Research on the discrepant inhibition mechanism of microcystin-LR disinfectant by-products target to protein phosphatase 1

Conformation and binding of 12 Microcystin (MC) congeners to PPP1 using molecular dynamics simulations: A potential approach in support of an improved MC risk assessment

Microcystins (MCs) occur frequently during cyanobacterial blooms worldwide, representing a group of currently about 300 known MC congeners, which are structurally highly similar. Human exposure to MCs via contaminated water, food or dietary supplements can lead to severe intoxications with ensuing high morbidity and in some cases mortality. Currently, one MC congener (MC-LR) is almost exclusively considered for risk assessment (RA) by the WHO. Many MC congeners co-occur during bloom events, of which MC-LR is not the most toxic. Indeed, MC congeners differ dramatically in their inherent toxicity, consequently raising question about the reliability of the WHO RA and the derived guidance values. Molecular dynamics (MD) simulation can aid in understanding differences in toxicity, as experimental validation for all known MC congeners is not feasible. Therefore, we present MD simulations of a total of twelve MC congeners, of which eight MC congeners were simulated for the first time. We show that depending on their structure and toxicity class, MCs adapt to different backbone conformations. These backbone conformations are specific to certain MC congeners and can change or shift to other conformations upon binding to PPP1, affecting the stability of the binding. Analysis of the interactions with PPP1 demonstrated that there are frequently occurring patterns for individual MC congeners, and that published PPP interactions could be reproduced. In addition, common but also unique patterns were found for individual MC congeners, suggesting differences in binding behaviour. The MD simulations presented here therefore enhance our understanding of MC congener-specific differences and demonstrated that congener-specific investigations are prerequisite for allowing characterisation of yet untested or even unknown MC congeners, thereby allowing for a novel potential approach in support of an improved RA of microcystins in humans.

Non-negligible inhibition effect of microcystin-LR biodegradation products target to protein phosphatase 2A

Though biodegradation is an important regulation pathway for microcystins (MCs) pollution, more consideration needs to be given to the potential risk associated with related biodegradation products (MC-BDPs). In this work, typical MCLR-BDPs were prepared and their toxicity was evaluated by protein phosphatases (PPs) inhibition assay. Results showed the initial ring opening of MCLR played a crucial role in detoxification. However, partial MCLR-BDPs still retained the critical structures and thus exhibited certain toxicity (2.8-43.5% of MCLR). With the aid of molecular simulation, the mechanism for the potential toxicity of BDPs targeting PP2A was elucidated. The initial ring opening made the loss of hydrogen bond Leu2←Arg89, and pi-H bond Adda5-His191, which was responsible for the significant reduction in the toxicity of MCLR-BDP. However, the key hydrogen bonds MeAsp3←Arg89, Glu6←Arg89, Adda5←Asn117, Adda5←His118, Arg4→Pro213, Arg4←Arg214, Ala1←Arg268, and Mdha7←Arg268, metal bond Glu6-Mn12+, and ionic bonds Glu6-Arg89, and Glu6-Mn22+ were preserved in varying degrees. Above preserved interactions maintained the interactions between PP2A and Mn2+ ions (reducing the exposure of Mn2+ ions). Above preserved interactions also hindered the combination of phosphate groups to Arg214 residual and thus exhibited potential toxicity.

Mechanism for the Potential Inhibition Effect of Microcystin-LR Disinfectant By-Products on Protein Phosphatase 2A

The secondary contamination of microcystin disinfection by-products (MC-DBPs) is of concern due to the residual structure similar to their original toxin. Based on identification and preparation, the potential inhibition effect of typical MCLR-DBPs (associated with the oxidation of Adda⁵) on PP2A was confirmed in the sequence of MCLR > P1 > P4 > P3 ≈ P2 > P7 ≈ P6 ≈ P5 > P8. To elucidate the molecular mechanism underlying the inhibition effect, the interaction models for typical MCLR-DBPs and PP2A were constructed using a modeling-based-on-ligand-similarity approach, and the candidate interaction parameters between typical MCLR-DBPs and PP2A were obtained by molecular docking. By analyzing the correlation between inhibition data and candidate interaction parameters, the key interaction parameters were filtered as hydrogen bonds "Adda⁵"←Asn₁₁₇, "Adda⁵"←His₁₁₈, MeAsp³←Arg₈₉, Arg⁴←Arg₂₁₄, Arg⁴→Pro₂₁₃; ionic bonds Glu⁶-Arg₈₉, Asp₈₅-Mn₁²⁺, Asp₅₇-Mn₂²⁺; and metal bonds Glu⁶-Mn₁²⁺, Glu⁶-Mn₂²⁺. With the gradual intensification of chlorination, Adda⁵ was destroyed to varying degrees. The key interactions changed correspondingly, resulting in the discrepant inhibition effects of typical MCLR-DBPs on PP2A.