CO2-Fixation Strategies in Energy Extremophiles: What Can We Learn From Acetogens?

Domestication of CO2-fixation became a worldwide priority enhanced by the will to convert this greenhouse gas into fuels and valuable chemicals. Because of its high stability, CO2-activation/fixation represents a true challenge for chemists. Autotrophic microbial communities, however, perform these reactions under standard temperature and pressure. Recent discoveries shine light on autotrophic acetogenic bacteria and hydrogenotrophic methanogens, as these anaerobes use a particularly efficient CO2-capture system to fulfill their carbon and energy needs. While other autotrophs assimilate CO2 via carboxylation followed by a reduction, acetogens and methanogens do the opposite. They first generate formate and CO by CO2-reduction, which are subsequently fixed to funnel the carbon toward their central metabolism. Yet their CO2-reduction pathways, with acetate or methane as end-products, constrain them to thrive at the "thermodynamic limits of Life". Despite this energy restriction acetogens and methanogens are growing at unexpected fast rates. To overcome the thermodynamic barrier of CO2-reduction they apply different ingenious chemical tricks such as the use of flavin-based electron-bifurcation or coupled reactions. This mini-review summarizes the current knowledge gathered on the CO2-fixation strategies among acetogens. While extensive biochemical characterization of the acetogenic formate-generating machineries has been done, there is no structural data available. Based on their shared mechanistic similarities, we apply the structural information obtained from hydrogenotrophic methanogens to highlight common features, as well as the specific differences of their CO2-fixation systems. We discuss the consequences of their CO2-reduction strategies on the evolution of Life, their wide distribution and their impact in biotechnological applications

Structural Rearrangements of a Dodecameric Ketol-Acid Reductoisomerase Isolated from a Marine Thermophilic Methanogen

Ketol-acid reductoisomerase (KARI) orchestrates the biosynthesis of branched-chain amino acids, an elementary reaction in prototrophic organisms as well as a valuable process in biotechnology. Bacterial KARIs belonging to class I organise as dimers or dodecamers and were intensively studied to understand their remarkable specificity towards NADH or NADPH, but also to develop antibiotics. Here, we present the first structural study on a KARI natively isolated from a methanogenic archaea. The dodecameric structure of 0.44-MDa was obtained in two different conformations, an open and close state refined to a resolution of 2.2-Å and 2.1-Å, respectively. These structures illustrate the conformational movement required for substrate and coenzyme binding. While the close state presents the complete NADP bound in front of a partially occupied Mg2+-site, the Mg2+-free open state contains a tartrate at the nicotinamide location and a bound NADP with the adenine-nicotinamide protruding out of the active site. Structural comparisons show a very high conservation of the active site environment and detailed analyses point towards few specific residues required for the dodecamerisation. These residues are not conserved in other dodecameric KARIs that stabilise their trimeric interface differently, suggesting that dodecamerisation, the cellular role of which is still unknown, might have occurred several times in the evolution of KARIs

Bone orthotropic remodeling as a thermodynamically-driven evolution

International audienceIn this contribution we present and discuss a model of bone remodeling set up in the framework of the theory of generalized continuum mechanics and first introduced by DiCarlo et al.[1]. Bone is described as an orthotropic body experiencing remodeling as a rotation of its microstruc-ture. Thus, the complete kinematic description of a material point is provided by its position in space and a rotation tensor describing the orientation of its microstructure. Material motion is driven by energetic considerations , namely by the application of the Clausius-Duhem inequality to the microstructured material. Within this framework of orthotropic re-modeling, some key features of the remodeling equilibrium configurations are deduced in the case of homogeneous strain or stress loading conditions. First, it is shown that remodeling equilibrium configurations correspond to energy extrema. Second, stability of the remodeling equilibrium configurations is assessed in terms of the local convexity of the strain and complementary energy functionals hence recovering some classical energy theorems. Eventually, it is shown that the remodeling equilibrium configurations are not only highly dependent on the loading conditions, but also on the material properties

Short-range repulsion and isospin dependence in the KN system

The short-range properties of the KN interaction are studied within the meson-exchange model of the Juelich group. Specifically, dynamical explanations for the phenomenological short-range repulsion, required in this model for achieving agreement with the empirical KN data, are explored. Evidence is found that contributions from the exchange of a heavy scalar-isovector meson (a_0(980)) as well as from genuine quark-gluon exchange processes are needed. Taking both mechanisms into account a satisfactory description of the KN phase shifts can be obtained without resorting to phenomenological pieces.Comment: 26 pages, 5 figure

ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio

We have observed the S(0), S(1), S(2), S(3), S(4) and S(5) rotational lines of molecular hydrogen (H2) towards the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300 - 700 K. However, the data cannot be fitted by an ortho- to para- (OTP) ratio of 3. An OTP ratio in the range ~1.5 - 2 is necessary to explain our observations. This is the first detection of a non-equilibrium OTP ratio measured from the H2 pure-rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.Comment: 4 pages, 3 figure

Far-ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro-2

Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are dominated by emission of the CO Fourth Positive band system and strong OI and CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and C - X (0,0) at 1088 A, are detected for the first time, and there is a weak indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B - X band is blended with emission from OI 1152. Modeling the relative intensities of these bands suggests that resonance fluorescence of CO is the dominant source of the emission, as it is for the Fourth Positive system. Shortward of Lyman-alpha, other emission features detected include OII 834, OI lambda 989, HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses of the OI, OII, and HI features are about one-half of those reported by Hord et al. (1991) from Galileo EUV measurements made in February 1990. This result is consistent with the expected variation from solar maximum to solar minimum. The ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived mixing ratio of Ar is of the order of 2%, consistent with previous determinations.Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 200