Redox regulation of maize NADP-malic enzyme by thiol-disulfide interchange: effect of reduced thioredoxin on activity

Engineering of Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) has evolved from a C₃ ground state to increase water use efficiency of photosynthesis. During CAM evolution, selective pressures altered the abundance and expression patterns of C₃ genes and their regulators to enable the trait. The circadian pattern of CO₂ fixation and the stomatal opening pattern observed in CAM can be explained largely with a regulatory architecture already present in C₃ plants. The metabolic CAM cycle relies on enzymes and transporters that exist in C₃ plants and requires tight regulatory control to avoid futile cycles between carboxylation and decarboxylation. Ecological observations and modeling point to mesophyll conductance as a major factor during CAM evolution. The present state of knowledge enables suggestions for genes for a minimal CAM cycle for proof-of-concept engineering, assuming altered regulation of starch synthesis and degradation are not critical elements of CAM photosynthesis and sufficient malic acid export from the vacuole is possible.

Inhibition of NADP-malic enzyme activity by H2 S and NO in sweet pepper (Capsicum annuum L.) fruits

NADPH is an essential cofactor in many physiological processes. Fruit ripening is caused by multiple biochemical pathways in which, reactive oxygen and nitrogen species (ROS/RNS) metabolism is involved. Previous studies have demonstrated the differential modulation of nitric oxide (NO) and hydrogen sulfide (H₂ S) content during sweet pepper (Capsicum annuum L.) fruit ripening, both of which regulate NADP-isocitrate dehydrogenase activity. To gain a deeper understanding of the potential functions of other NADPH-generating components, we analyzed glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH), which are involved in the oxidative phase of the pentose phosphate pathway (OxPPP) and NADP-malic enzyme (NADP-ME). During fruit ripening, G6PDH activity diminished by 38%, while 6PGDH and NADP-ME activity increased 1.5- and 2.6-fold, respectively. To better understand the potential regulation of these NADP-dehydrogenases by H₂ S, we obtained a 50-75% ammonium-sulfate-enriched protein fraction containing these proteins. With the aid of in vitro assays, in the presence of H₂ S, we observed that, while NADP-ME activity was inhibited by up to 29-32% using 2 and 5 mM Na₂ S as H₂ S donor, G6PDH and 6PGDH activities were unaffected. On the other hand, NO donors, S-nitrosocyteine (CysNO) and DETA NONOate also inhibited NADP-ME activity by 35%. These findings suggest that both NADP-ME and 6PGDH play an important role in maintaining the supply of NADPH during pepper fruit ripening and that H₂ S and NO partially modulate the NADPH-generating system.

Engineering oxidative stress defense pathways to build a robust lipid production platform in Yarrowia lipolytica

Microbially derived lipids have recently attracted renewed interests due to their broad applications in production of green diesels, cosmetic additives, and oleochemicals. Metabolic engineering efforts have targeted a large portfolio of biosynthetic pathways to efficiently convert sugar to lipids in oleaginous yeast. In the engineered overproducing strains, endogenous cell metabolism typically generates harmful electrophilic molecules that compromise cell fitness and productivity. Lipids, particularly unsaturated fatty acids, are highly susceptible to oxygen radical attack and the resulting oxidative species are detrimental to cell metabolism and limit lipid productivity. In this study, we investigated cellular oxidative stress defense pathways in Yarrowia lipolytica to further improve the lipid titer, yield, and productivity. Specifically, we determined that coupling glutathione disulfide reductase and glucose-6-phosphate dehydrogenase along with aldehyde dehydrogenase are efficient solutions to combat reactive oxygen and aldehyde stress in Y. lipolytica. With the reported engineering strategies, we were able to synchronize cell growth and lipid production, improve cell fitness and morphology, and achieved industrially-relevant level of lipid titer (72.7 g/L), oil content (81.4%) and productivity (0.97 g/L/h) in controlled bench-top bioreactors. The strategies reported here represent viable steps in the development of sustainable biorefinery platforms that potentially upgrade low value carbons to high value oleochemicals and biofuels. Biotechnol. Bioeng. 2017;114: 1521-1530. © 2017 Wiley Periodicals, Inc.

