Abstract
Phycocyanin, which covalently binds phycocyanobilin chromophores, is not only a candidate fluorescent probe for biological imaging, but also a potential antioxidative agent for healthcare. Herein, a plasmid harboring two cassettes was constructed, with cpcB from Spirulina subsalsa in one cassette and the fusion gene cpcS::ho1::pcyA in the other, and then expressed in Escherichia coli. PCB-CpcB(C-82), a fluorescent phycocyanin β subunit, was biosynthesized in E. coli, exhibiting an absorption maximum at 620 nm and fluorescence emission maximum at 640 nm. When cpcS was replaced by cpcT, PCB-CpcB(C-153), another fluorescent phycocyanin β subunit, was produced, exhibiting an absorption maximum at 590 nm and fluorescence emission maximum at 620 nm. These two fluorescent biliproteins showed stronger scavenging activity toward hydroxyl and DPPH free radicals than apo-CpcB. The IC50 values for hydroxyl radical scavenging by PCB-CpcB(C-82), PCB-CpcB(C-153), and apo-CpcB were 38.72 ± 2.48 µg/mL, 51.06 ± 6.74 µg/mL, and 81.82 ± 0.67 µg/mL, respectively, and the values for DPPH radical scavenging were 201.00 ± 5.86 µg/mL, 240.34 ± 4.03 µg/mL, and 352.93 ± 26.30 µg/mL, respectively. The comparative antioxidant capacities of the proteins were PCB-CpcB(C-82) > PCB-CpcB(C-153) > apo-CpcB, due to bilin binding. The two fluorescent biliproteins exhibited a significant effect on relieving the growth of E. coli cells injured by H2O2. The results of this study suggest that the fluorescent phycocyanin β subunits of S. subsalsa were reconstructed by one expression vector in E. coli, and could be developed as potential antioxidants.
Highlights
Phycobiliproteins are a type of light-harvesting biliprotein involved in photosynthesis in cyanobacteria and some eukaryotic alga [1,2,3]
The results of this study suggest that the fluorescent phycocyanin β subunits of S. subsalsa were reconstructed by one expression vector in E. coli, and could be developed as potential antioxidants
The results showed that recombinant phycocyanins had DPPH free radical scavenging activity, and this showed that recombinant phycocyanins had free radical scavenging activity, and this activity activity increased with increasing protein concentration
Summary
Phycobiliproteins are a type of light-harvesting biliprotein involved in photosynthesis in cyanobacteria and some eukaryotic alga [1,2,3]. Phycobiliproteins consist of at least two dissimilar types of peptides, with linear tetrapyrrole prosthetic groups covalently attached to the apoprotein via cysteine thioether linkages [4,5]. Phycobiliproteins exhibit unique absorbance and fluorescence properties and can be classified into three types, based on their absorbance maxima: phycoerythrin (PE, λmax : 540–570 nm), phycocyanin (PC, λmax : 610–620 nm), and allophycocyanin (APC, λmax : 650–655 nm) [6]. C-phycocyanin (CPC), one of the major phycobiliproteins, is composed of α and β subunits in the form of (αβ) or (αβ) aggregates. The α or β subunits of CPC can be assembled by apoproteins and chromophore groups in vivo or in vitro, with catalysis by the chromophore lyase. PCB is covalently attached to the apoprotein, which is catalyzed by specific lyases to generate fluorescent CPC. The lyases CpcE and CpcF are responsible for the Molecules 2018, 23, 1369; doi:10.3390/molecules23061369 www.mdpi.com/journal/molecules
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