Several computational models of chemical reaction networks have been presented in the literature in the past, showing the appearance and (potential) evolution of autocatalytic sets. However, the notion of autocatalytic sets has been defined differently in different modeling contexts, each one having some shortcoming or limitation. Here, we review four such models and definitions, and then formally describe and analyze them in the context of a mathematical framework for studying autocatalytic sets known as RAF theory. The main results are that: (1) RAF theory can capture the various previous definitions of autocatalytic sets and is therefore more complete and general, (2) the formal framework can be used to efficiently detect and analyze autocatalytic sets in all of these different computational models, (3) autocatalytic (RAF) sets are indeed likely to appear and evolve in such models, and (4) this could have important implications for a possible metabolism-first scenario for the origin of life. 相似文献
Understanding the changes occurring in the oral ecosystem during development of gingivitis could help improve prevention and treatment strategies for oral health. Erythritol is a non-caloric polyol proposed to have beneficial effects on oral health.
Objectives
To examine the effect of experimental gingivitis and the effect of erythritol on the salivary metabolome and salivary functional biochemistry.
Methods
In a two-week experimental gingivitis challenge intervention study, non-targeted, mass spectrometry-based metabolomic profiling was performed on saliva samples from 61 healthy adults, collected at five time-points. The effect of erythritol was studied in a randomized, controlled trial setting. Fourteen salivary biochemistry variables were measured with antibody- or enzymatic activity-based assays.
Results
Bacterial amino acid catabolites (cadaverine, N-acetylcadaverine, and α-hydroxyisovalerate) and end-products of bacterial alkali-producing pathways (N-α-acetylornithine and γ-aminobutyrate) increased significantly during the experimental gingivitis. Significant changes were found in a set of 13 salivary metabolite ratios composed of host cell membrane lipids involved in cell signaling, host responses to bacteria, and defense against free radicals. An increase in mevalonate was also observed. There were no significant effects of erythritol. No significant changes were found in functional salivary biochemistry.
Conclusions
The findings underline a dynamic interaction between the host and the oral microbial biofilm during an experimental induction of gingivitis.
Recent demonstrations of the role of plant–soil biota interactions have challenged the conventional view that vegetation changes are mainly driven by changing abiotic conditions. However, while this concept has been validated under natural conditions, our understanding of the long‐term consequences of plant–soil interactions for above‐belowground community assembly is restricted to mathematical and conceptual model projections. Here, we demonstrate experimentally that one‐time additions of soil biota and plant seeds alter soil‐borne nematode and plant community composition in semi‐natural grassland for 20 years. Over time, aboveground and belowground community composition became increasingly correlated, suggesting an increasing connectedness of soil biota and plants. We conclude that the initial composition of not only plant communities, but also soil communities has a long‐lasting impact on the trajectory of community assembly. 相似文献
There is strong interest in creating an enzyme that can deacylate natural cephalosporins such as cephalosporin C in order to efficiently acquire the starting compound for the industrial production of semisynthetic cephalosporin antibiotics. In this study, the active site of the glutaryl acylase from Pseudomonas SY-77 was randomized rationally. Several mutations that were found in previous studies to enhance the activity of the enzyme towards adipyl-7-aminodesacetoxycephalosporanic acid (ADCA) and cephalosporin C have now been combined, and libraries have been made in which random amino acid substitutions at these positions are joined. The mutants were expressed in a leucine-deficient Escherichia coli strain and subjected to growth selection with adipyl-leucine or amino-adipyl-leucine as sole leucine source. The mutants growing on these media were selected and purified, and their hydrolysis activities towards adipyl-7-ADCA and cephalosporin C were tested. Several mutants with highly improved activities towards the desired substrates were found in these rationally randomized libraries. The best mutant was selected from a library of totally randomized residues: 178, 266, and 375. This mutant comprises two mutations, Y178F + F375H, which synergistically improve the catalytic efficiency towards adipyl-7-ADCA 36-fold. The activity of this mutant towards adipyl-7-ADCA is 50% of the activity of the wild-type enzyme towards the preferred substrate glutaryl-7-aminocephalosporanic acid, and therefore the characteristics of this mutant approach those needed for industrial application. 相似文献
