Trehalose Interferes with the Photosynthetic Electron Transfer Chain of Cereibacter (Rhodobacter) sphaeroides Permeating the Bacterial Chromatophore Membrane

Disaccharide trehalose has been proven in many cases to be particularly effective in preserving the functional and structural integrity of biological macromolecules. In this work, we studied its effect on the electron transfer reactions that occur in the chromatophores of the photosynthetic bacterium Cereibacter sphaeroides. In the presence of a high concentration of trehalose, following the activation of the photochemistry by flashes of light, a slowdown of the electrogenic reactions related to the activity of the photosynthetic reaction center and cytochtome (cyt) bc1 complexes is observable. The kinetics of the third phase of the electrochromic carotenoid shift, due to electrogenic events linked to the reduction in cyt bH heme via the low-potential branch of the cyt bc1 complex and its oxidation by quinone molecule on the Qi site, is about four times slower in the presence of trehalose. In parallel, the reduction in oxidized cyt (c1 + c2) and high-potential cyt bH are strongly slowed down, suggesting that the disaccharide interferes with the electron transfer reactions of the high-potential branch of the bc1 complex. A slowing effect of trehalose on the kinetics of the electrogenic protonation of the secondary quinone acceptor QB in the reaction center complex, measured by direct electrometrical methods, was also found, but was much less pronounced. The direct detection of carbohydrate content indicates that trehalose, at high concentrations, permeates the membrane of chromatophores. The possible mechanisms underlying the observed effect of trehalose on the electron/proton transfer process are discussed in terms of trehalose's propensity to form strong hydrogen bonds with its surroundings.

Trehalose counteracts the dissociation of tetrameric rabbit lactate dehydrogenase induced by acidic pH conditions

The lactate dehydrogenase from rabbit skeletal muscle (rbLDH) is a tetrameric enzyme, known to undergo dissociation when exposed to acidic pH conditions. Moreover, it should be mentioned that this dissociation translates into a pronounced loss of enzyme activity. Notably, among the compounds able to stabilize proteins and enzymes, the disaccharide trehalose represents an outperformer. In particular, trehalose was shown to efficiently counteract quite a number of physical and chemical agents inducing protein denaturation. However, no information is available on the effect, if any, exerted by trehalose against the dissociation of protein oligomers. Accordingly, we thought it of interest to investigate whether this disaccharide is competent in preventing the dissociation of rbLDH induced by acidic pH conditions. Further, we compared the action of trehalose with the effects triggered by maltose and cellobiose. Surprisingly, both these disaccharides enhanced the dissociation of rbLDH, with maltose being responsible for a major effect when compared to cellobiose. On the contrary, trehalose was effective in preventing enzyme dissociation, as revealed by activity assays and by Dynamic Light Scattering (DLS) experiments. Moreover, we detected a significant decrease of both K_0.5 and V_max when the rbLDH activity was tested (at pH 7.5 and 6.5) as a function of pyruvate concentration in the presence of trehalose. Further, we found that trehalose induces a remarkable increase of V_max when the enzyme is exposed to pH 5. Overall, our observations suggest that trehalose triggers conformational rearrangements of tetrameric rbLDH mirrored by resistance to dissociation and peculiar catalytic features.

Exogenous trehalose protects photosystem II by promoting cyclic electron flow under heat and drought stresses in winter wheat

Drought and rising global temperatures are important factors that reduce wheat production. Trehalose protects the reaction centres and improves photosystem II (PSII) activity under diverse stress conditions. However, the underlying mechanism remains unknown. Cyclic electron flow (CEF) plays an important role in protecting PSII under environmental stresses. Our study focused on the effects of exogenous trehalose on the activity of PSII, D1 protein content, plastoquinone (PQ) pool and ATP synthase activity in wheat seedlings under heat and drought stresses to explore the relationship between trehalose and CEF. The results indicated that heat and drought stresses decreased maximum photochemical efficiency of PSII (F_v /F_m ) and electron transport rate of PSII (EFR(II)), whereas the trehalose pretreatment improved photochemical efficiency and electron transport rate of PSII. The trehalose pretreatment stimulated CEF under heat and drought stresses. Furthermore, the proton gradient (ΔpH) across the thylakoid membrane and ATPase activity increased. The higher ΔpH and ATPase activity played a key role in protecting PSII under stresses. Trehalose pretreatment could reduce inhibition caused by heat and drought stresses on the PQ pool. Thus, our results indicated that photoinhibition in heat- and drought-stressed plants was alleviated by the trehalose pretreatment, which was mediated by CEF and the PQ pool.

