32 research outputs found
Global organization of metabolic fluxes in the bacterium, Escherichia coli
Cellular metabolism, the integrated interconversion of thousands of metabolic
substrates through enzyme-catalyzed biochemical reactions, is the most
investigated complex intercellular web of molecular interactions. While the
topological organization of individual reactions into metabolic networks is
increasingly well understood, the principles governing their global functional
utilization under different growth conditions pose many open questions. We
implement a flux balance analysis of the E. coli MG1655 metabolism, finding
that the network utilization is highly uneven: while most metabolic reactions
have small fluxes, the metabolism's activity is dominated by several reactions
with very high fluxes. E. coli responds to changes in growth conditions by
reorganizing the rates of selected fluxes predominantly within this high flux
backbone. The identified behavior likely represents a universal feature of
metabolic activity in all cells, with potential implications to metabolic
engineering.Comment: 15 pages 4 figure
A New Role for Translation Initiation Factor 2 in Maintaining Genome Integrity
Escherichia coli translation initiation factor 2 (IF2) performs the unexpected function of promoting transition from recombination to replication during bacteriophage Mu transposition in vitro, leading to initiation by replication restart proteins. This function has suggested a role of IF2 in engaging cellular restart mechanisms and regulating the maintenance of genome integrity. To examine the potential effect of IF2 on restart mechanisms, we characterized its influence on cellular recovery following DNA damage by methyl methanesulfonate (MMS) and UV damage. Mutations that prevent expression of full-length IF2-1 or truncated IF2-2 and IF2-3 isoforms affected cellular growth or recovery following DNA damage differently, influencing different restart mechanisms. A deletion mutant (del1) expressing only IF2-2/3 was severely sensitive to growth in the presence of DNA-damaging agent MMS. Proficient as wild type in repairing DNA lesions and promoting replication restart upon removal of MMS, this mutant was nevertheless unable to sustain cell growth in the presence of MMS; however, growth in MMS could be partly restored by disruption of sulA, which encodes a cell division inhibitor induced during replication fork arrest. Moreover, such characteristics of del1 MMS sensitivity were shared by restart mutant priA300, which encodes a helicase-deficient restart protein. Epistasis analysis indicated that del1 in combination with priA300 had no further effects on cellular recovery from MMS and UV treatment; however, the del2/3 mutation, which allows expression of only IF2-1, synergistically increased UV sensitivity in combination with priA300. The results indicate that full-length IF2, in a function distinct from truncated forms, influences the engagement or activity of restart functions dependent on PriA helicase, allowing cellular growth when a DNA–damaging agent is present
The creatine kinase system and pleiotropic effects of creatine
The pleiotropic effects of creatine (Cr) are based mostly on the functions of the enzyme creatine kinase (CK) and its high-energy product phosphocreatine (PCr). Multidisciplinary studies have established molecular, cellular, organ and somatic functions of the CK/PCr system, in particular for cells and tissues with high and intermittent energy fluctuations. These studies include tissue-specific expression and subcellular localization of CK isoforms, high-resolution molecular structures and structure–function relationships, transgenic CK abrogation and reverse genetic approaches. Three energy-related physiological principles emerge, namely that the CK/PCr systems functions as (a) an immediately available temporal energy buffer, (b) a spatial energy buffer or intracellular energy transport system (the CK/PCr energy shuttle or circuit) and (c) a metabolic regulator. The CK/PCr energy shuttle connects sites of ATP production (glycolysis and mitochondrial oxidative phosphorylation) with subcellular sites of ATP utilization (ATPases). Thus, diffusion limitations of ADP and ATP are overcome by PCr/Cr shuttling, as most clearly seen in polar cells such as spermatozoa, retina photoreceptor cells and sensory hair bundles of the inner ear. The CK/PCr system relies on the close exchange of substrates and products between CK isoforms and ATP-generating or -consuming processes. Mitochondrial CK in the mitochondrial outer compartment, for example, is tightly coupled to ATP export via adenine nucleotide transporter or carrier (ANT) and thus ATP-synthesis and respiratory chain activity, releasing PCr