Systems biology and metabolic modelling unveils limitations to polyhydroxybutyrate accumulation in sugarcane leaves; lessons for C4 engineering

In planta production of the bioplastic polyhydroxybutyrate (PHB) is one important way in which plant biotechnology can address environmental problems and emerging issues related to peak oil. However, high biomass C4 plants such as maize, switch grass and sugarcane develop adverse phenotypes including stunting, chlorosis and reduced biomass as PHB levels in leaves increase. In this study, we explore limitations to PHB accumulation in sugarcane chloroplasts using a systems biology approach, coupled with a metabolic model of C4 photosynthesis. Decreased assimilation was evident in high PHB-producing sugarcane plants, which also showed a dramatic decrease in sucrose and starch content of leaves. A subtle decrease in the C/N ratio was found which was not associated with a decrease in total protein content. An increase in amino acids used for nitrogen recapture was also observed. Based on the accumulation of substrates of ATP-dependent reactions, we hypothesized ATP starvation in bundle sheath chloroplasts. This was supported by mRNA differential expression patterns. The disruption in ATP supply in bundle sheath cells appears to be linked to the physical presence of the PHB polymer which may disrupt photosynthesis by scattering photosynthetically active radiation and/or physically disrupting thylakoid membranes.

Light activation of NADP malic enzyme in leaves of maize: marginal increase in activity, but marked change in regulatory properties of enzyme

This article reports the characteristics of light activation of NADP-malic enzyme (NADP-ME, EC 1.1.1.40) in leaf discs of maize (Zea mays cv. VMH 404) for the first time. The leaf discs were illuminated in the presence of 2 mmol/L bicarbonate, as light activation increases in the presence of bicarbonate. Upon illumination, the Vmax of NADP-ME increased by about 30%. Although small, the increase was consistent and significant. The changes in regulatory properties of NADP-ME were quite pronounced. The extent of light activation was similar when substrate (malate) concentration was either 4 mmol/L (saturating) or 0.01 mmol/L (limiting). There was only a marginal change in the Km for malate, but there was marked change in the response of NADP-ME to activators or inhibitors. The Ki for pyruvate and oxalate increased by 100 and 67% respectively, while the Ka for the citrate and succinate increased by 36 and 32% respectively. These results suggest that the NADP-ME becomes less sensitive to feedback inhibition on illumination. The light-induced change seems to be due, at least partially, to the reduction of dithiols, as incubation of leaf extracts with DTE dampened light activation of NADP-ME. We conclude that the properties of NADP-ME do change on illumination. Although there was only a marginal increase in the activity of the enzyme on illumination of leaf discs, the changes in regulatory properties of NADP-ME were marked.

Thioltransferase activity of bovine lens glutathione S-transferase

A Mu-class glutathione S-transferase purified to electrophoretic homogeneity from bovine lens displayed thioltransferase activity, catalysing the transthiolation reaction between GSH and hydroxyethyldisulphide. The thiol-transfer reaction is composed of two steps, the formation of GSSG occurring through the generation of an intermediate mixed disulphide between GSH and the target disulphide. Unlike glutaredoxin, which is only able to catalyse the second step of the transthiolation process, glutathioneS-transferase catalyses both steps of the reaction. Data are presented showing that bovine lens glutathione S-transferase and rat liver glutaredoxin, which was used as a thioltransferase enzyme model, can operate in synergy to catalyse the GSH-dependent reduction of hydroxyethyldisulphide.

Superoxide: a two-edged sword

Superoxide (O2-) is the compound obtained when oxygen is reduced by one electron. For a molecule with an unpaired electron, O2- is surprisingly inert, its chief reaction being a dismutation in which it reacts with itself to form H2O2 and oxygen. The involvement of O2- in biological systems was first revealed by the discovery in 1969 of superoxide dismutase, an enzyme that catalyzes the dismutation of O2-. Since then it has been found that biological systems produce a bewildering variety of reactive oxidants, all but a few arising ultimately from O2-. These oxidants include O2- itself, H2O2 and alkyl peroxides, hydroxyl radical and other reactive oxidizing radicals, oxidized halogens and halamines, singlet oxygen, and peroxynitrite. These various oxidants are able to damage molecules in their environment, and are therefore very dangerous. They are thought to participate in the pathogenesis of a number of common diseases, including among others malignancy, by their ability to mutate the genome, and atherosclerosis, by their capacity for oxidizing lipoproteins. Their properties are put to good use, however, in host defense, where they serve as microbicidal and parasiticidal agents, and in biological signalling, where their liberation in small quantities results in redox-mediated changes in the functions of enzymes and other proteins.