Abstract
Harmful cyanobacterial blooms in waters have become a global environmental problem, this mainly due to the production and release of various microalgal toxins, in which microcystins (MCs) are distributed widely. Here, we focused on the study of a typical form of microcystins called microcystin-YR (MC-YR). It was found that initial 14.8 mg/L of MC-YR could be completely eliminated within 10 hr by the crude enzymes (CEs) of Sphingopyxis sp. USTB-05, a promising bacterial strain we isolated and identified in our previous study. During the enzymatic biodegradation of MC-YR with time course, the peaks of two intermediate and two final products were observed on the profiles of HPLC at the wavelengths of 238 nm and 230 nm, respectively. Based on the analysis of m/z ratios of MC-YR and its four products by LC-MS/MS, we suggested that at least four enzymes were involved in the biodegradation of MC-YR by Sphingopyxis sp. USTB-05. The first enzyme microcystinase converted cyclic MC-YR to linear MC-YR as the first product. Then the second enzyme serine protease was found to cleave the target peptide bond between alanine (Ala) and tyrosine (Tyr) of linearized MC-YR, producing a tetrapeptide and a tripeptide as second products, which were Adda-Glu-Mdha-Ala and Tyr-Masp-Arg, respectively. Next, the third enzyme peptidase converted the tetrapeptide of Adda-Glu-Mdha-Ala to Adda. And the fourth enzyme cleaved the tripeptide of Tyr-Masp-Arg to produce Tyr and dipeptide (Masp-Arg), which has never been reported. These findings will help us better understand the biodegradation pathway of MC-YR by Sphingopyxis sp. USTB-05.
Highlights
With the development of industry and agriculture, a great amount of wastewater containing nitrogen and phosphorus has been discharged into rivers and lakes
The first enzyme microcystinase was active in cleaving the target peptide bond between Adda and Arg of MC-YR (m/z 1045) (Fig 4) and converting cyclic MC-YR to linear MC-YR (m/z 1063) (Fig 5) as the first product
USTB-05 for the biodegradation of MCs, we successfully investigated its enzymatic biodegradation pathway toward MC-YR, which contains more complex Tyr in the ring structure of heptapeptide
Summary
With the development of industry and agriculture, a great amount of wastewater containing nitrogen and phosphorus has been discharged into rivers and lakes. Driven by the global warming, harmful cyanobacterial blooms have become more ubiquitous. About 70% of cyanobacterial blooms produce toxins, in which microcystins (MCs) are the most commonly detected and widely distributed around the world [1]. MCs have been studied extensively, for their ability to cause acute poisonings [2,3], and for their existence in drinking water which can PLOS ONE | DOI:10.1371/journal.pone.0124425. Pathway for Biodegrading MC-YR by Sphingopyxis sp. MCs are so poisonous [5] that they have severely threaten the security of water resources and the stability of aquatic ecosystems
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