Insights into the role of magnesium triad in myo-inositol monophosphatase: metal mechanism, substrate binding, and lithium therapy.
myo-Inositol monophosphatase (IMPase) plays a pivotal role in the intracellular phosphatidylinositol cell signaling pathway. It has attracted considerable attention as a putative therapeutic target for lithium therapy in the treatment of bipolar disorder. A trio of activated cofactor Mg²⁺ ions is required for inositol monophosphate hydrolysis by IMPase. In the present study, computational studies, including two-layered ONIOM-based quantum mechanics/mechanical mechanics (QM/MM) calculations, molecular modeling, and molecular dynamics (MD) simulations, were performed to ascertain the role of the Mg²⁺ triad in the IMPase active site. The QM/MM calculations show that the structural identity of the nucleophilic water molecule W1 shared by Mg²⁺-1 and Mg²⁺-3, activated by Thr95/Asp47 dyad, is a hydroxide ion. Moreover, Mg²⁺-3 needs to be conjugated with Mg²⁺-1 in the binding site to create the activated nucleophilic hydroxide ion in accordance with the three-metal ion catalytic mechanism. The MD simulation of the IMPase-substrate-Mg²⁺ complex shows that the three Mg²⁺ ions promote substrate binding and help fix the phosphate moiety of the substrate for nucleophilic attack by the hydroxide ion. When Mg²⁺-2 is displaced with Li⁺, the MD simulations of the postreaction complex indicate that the conformation of the catalytic loop (residues 33 to 44) is disrupted and water molecule W2 does not coordinate with Li⁺. This disruption traps the inorganic phosphate and inositolate in the active site, which lead to IMPase inhibition. By contrast, in the native Mg²⁺ system, the W2 ligated by Mg²⁺-2 and Asp200 aids in protonation of the leaving inositolate moiety.
Department of Pathophysiology, Key Laboratory of Cell Differentiation and Apoptosis of Chinese Ministry of Education, Shanghai Jiaotong University, School of Medicine, Shanghai 200025, China.
Shaoyong
S
Lu
Wenkang
W
Huang
Xiaobai
X
Li
Zhimin
Z
Huang
Xinyi
X
Liu
Yingyi
Y
Chen
Ting
T
Shi
Jian
J
Zhang
D001665:Binding Sites; D007295:Inositol Phosphates / Q000378:metabolism*; D018020:Lithium Compounds / Q000627:therapeutic use*; D008274:Magnesium / Q000378:metabolism*; D008958:Models, Molecular; D011789:Quantum Theory; D013379:Substrate Specificity
D007295:Inositol Phosphates; D018020:Lithium Compounds; D008274:Magnesium