Date Published: September 1, 2011
Author(s): Rio Yamanaka, Kaoru Nakamura, Akio Murakami.
Effective utilization of photosynthetic microorganisms as potential biocatalysts is favorable for the production of useful biomaterials and the reduction of atmospheric CO2. For example, biocatalytic transformations are used in the synthesis of optically active alcohols. We previously found that ketone reduction in cells of the cyanobacterium Synechococcus PCC 7942 is highly enantioselective and remarkably enhanced under light illumination. In this study, the mechanism of light-enhanced ketone reduction was investigated in detail using several inhibitors of photosynthetic electron transport and of enzymes of the Calvin cycle. It is demonstrated that light intensity and photosynthesis inhibitors significantly affect the ketone reduction activity in Synechococcus. This indicates that the reduction correlates well with photosynthetic activity. Moreover, ketone reduction in Synechococcus specifically depends upon NADPH and not NADH. These results also suggest that cyanobacteria have the potential to be utilized as biocatalytic systems for direct usage of light energy in various applications such as syntheses of useful compounds and remediation of environmental pollutants.
Most chlorophyll-containing organisms are photoautotrophic and manufacture various organic compounds from inorganic carbon in the form of CO2 using solar light energy. All other organisms except for chemosynthetic bacteria are heterotrophic and ultimately depend upon photoautotrophic organisms to provide their energy and nutrients (Anemaet et al. 2010). Oxygen-evolving photoautotrophic organisms, plants, algae, and cyanobacteria have significantly influenced the global environment and carbon cycle on Earth. Improved utilization of such photoautotrophic organisms will be essential in preventing acceleration of a rise in anthropogenic CO2, which is believed to cause global warming and ocean acidification (Zabochnicka-świątek 2010). In particular, aquatic microalgae and cyanobacteria are among the most promising candidates in efforts to reduce atmospheric CO2 because they have higher photosynthetic activity and proliferate faster than terrestrial plants (Kurano et al. 1998). For this purpose, large-scale mass-culture of microalgae and cyanobacteria has been accomplished using either outdoor open pond processes or enclosed bioreactor systems (Takano et al. 1992, Ugwu et al. 2005, Kumar et al. 2010).
In this study, we demonstrated that the reduction of exogenously added ketones in cyanobacterial cells is highly dependent upon photosynthetic activity (Figure 2 and Table 1). Similar correlation curves for the reduction activities of ketones and the photosynthetic activities of the cyanobacterium were obtained from two different modulation mechanisms of photosynthetic activity (Figure 3). Since cyanobacterial ketone reduction was expectedly found to be enhanced by inhibitors of the Calvin cycle (Table 2), we propose that the reducing power of NADPH generated through photosynthesis (which is primarily used in CO2 fixation), can also be used for the reduction of exogenous ketones to produce the corresponding alcohols.
DBMIB: 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (dibromothymoquinone); DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DMSO: dimethyl sulfoxide; GC: gas chromatography; IAA: iodoacetic acid; IAM: iodoacetamide; NAD(P)H: nicotinamide adenine dinucleotide (phosphate); PF: 2′, 3′, 4′, 5′, 6′-pentafluoroacetophenone; TFA: α, α, α-trifluoroacetophenone
The authors declare that they have no competing interests.