into the cytosol. This coupling also reduces formation of reactive oxygen species (ROS) and inhibits mitochondrial permeability transition, an early event in apoptosis. Cr itself may also act as a direct and/or indirect anti-oxidant, while PCr can interact with and protect cellular membranes. Collectively, these factors may well explain the beneficial effects of Cr supplementation. The stimulating effects of Cr for muscle and bone growth and maintenance, and especially in neuroprotection, are now recognized and the first clinical studies are underway. Novel socio-economically relevant applications of Cr supplementation are emerging, e.g. for senior people, intensive care units and dialysis patients, who are notoriously Cr-depleted. Also, Cr will likely be beneficial for the healthy development of premature infants, who after separation from the placenta depend on external Cr. Cr supplementation of pregnant and lactating women, as well as of babies and infants are likely to be of benefit for child development. Last but not least, Cr harbours a global ecological potential as an additive for animal feed, replacing meat- and fish meal for animal (poultry and swine) and fish aqua farming. This may help to alleviate human starvation and at the same time prevent over-fishing of oceans
Wintertime organic and inorganic aerosols in Lanzhou, China: sources, processes, and comparison with the results during summer
Lanzhou, which is located in a steep alpine valley in western China, is one
of the most polluted cities in China during the wintertime. In this study,
an Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS), a seven-wavelength aethalometer, and a scanning mobility
particle sizer (SMPS) were deployed during 10 January to 4 February 2014 to
study the mass concentrations, chemical processes, and sources of
submicrometer particulate matter (PM<sub>1</sub>). The average PM<sub>1</sub>
concentration during this study was 57.3 µg m<sup>−3</sup> (ranging from
2.1 to 229.7 µg m<sup>−3</sup> for hourly averages), with organic aerosol
(OA) accounting for 51.2 %, followed by nitrate (16.5 %), sulfate
(12.5 %), ammonium (10.3 %), black carbon (BC, 6.4 %), and chloride
(3.0 %). The mass concentration of PM<sub>1</sub> during winter was more than
twice the average value observed at the same site in summer 2012
(24.5 µg m<sup>−3</sup>), but the mass fraction of OA was similar in the two
seasons. Nitrate contributed a significantly higher fraction to the PM<sub>1</sub>
mass in winter than summer (16.5 % vs. 10 %), largely due to more
favored partitioning to the particle phase at low air temperature. The mass
fractions of both OA and nitrate increased by ∼ 5 % (47
to 52 for OA and 13 to 18 % for nitrate) with the increase of the
total PM<sub>1</sub> mass loading, while the average sulfate fraction decreased
by 6 % (17 to 11 %), indicating the importance of OA and nitrate for
the heavy air pollution events in Lanzhou. The size distributions of OA,
nitrate, sulfate, ammonium, and chloride all peaked at ∼ 500 nm, with OA being
slightly broader, suggesting that aerosol particles were
internally mixed during winter, likely due to frequently calm and stagnant
air conditions during wintertime in Lanzhou (average wind speed: 0.82 m s<sup>−1</sup>).<br><br>The average mass spectrum of OA showed a medium oxidation degree (average
O ∕ C ratio of 0.28), which was lower than that during summer 2012
(O ∕ C = 0.33). This is consistent with weaker photochemical processing during
winter. Positive matrix factorization (PMF) with the multi-linear engine
(ME-2) solver identified six OA sources, i.e., a hydrocarbon-like OA (HOA),
a biomass burning OA (BBOA), a cooking-emitted OA (COA), a coal combustion
OA (CCOA), and two oxygenated OA (OOA) factors. One of the OOAs was
less oxidized (LO-OOA), and the other one more oxidized (MO-OOA). LO-OOA
was the most abundant OA component (22.3 % of OA mass), followed by CCOA
(22.0 %), COA (20.2 %), MO-OOA (14.9 %), BBOA (10.8 %), and HOA
(9.8 %). The mass fraction of primary OA
( = HOA + BBOA + COA + CCOA) increased during high PM pollution periods, indicating that local
primary emissions were a main reason for the formation of air pollution
events in Lanzhou during winter. Radiocarbon (<sup>14</sup>C) measurement was
conducted on four PM<sub>2.5</sub> filter samples from this study, which allowed
for a quantitative source apportionment of organic carbon (OC). The
non-fossil sources on average accounted for 55 ± 3 % of OC, which
could be mainly from biomass burning and cooking activities, suggesting the
importance of non-fossil sources for the PM pollution in Lanzhou. Together
with the PMF results, we also found that a large fraction (66 ± 10 %) of the secondary OC was from non-fossil OC