Effect of Detergents and Osmolytes on Thermal Stability of Native and Mutant Rhodobacter sphaeroides Reaction Centers

Photosynthetic reaction center (RC) of the purple bacterium Rhodobacter sphaeroides is one of the most well-studied transmembrane pigment-protein complexes. It is a relatively stable protein with established conditions for its isolation from membranes, purification, and storage. However, it has been shown that some amino acid substitutions can affect stability of the RC, which results in a decrease of the RCs yield during its isolation and purification, disturbs spectral properties of the RCs during storage, and can lead to sample heterogeneity. To optimize conditions for studying mutant RCs, the effect of various detergents and osmolytes on thermal stability of the complex was examined. It was shown that trehalose and, to a lesser extent, sucrose, maltose, and hydroxyectoin at 1 M concentration slow down thermal denaturation of RCs. Sodium cholate was found to have significant stabilizing effect on the structure of native and genetically modified RCs. The use of sodium cholate as a detergent has several advantages and can be recommended for the storage and investigation of the unstable mutant membrane complexes of purple bacteria in long-term experiments.

Photovoltage generation by photosystem II core complexes immobilized onto a Millipore filter on an indium tin oxide electrode

The light-induced functioning of photosynthetic pigment-protein complex of photosystem II (PSII) is linked to the vectorial translocation of charges across the membrane, which results in the formation of voltage. Direct measurement of the light-induced voltage (∆V) generated by spinach oxygen-evolving PSII core complexes adsorbed onto a Millipore membrane filter (MF) on an indium tin oxide (ITO) electrode under continuous illumination has been performed. PSII was shown to participate in electron transfer from water to the ITO electrode, resulting in ∆V generation. No photovoltage was detected in PSII deprived of the water-oxidizing complex. The maximal and stable photoelectric signal was observed in the presence of disaccharide trehalose and 2,6-dichloro-1,4-benzoquinone, acting as a redox mediator between the primary quinone acceptor Q_A of PSII and electrode surface. Long time preservation of the steady-state photoactivity at room temperature in a simple in design ITO|PSII-MF|ITO system may be related to the retention of water molecules attached to the PSII surface in the presence of trehalose.

Control of electron transfer by protein dynamics in photosynthetic reaction centers

Trehalose and glycerol are low molecular mass sugars/polyols that have found widespread use in the protection of native protein states, in both short- and long-term storage of biological materials, and as a means of understanding protein dynamics. These myriad uses are often attributed to their ability to form an amorphous glassy matrix. In glycerol, the glass is formed only at cryogenic temperatures, while in trehalose, the glass is formed at room temperature, but only upon dehydration of the sample. While much work has been carried out to elucidate a mechanistic view of how each of these matrices interact with proteins to provide stability, rarely have the effects of these two independent systems been directly compared to each other. This review aims to compile decades of research on how different glassy matrices affect two types of photosynthetic proteins: (i) the Type II bacterial reaction center from Rhodobacter sphaeroides and (ii) the Type I Photosystem I reaction center from cyanobacteria. By comparing aggregate data on electron transfer, protein structure, and protein dynamics, it appears that the effects of these two distinct matrices are remarkably similar. Both seem to cause a "tightening" of the solvation shell when in a glassy state, resulting in severely restricted conformational mobility of the protein and associated water molecules. Thus, trehalose appears to be able to mimic, at room temperature, nearly all of the effects on protein dynamics observed in low temperature glycerol glasses.

