| The outbreak of cyanobacterial bloom is increasing all over the world.Its main harm is to produce and release various algal toxins,especially microsystins(MCs)and nodularins(NODs),which may threaten human health.Previous researches on hepatotoxin biodegradation were mainly focus on the screening of degrading bacteria,biodegradation characteristics and mlr degradation pathway.However,there are few studies on the biodegradation mechanism from the perspective of genome and enzyme structure analysis.In this study,the main results are as follows:(1)The genome length of Sphingopyxis sp.USTB-05 is 4.68 Mbp,contained 4312 predictive protein coding sequences.The genome sequence information has been submitted to GenBank with the search number CP084712.1.Sphingopyxis sp.USTB-05 has abundant pollutant degradation genes,and can effectively degrade hepatotoxin MCs and NOD in both cell and enzyme level.(2)The complete hepatotoxin biodegradation pathway of Sphingopyxis sp.USTB-05 was clarified.Hepatotoxins degrade into Adda through mlr degradation pathway.Adda is converted into PAA-CoA by PAAase enzyme,and then degrades into acetyl-CoA through paa phenylacetic acid degradation pathway.Finally,acetyl-CoA enters the tricarboxylic acid cycle and degrades into CO2.(3)MlrA was found to be a glutamate protease belonging to type II CAAX prenyl endopeptidases.Pure USTB-05-A enzyme was obtained.Its biodegradation mechanism is proposed:Glu172 and His205 activate a water molecule facilitating a nucleophilic attack on the Adda-Arg peptide bond of hepatotoxins to generate linear hepatotoxins;Trp176 and Trp201 contact the carboxylate side chain of Glu172 by raising its pKa potentially,accelerate the reaction rates;His260 and Asn264 function as an oxyanion hole to stabilize the transition states.(4)Pure USTB-05-C enzyme was obtained.Its biodegradation mechanism is proposed:Asp184,His186 and His208 coordinate with Zn2+,and Glu56 and Zn2+jointly activate a water molecule facilitating a nucleophilic attack on the Adda-Glu peptide bond of linear hepatotoxins to generate Adda. |
