was not previously discovered because 3V-Bchlide a does not accumulate during the growth of WT cells synthesizing Bchl a. The levels of reactive oxygen species increased with the onset of photoheterotrophic growth in BF, resulting in a long lag period. Although the inhibitor of BchF is unknown, the V-RC may act as a substitute for the WT-RC when BchF is completely inhibited. Alternatively, it may act synergistically with WT-RC at low levels of BchF activity. The V-RC may broaden the absorption spectra of R. sphaeroides and supplement its photosynthetic ability at various wavelengths of visible light to a greater extent than that by the WT-RC alone.

Photoautotrophic Growth Rate Enhancement of Synechocystis sp. PCC6803 by Heterologous Production of 2-Oxoglutarate:Ferredoxin Oxidoreductase from Chlorobaculum tepidum

2-Oxoglutarate:ferredoxin oxidoreductase from Chlorobaculum tepidum (CtOGOR) is a carbon-fixing enzyme in the reductive TCA cycle that reversibly carboxylates succinyl-CoA to yield 2-oxoglutarate. CtOGOR is a heterotetramer of two large (α = 68 kDa) and two small (β = 38 kDa) subunits. The αβ protomer harbors one thiamine pyrophosphate and two 4Fe-4S clusters. Nonetheless, the enzyme has a considerable oxygen tolerance with a half-life of 143 min at 215 μM dissolved oxygen. Kinetic analyses of the purified recombinant CtOGOR revealed a lower Km for succinyl-CoA than for 2-oxoglutarate. Cellular levels of 2-oxoglutarate and glutamate—a product of glutamine oxoglutarate aminotransferase and glutamate dehydrogenase—increased more than twofold in the exponential phase compared with the control strain, leading to an approximately >30% increase in the photoautotrophic growth rate. Thus, CtOGOR was successfully produced in Synechocystis, thereby boosting carboxylation, resulting in enhanced photoautotrophic growth.

Sustainability of in vitro light-dependent NADPH generation by the thylakoid membrane of Synechocystis sp. PCC6803

BACKGROUND

NADPH is used as a reductant in various biosynthetic reactions. Cell-free bio-systems have gained considerable attention owing to their high energy utilization and time efficiency. Efforts have been made to continuously supply reducing power to the reaction mixture in a cyclical manner. The thylakoid membrane (TM) is a promising molecular energy generator, producing NADPH under light. Thus, TM sustainability is of major relevance for its in vitro utilization.

RESULTS

Over 70% of TMs prepared from Synechocystis sp. PCC6803 existed in a sealed vesicular structure, with the F1 complex of ATP synthase facing outward (right-side-out), producing NADPH and ATP under light. The NADPH generation activity of TM increased approximately two-fold with the addition of carbonyl cyanide-p-(trifluoromethoxy) phenylhydrazone (FCCP) or removal of the F1 complex using EDTA. Thus, the uncoupling of proton translocation from the electron transport chain or proton leakage through the Fo complex resulted in greater NADPH generation. Biosilicified TM retained more than 80% of its NADPH generation activity after a week at 30°C in the dark. However, activity declined sharply to below 30% after two days in light. The introduction of engineered water-forming NADPH oxidase (Noxm) to keep the electron transport chain of TM working resulted in the improved sustainability of NADPH generation activity in a ratio (Noxm to TM)-dependent manner, which correlated with the decrease of singlet oxygen generation. Removal of reactive oxygen species (ROS) by catalase further highlighted the sustainable NADPH generation activity of up to 80% in two days under light.

CONCLUSION

Reducing power generated by light energy has to be consumed for TM sustainability. Otherwise, TM can generate singlet oxygen, causing oxidative damage. Thus, TMs should be kept in the dark when not in use. Although NADPH generation activity by TM can be extended via silica encapsulation, further removal of hydrogen peroxide results in an improvement of TM sustainability. Therefore, as long as ROS formation by TM in light is properly handled, it can be used as a promising source of reducing power for in vitro biochemical reactions.