Biodesulfurization
[4] The combustion of crude oil releases sulfur oxides (SOx) to the atmosphere, which are harmful to public health and contribute to serious environmental effects such as air pollution and acid rains.
[2][3] In addition, the sulfur content in crude oil is a major problem for refineries, as it promotes the corrosion of the equipment and the poisoning of the noble metal catalysts.
The reduction of the concentration of sulfur in crude oil becomes necessary to mitigate one of the leading sources of the harmful health and environmental effects caused by its combustion.
[12] Despite their efficiency at reducing sulfur content, the conventional desulfurization methods are still accountable for a significant amount of the CO2 emissions associated with the crude oil refining process, releasing up to 9000 metric tons per year.
[18] However, they lacked the scalability desired for an industrial setup due to overall low enzyme efficiency, product feedback inhibition mechanisms and toxicity, or inadequate conditions for long-term bacterial growth.
The ring destructive pathway consists of the selective cleavage of carbon-carbon bonds with release of small organic sulfides soluble in the surrounding aqueous environment, whereas the sulfur-specific pathways rely on successive sulfur redox reactions to release sulfur either as sulfide or sulfite anions as byproducts.
[3] The latter have thus been considered as a very promising pathway to produce sulfur-free compounds with a high calorific content, in particular in the desulfurization of sulfur heterocycles abundant in sour crude oil fractions.
The reaction catalyzed by DszC involves three phases: 1) molecular oxygen activation leading to the formation of a hydroperoxyflavin-intermediate (C4aOOH); 2) oxidation of DBT to DBTO; and 3) dehydration of FMN.
[24] It is also severely affected from feedback inhibition caused mostly by HPBS and 2-HBP, the products of DszA and DszB respectively,[24] For that reason, it has been targeted for optimization through enzyme engineering.
[24] The NADH-FMN oxidoreductase (DszD) regenerates the FMNH2 cofactor needed for the reactions catalyzed by DszC and DszA, through the oxidation of NADH to NAD+ in a two step mechanism.
[32] Directed evolution, rational design or a combination of both strategies are some of the approaches that have been applied to tackle the lack of catalytic efficiency and stability of the 4S enzymes.
After 40 subculturing events in a medium in which DBT was the sole sulfur source, the modified Rhodococcus strains presented a 35-fold improvement.
[36] A computational study demonstrated that substitutions in position 62 of DszD sequence have a major impact in the activation energy for the hydride transfer reaction from NADH to FAD.