In this study, we developed a microfluidics method, using a so-called H-cell microfluidics device, for the determination of protein diffusion coefficients at different concentrations, pHs, ionic strengths, and solvent viscosities. Protein transfer takes place in the H-cell channels between two laminarly flowing streams with each containing a different initial protein concentration. The protein diffusion coefficients are calculated based on the measured protein mass transfer, the channel dimensions, and the contact time between the two streams. The diffusion rates of lysozyme, cytochrome c, myoglobin, ovalbumin, bovine serum albumin, and etanercept were investigated. The accuracy of the presented methodology was demonstrated by comparing the measured diffusion coefficients with literature values measured under similar solvent conditions using other techniques. At low pH and ionic strength, the measured lysozyme diffusion coefficient increased with the protein concentration gradient, suggesting stronger and more frequent intermolecular interactions. At comparable concentration gradients, the measured lysozyme diffusion coefficient decreased drastically as a function of increasing ionic strength (from zero onwards) and increasing medium viscosity. Additionally, a particle tracing numerical simulation was performed to achieve a better understanding of the macromolecular displacement in the H-cell microchannels. It was found that particle transfer between the two channels tends to speed up at low ionic strength and high concentration gradient. This confirms the corresponding experimental observation of protein diffusion measured via the H-cell microfluidics. 相似文献
1. Comparative studies were made on the fluorescence characteristics of chlorophyll a at 20° and −193°, and quantum efficiencies for P 700 oxidation and NADP+ reduction were measured in chloroplasts and chloroplast fragments obtained after incubation with 0.5% digitonin.
2. Differences in the flurescence yield of chlorophyll a in flowing and stationary suspensions of untreated chloroplasts and of the large fragments are indicative of light-induced photoreduction of the quencher Q of chlorophyll a, associated with pigment System 2 (chlorophyll a2). The relatively low constant fluorescence yield of chlorophyll a in the small fragments indicates the absence of fluorescent chlorophyll a2 from these fragments and suggests that the low fluorescence is due to chlorophyll a, associated with pigmen System 1 (chlorophyll a1). The ratio of the fluorescence yields of chlorophyll a1 and chlorophyll a2 is 0.45:1. In the large particles the concentration ratio of pigment System 1 and System 2 is 1:3.
3. The efficiencies of quanta absorbed at 673, 683 and 705 nm for NADP+ reduction and P 700 oxidation in untreated chloroplasts and chloroplast fragments indicate that digitonin treatment results in a separation of System 2 from System 1 in the small fragments. Sonication does not cause such a separation. Under the conditions used P 700 oxidation and NADP+ reduction in the small fragments separated after digitonin treatment, occurred with maximal efficiency of 0.7 to 1.0 and 0.7, respectively.
4. The constancy of the fluorescence yield of chlorophyll a1 in the small fragments, under conditions at which P 700 is oxidized and NADP+ is reduced, is interpreted as evidence either for the hypothesis that the fluorescence of chlorophyll a1 is controlled by the redox state of the primary photoreductant XH, or alternatively for the hypothesis that energy transfer from fluorescent chlorophyll a1 to P 700 goes via an intrinsically weak fluorescent, still unknown, chlorophyll-like pigment.
5. The low-temperature emission band around 730 nm is argued not to be due to excitation by System 1 only; the relatively large half width of the band, as compared to the emission bands at 683 and 696 nm, suggests that it is possibly due to overlapping emission bands of different pigments. 相似文献
XPG is the human endonuclease that cuts 3' to DNA lesions during nucleotide excision repair. Missense mutations in XPG can lead to xeroderma pigmentosum (XP), whereas truncated or unstable XPG proteins cause Cockayne syndrome (CS), normally yielding life spans of <7 years. One XP-G individual who had advanced XP/CS symptoms at 28 years has been identified. The genetic, biochemical, and cellular defects in this remarkable case provide insight into the onset of XP and CS, and they reveal a previously unrecognized property of XPG. Both of this individual's XPG alleles produce a severely truncated protein, but an infrequent alternative splice generates an XPG protein lacking seven internal amino acids, which can account for his very slight cellular UV resistance. Deletion of XPG amino acids 225 to 231 does not abolish structure-specific endonuclease activity. Instead, this region is essential for interaction with TFIIH and for the stable recruitment of XPG to sites of local UV damage after the prior recruitment of TFIIH. These results define a new functional domain of XPG, and they demonstrate that recruitment of DNA repair proteins to sites of damage does not necessarily lead to productive repair reactions. This observation has potential implications that extend beyond nucleotide excision repair. 相似文献