Carbon and nitrogen metabolism in legume root nodules

The literature concerning the metabolism of carbon compounds during the reduction, assimilation and translocation of nitrogen in root nodules of leguminous plants is reviewed. The reduction of dinitrogen requires an energy source (ATP) and a reluctant which are both supplied by respiratory catabolism of carbohydrates produced by the host plant. Photosynthates are also required to generate the carbon skeletons for amino acid or urcide synthesis during the assimilation of ammonia produced by the bacteria within the nodule tissue. Competition for photosynthates occurs between the bacteroids, nodule tissue and the various vegetative and reproductive sinks in the host plant. The nature of carbon compounds involved in these processes, their routes of metabolism, the mechanisms of control and the partitioning of metabolises between the various sites of utilization are only poorly understood. It is apparent that dinitrogen is reduced to ammonia in the bacteroids. Both fast- and slow-growing strains of Rhizobium possess the Entner-Doudoroff pathway of glucose catabolism, and some, if not all, enzymes of the Emden-Meyerhof pathway. Some bacterial cultures also metabolize carbon through the ketogluconate pathway but only the fast-growing strains of cultured rhizobia possess the key enzyme of the pentose phosphate pathway (6-phosphogluconate dehydrogenase). The host cells are thought to contain the complete Emden-Meyerhof pathway and tricarboxylic acid cycle, which provides the carbon skeletons for assimilation of the ammonia, formed by the bacteroids, into α-amino acids. A pathway of anapleurotic carbon conservation, operative in the host cells, synthesizes oxaloacetic acid through β-carboxylation of phosphoenol pyruvate. This process could be important in the recapture and assimilation of respired CO₂ in the rhizosphere. The main route of assimilation of ammonia produced by the bacteroids would appear to be via the glutamine synthetase-glutamate synthase pathway in the host cells. However, glutamate dehydrogenase may also be involved in ammonia assimilation. These enzymes also occur in in vitro cultures of Rhizobium and in bacteroids where they presumably participate in the synthesis of amino acids for growth of the bacteria or bacteroids. Nitrogen assimilated into glutamine or glutamate is exported from the nodules in a variety of forms, which include asparagine, glutamine, aspartate, homoserine and allantoates, in proportions which depend on the legume species. Studies on regulation of the overall process have focussed on expression of bacteroid genes and on the control of enzyme activity, at the level of nitrogenase and enzymes of nitrogen assimilation in particular. However, due to the wide range of experimental techniques, environmental conditions and plant species which have been used, no clear conclusions can yet be drawn. The pathways of carbon flow in nitrogen metabolism, particularly in relation to the synthesis of ureides and the regulation of carbon metabolism, remain key areas for future research in symbiotic nitrogen fixation.     

Molecular and structural basis for redox regulation of beta-actin

An essential consequence of growth factor-mediated signal transduction is the generation of intracellular H₂O₂. It operates as a second messenger in the control of actin microfilament dynamics, causing rapid and dramatic changes in the morphology and motile activity of stimulated cells. Little is understood about the molecular mechanisms causing these changes in the actin system. Here, it is shown that H₂O₂ acts directly upon several levels of this system, and some of the mechanistic effects are detailed. We describe the impact of oxidation on the polymerizability of non-muscle β/γ-actin and compare with that of muscle α-actin. Oxidation of β/γ-actin can cause a complete loss of polymerizability, crucially, reversible by the thioredoxin system. Further, oxidation of the actin impedes its interaction with profilin and causes depolymerization of filamentous actin. The effects of oxidation are critically dependent on the nucleotide state and the concentration of Ca²⁺. We have determined the crystal structure of oxidized β-actin to a resolution of 2.6 Å. The arrangement in the crystal implies an antiparallel homodimer connected by an intermolecular disulfide bond involving cysteine 374. Our data indicate that this dimer forms under non-polymerizing and oxidizing conditions. We identify oxidation of cysteine 272 in the crystallized actin dimer, likely to a cysteine sulfinic acid. In β/γ-actin, this is the cysteine residue most reactive towards H₂O₂ in solution, and we suggest plausible structural determinants for its reactivity. No other oxidative modification was obvious in the structure, highlighting the specificity of the oxidation by H₂O₂. Possible consequences of the observed effects in a cellular context and their potential relevance are discussed.     