Trehalose protects Mn-depleted photosystem 2 preparations against the donor-side photoinhibition

Recently, it has been shown that the addition of 1M trehalose leads to the increase of the rate of oxygen photoconsumption associated with activation of electron transport in the reaction center of photosystem 2 (PS2) in Mn-depleted PS2 membranes (apo-WOC-PS2) [37]. In the present work the effect of trehalose on photoinhibition of apo-WOC-PS2 preparations (which are characterized by a high sensitivity to the donor side photoinhibition of PS2) was investigated. The degree of photoinhibition was estimated by the loss of the capability of exogenous electron donor (sodium ascorbate) to reactivate the electron transport (measured by light-induced changes of chlorophyll fluorescence yield (∆F)) in apo-WOC-PS2. It was found that 1M trehalose enhanced the Mn²⁺-dependent suppression of photoinhibition of apo-WOC-PS2: in the presence of trehalose the addition of 0.2μM Mn²⁺ (corresponding to 2 Mn²⁺ per one reaction center) was sufficient for an almost complete suppression of the donor side photoinhibition of the complex. In the absence of trehalose it was necessary to add 100μM Mn²⁺ to achieve a similar result. The effect of trehalose was observed during photoinhibition of apo-WOC-PS2 at low (15μmolphotons^-1m^-2) and high (200μmolphotons^-1m^-2) light intensity. When Mn²⁺ was replaced by other PS2 electron donors (ferrocyanide, DPC) as well as by Ca²⁺ the protective effect of trehalose was not observed. It was also found that 1M trehalose decreased photoinhibition of apo-WOC-PS2 if the samples contained endogenous manganese (1-2 Mn ions per one RC was enough for the maximum protection effect). It is concluded that structural changes in PS2 caused by the addition of trehalose enhance the capability of photochemical reaction centers of apo-WOC-PS2 to accept electrons from manganese (both exogenous and endogenous), which in turn leads to a considerable suppression of the donor side photoinhibition of PS2.

Trehalose stimulation of photoinduced electron transfer and oxygen photoconsumption in Mn-depleted photosystem 2 membrane fragments

It is known that the removal of manganese from the water-oxidizing complex (WOC) of photosystem 2 (PS2) leads to activation of oxygen photoconsumption (OPC) [Khorobrykh et al., 2002; Yanykin et al., 2010] that is accompanied by the formation of organic hydroperoxides on the electron-donor side of PS2 [Khorobrykh et al., 2011]. In the present work the effect of trehalose on the OPC in Mn-depleted PS2 preparations (apo-WOC-PS2) was investigated. A more than two-fold increase of the OPC is revealed upon the addition of 1M trehalose. Drastic (30%-70%) inhibition of the OPC upon the addition of either electron acceptor or electron donor indicates that the trehalose-induced activation of the OPC occurs on both donor and acceptor sides of PS2. A two-fold increase in the rate of superoxide-anion radical photoproduction on the electron-acceptor side of PS2 was also shown. Applying the "variable" chlorophyll fluorescence (ΔF) it was shown that the addition of trehalose induces: (i) a significant increase in the ability of exogenous Mn(2+) to donate electrons to the reaction center of PS2, (ii) slowing down the photoaccumulation of the primary quinone electron acceptor of PS2 (QA(-)) under aerobic conditions, (iii) acceleration of the reoxidation of QA(-) by QB (and by QB(-)) as well as the replacement of QB(2-) by a fully oxidized plastoquinone, and (iv) restoration of the electron transfer between the quinone electron carriers in the so-called "closed reaction centers of PS2" (their content in the apo-WOC-PS2 is 41%). It is suggested that the trehalose-induced increase in efficiency of the O2 interaction with the electron-donor and electron-acceptor sides of apo-WOC-PS2 is due to structural changes leading to both a decrease in the proportion of the "closed PS2 reaction centers" and an increase in the electron transfer rate in PS2.