Narrow Results

Cascade-type regulation, where certain enzymes in response to physiological signals modify the activity of other enzymes by covalent modification, is found in many organisms. We study the covalent regulation of glutamine synthetase which is involved in ammonia fixation in the bacterium Escherichia coli. In this paper we pose the question whether this type of regulation of glutamine synthetase has, under certain growth conditions an advantage over other types of regulation, e.g., allosteric regulation. We propose that the relatively slow dynamics of cascade-type regulation has an evolutionary advantage under conditions of fluctuating ammonia concentrations.

BiOBr-based photocatalysts have received extensive interest for photocatalytic nitrogen fixation due to their layered structure recently. In this study, BiOBr with enhanced photocatalytic nitrogen fixation performance was synthesized via a facile ozone modification method. Characterization results demonstrated that the ozone treated BiOBr photocatalysts have identical crystal structures, morphologies, and enhanced photocatalytic nitrogen fixation performance. In addition, due to the strong oxidation of ozone, more oxygen vacancies and active sites form on the BiOBr surface, which could improve the nitrogen fixation efficiency. Furthermore, a supposed photocatalytic nitrogen fixation mechanism of the ozone treated BiOBr is proposed.

A photocatalyst of high-performance hierarchical nitrogen-doped MoS₂ (N-MoS₂) microsphere was fabricated by an in situ hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB). The as-prepared N-MoS₂ microsphere was self-assembled by extremely thin interleaving petals, where CTAB acts as a nucleation site for the formation of the interleaving petals due to the strong interaction between CTA⁺ and ${\text{MoO}}_4^{2-}$. N-MoS₂ showed higher N₂ fixation ability (101.2 $\mu$ mol/g_(cat)h) than the non-doped MoS₂ under the visible light irradiation, and the improved photocatalytic activity could be ascribed to that the doped N narrows the band gap, and the surface reflecting and scattering effect caused by the hierarchical structure enhance the light adsorption. The trapping experiment of active species was also investigated to evaluate the role of photogenerated electrons in the photocatalytic reaction process. Meanwhile, the possible mechanism for the formation and excellent photocatalytic performance of N-MoS₂ microsphere were also presented.

In Hong Kong (as in much of SE Asia), rainfall surpasses 2200 millimetres per year. In addition, local granite-derived soils are poorly retentive of nutrients, creating oligotrophic conditions with < 0.47% nitrogen in surface soil levels and negligible concentrations below. Despite such challenges, numerous non-leguminous (and often lithophytic) plants, such as Ficus microcarpa and Glochidion hongkongense, are not only abundant but also grow rapidly to great size. These very high levels of carbon sequestration imply correspondingly large rates of nitrogen uptake that can only be explained by microbial nitrogen fixation. In this project, samples of aerial roots were recovered from a number of common Hong Kong plant species. nitrogen free (NF) media was used to identify potentially nitrogen fixing bacteria, which were then streaked to purity before DNA extraction and sequencing. Complete genomes of three isolates, 9ba2, Kosakonia radicincitans JS2a2 and Gluconobacter thailandicus ISBL3, were generated by hybrid assembly, using both Illumina MiSeq and Oxford Nanopore MinION platforms. Genomic analysis revealed numerous highly-conserved nitrogenase related (nif) genes across the different genera. Mineral transport systems, particularly for iron and molybdate, were also strongly represented. For example, in K. variicola 9bα-2, a 166 kbp plasmid not only encodes the fec(RABCDE) operon for ferric citrate uptake, but also the molybdate-responsive ModE transcription factor. The probiotic use of nitrogen fixing bacteria such as these has been proposed as a more sustainable measure for plant growth promotion that could reduce requirements for chemical fertilisers. Significantly, the extent of microbial nitrogen fixation that we estimate in Hong Kong suggests that the contribution to the nitrogen cycle of non-legume-associated microbes may be greatly underestimated.

Living organisms evolved by exploiting molybdenum as a catalyst, incorporating it in the active site of oxidoreductase enzymes that control a diverse array of oxygen, hydrogen and sulfur atom transfer reactions. The chemical versatility of this successful partnership, protein–molybdenum, enables the fulfillment of distinct physiological roles, from bacterial atmospheric dinitrogen and carbon dioxide fixation to human sulfite detoxification. After several decades of research, our present comprehensive understanding of the structure and function of molybdoenzymes renders this an opportune time to draw attention to the largely disregarded biological molybdenum reactivity and bring to light its biotechnological potential and health-related challenges. In this chapter, we provide a concise overview of the molybdoenzymes catalytic features, followed by an outline of selected biotechnological applications to tackle some of the challenges that our modern society faces in the fields of environment, agriculture, climate and energy, namely, nitrate remediation, dinitrogen fixation and carbon dioxide capture and utilization. A brief account of molybdenum’s human health implications is also included to highlight the relevance of these metalloenzymes to medicine and the pharma industry.