Comparison of the Effects of Orally Administered Linoleic Acid,and Its Hydroperoxides and Secondary Autoxidation Products on Hepatic Lipid Metabolism in Rats

Linoleic acid, and its hydroperoxides and secondary autoxidation products were orally administered to rats (400 mg/rat). Their effects on hepatic lipid metabolism were examined. Linoleic acid reduced the activities of de novo synthesis of fatty acids and acetyl-CoA carboxylase. It decreased the CoASH level and caused the accumulation of long-chain acyl-CoA. Hydroperoxides changed the compositions of unsaturated fatty acids in the hepatic lipids and lowered the content of neutral lipids. Secondary products stimulated carnitine palmitoyltransferase and decreased the content of neutral lipids. They reduced the activities of de novo synthesis of fatty acids and acetyl-CoA carboxylase, and the levels of CoASH and acetyl-CoA. Thus, the effect of secondary products was apparently different from those of linoleic acid and its hydroperoxides.     

Catalytic reduction of acetylene in the presence of molybdenum and iron clusters, including FeMo cofactor of nitrogenase

Acetylene was reduced by zinc amalgam in the presence of three synthetic polynuclear complexes: {[Mg₂Mo₈O₂₂(OMe)₆(MeOH)₄]⁻²·[Mg(MeOH)₆]²⁺}6MeOH (I), (Bu₄N)₂[Fe₄S₄(SPh)₄] (II), [Me₄N][VFe₃S₄Cl₃(DMF)₃]·2DMF (III) and the iron-molybdenum cofactor of nitrogenase Azotobacter vinelandii MoFe₇(S²⁻)₉·homocitrate, FeMo-co (IV). Thiophenol was found to greatly facilitate the reaction in the presence of complexes I, II, IV. The reaction is catalytic and for I and IV proceeds at the amalgam surface. Thiophenol seems to increase the adsorption of the complexes, serving as an electron bridge to transfer electrons to the catalyst. In the case of II a homogeneous reduction of the substrate occurs presumably after the cluster reduction at the surface and with III the catalytic reduction proceeds only under the action of sodium amalgam; no thiophenol cocatalytic action is observed. Relevance to N₂ enzymatic reduction is discussed.     

Promises of Main Group Metal–Based Nanostructured Materials for Electrochemical CO2 Reduction to Formate

Selective CO₂ reduction to formic acid or formate is the most technologically and economically viable approach to realize electrochemical CO₂ valorization. Main group metal–based (Sn, Bi, In, Pb, and Sb) nanostructured materials hold great promise, but are still confronted with several challenges. Here, the current status, challenges, and future opportunities of main group metal–based nanostructured materials for electrochemical CO₂ reduction to formate are reviewed. Firstly, the fundamentals of electrochemical CO₂ reduction are presented, including the technoeconomic viability of different products, possible reaction pathways, standard experimental procedure, and performance figures of merit. This is then followed by detailed discussions about different types of main group metal–based electrocatalyst materials, with an emphasis on underlying material design principles for promoting the reaction activity, selectivity, and stability. Subsequently, recent efforts on flow cells and membrane electrode assembly cells are reviewed so as to promote the current density as well as mechanistic studies using in situ characterization techniques. To conclude a short perspective is offered about the future opportunities and directions of this exciting field.     

BAmSA: Visualising transmembrane regions in protein complexes using biotinylated amphipols and electron microscopy

Membrane protein (MP) complexes play key roles in all living cells. Their structural characterisation is hampered by difficulties in purifying and crystallising them. Recent progress in electron microscopy (EM) have revolutionised the field, not only by providing higher-resolution structures for previously characterised MPs but also by yielding first glimpses into the structure of larger and more challenging complexes, such as bacterial secretion systems. However, the resolution of pioneering EM structures may be difficult and their interpretation requires clues regarding the overall organisation of the complexes. In this context, we present BAmSA, a new method for localising transmembrane (TM) regions in MP complexes, using a general procedure that allows tagging them without resorting to neither genetic nor chemical modification. Labels bound to TM regions can be visualised directly on raw negative-stain EM images, on class averages, or on three-dimensional reconstructions, providing a novel strategy to explore the organisation of MP complexes.