Applications

As explained in the theory section, VHb can be used to increase the yield of proteins under oxygen-limited conditions. In this section, we present a review of some applications were the technology has proven itself useful. The practical benefits of VHb co-expression have been harnessed in order to increase the production of proteins [1][2] and growth of rice [3], maize [4] and the tobacco plant [5]. VHb can also be used in bioremediation of toxic compounds such as textile dyes [6] and aromatic compounds [7].

β-galactosidase is an enzyme commonly used for removal of lactose from milk, which is required to make it drinkable for more than half of the world’s population [8]. Wu et al. [1] managed to increase the production of this enzyme by 9.9% under non-limiting aeration, proving the benefit of VHb co-expression. The host organism Pichia pastoris demonstrated an improvement of 28.2% in oxygen uptake rate when grown in shake flasks. Another industrial enzyme, ɑ-amylase, was also co-expressed with VHb [2]. An 18% increase of the enzyme concentration was noted when cultivated in a bioreactor under non-limiting aeration. This strengthens the argument that VHb can emit similar benefits when scaling up the process.

Expression of VHb has also been studied in plants and shown to induce physiological changes. Wang et al. [9] managed to successfully integrate the VHb gene into Arabidopsis thaliana, which resulted in multiple positive effects: substantially taller plants, faster generation time and increased resistance to flooding compared to the wild-type.

Water-logging, where water saturates the roots of the plant, is a common issue in agriculture with about 6 to 10 million hectares of land affected in India alone [10]. In 2011, the average crop yield of rice, cereals and wheat in India was 2636 kg/hectare according to the Department of Agriculture & Cooperation [11]. To put this in perspective, 6 to 10 millions of hectares equals approximately 16 000 to 26 000 million kg of food affected by water-logging every year. VHb co-expression can be a viable solution to alleviate this issue, while also providing the additional benefit of increasing the biomass content of plants [5][12].

Contamination of soil and groundwater stems from pharmaceutical residues [13], petroleum [14], aromatic hydrocarbons [15], organic dyes [16], pesticides [17] and heavy metals [18] Many of these contaminants can lead to serious disorders in the domestic microbial flora [19] and human population [20]. Bioremediation is an environmentally friendly solution to this issue and is used to treat heavily contaminated sites [21]. A frequent issue during bioremediation is the lack of oxygen in the environment where it is taking place. VHb has been used in various bioremediation scenarios for exactly this reason.

Kahraman and Geckil [7] proposed a potential solution to the problem by expressing VHb in yeast capable of using benzene, toluene and xylene as carbon sources. Hypoxic conditions were emulated by limiting the oxygen supply and a significant increase in degradation of the contaminants was observed.

To prevent the release of genetically engineered bioremediating organisms into the environment, Urgun-Demirtas [22] used a membrane bioreactor commonly used in waste-water treatment. They studied the degradation of 2-chlorobenzoic acid by Burkholderia cepacia modified to express VHb. The transformed cells reached higher cell densities compared to their native counterparts, 3.2–5.4 g/L and 2.8–4.7 g/L respectively. Survivability in hypoxic oxygen concentrations also improved.

CO2 emission is expected to increase by approximately 20% before 2035 according to the Intergovernmental panel on Climate Change [23]. Biofuels have become a feasible alternative to fossil-based fuels and more car owners are committing to the change. The consumption of biofuels, mainly ethanol, is predicted to rise from 1.3 barrels/day in 2011 to 4.1 barrels/day by 2035 [24]. In turn, the demand for more efficient biofuel production will increase.

Ethanol makes up three fourths of biofuel use [24]. Until now it has been produced using cornstarch, sugar canes and sugar beets, however a cost-effective production of bioethanol will necessitate the development of alternative fermentation feedstocks and increased productivity [25]. Sanny et al. [26] and Arnaldos et al. [27] demonstrate that the expression of small amounts of VHb, despite of being counterintuitive for a fermentation process, resulted in a significant increase in ethanol production.

The delivery of small amounts of oxygen to the bacteria have generated an increase in ethanol production in E. coli using feedstocks as diverse as glucose (30% increase) [26], xylose (119%) [26], corn stover hydrolysate (59%) [26], potato processing waste water (5-18%) [28], cheese whey (21-419%) [29] and whey powder (17-362%) [29] among others. VHb have also been shown to increase ethanol production in yeast [30]. Also investigators in Hong Kong have shown that its expression in Aurantiochytrium sp. enhanced the production of fatty acids that could be used as precursors of the synthesis of biodiesel [31].

References