Genetic Engineering for Removal of Sulfur from Fuel Aromatic Heterocycles
Aromatic sulfur heterocyclic (ASH) compounds are among the most toxic and recalcitrant contaminants of fossil fuels and may cause serious environmental (e.g., acid precipitation), industrial (e.g., catalyst poisoning), and health problems (e.g., cardiopulmonary diseases). Different biochemical pathways for ASH degradation have been described in a wide variety of microorganisms. These pathways, which are encoded either on plasmids or on the host chromosome, usually are not essential for growth but rather allow to exploit a specific environmental niche or condition. Dibenzothiophene (DBT) is widely use as model ASH compound. A sulfur-specific pathway for DBT biodesulfurization (dsz pathway or 4S pathway) has been extensively studied at the physiological, biochemical, and genetic levels. The distribution and conservation of the dsz genes in a wide variety of bacteria strongly suggest that these genes are commonly subjected to horizontal gene transfer in nature. Despite the fact that an efficient ASH biodesulfurization depends on the expression, activity, feedback inhibition, and substrate range of the dsz gene products, host cell contributions also play an essential role in achieving higher activities, which are pivotal from a biotechnological point of view to develop a commercially viable biodesulfurization process. Factors, such as the cell-reducing power, cytoplasmic oxygen levels, transmembrane trafficking of substrates and products, solvent tolerance, and the ability of the cells to access and uptake the aromatic compounds, may influence strongly the biodesulfurization efficiency. A large number of recombinant bacteria have been engineered to overcome the major bottlenecks of the biodesulfurization process. The increased use of high-throughput omic techniques, as well as systems biology and synthetic biology approaches, is contributing significantly to unravel the intricate regulatory and metabolic networks that govern the degradation of ASHs. These studies will pave the way for further metabolic flux modeling and for the rational design of synthetic metabolic pathways or bacterial consortia for upgrading large volumes of fossil fuels, one of the greatest challenges addressed by current biotechnology. Moreover, existing desulfurization biocatalysts can also potentially be used in a variety of applications, e.g., synthesis of higher-value oil-based chemicals that have barely begun to be explored.
Work was supported by the Ministry of Economy and Competitiveness of Spain Grants BIO2012-39501, BIO2012-39695-C02-01, BIO2015-66960-C3-3-R, and PCIN2014-113, European Union Grants FP7-KBBE 6-311815 and H2020-FET-OPEN 686585, and Fundación Ramón Areces XVII Concurso Nacional.
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