Inositol monophosphatase

Structure

The enzyme is a dimer comprising 277 amino acid residues per subunit. Each dimer exists in 5 layers of alternating α-helices and β-sheets, totaling to 9 α-helices and β-sheets per subunit. IMPase has three hydrophilic hollow active sites, each of which bind water and magnesium molecules. These binding sites appear to be conserved in other phosphodiesterases such as fructose 1,6-bisphosphatase (FBPase) and inositol polyphosphate 1-phosphatase.

Catalytic mechanism

It was previously reported that the hydrolysis of inositol monophosphate was catalyzed by IMPase through a 2-magnesium ion mechanism. However a recent 1.4 A resolution crystal structure shows 3 magnesium ions coordinating in each active binding site of the 2 dimers, supporting a 3-magnesium ion mechanism. The mechanism for hydrolysis is now thought to proceed as such: the enzyme is activated by a magnesium ion binding to binding site I, containing three water molecules, and stabilized by the negative charges on the carboxylates of Glu70 and Asp90, and the carbonyl of Ile92. Another magnesium ion then cooperatively binds to binding site 2, which has of carboxylates of Asp90, Asp93, Asp220, and three water molecules, one of which is shared by binding site 1. Then, a third magnesium weakly and non-cooperatively to the third binding site, which has 5 water molecules and residue Glu70. After all three magnesium ions have bound, the inositol monophosphatase can bind, the negatively charge phosphate group stabilized by the three positively charged magnesium ions. Finally an activated water molecule acts a nucleophile and hydrolyzes the substrate, giving inositol and inorganic phosphate.

Function

Inositol monophosphatase plays an important role in maintaining intracellular levels of myo-inositol, a molecule that forms the structural basis of several secondary messengers in eukaryotic cells. IMPase dephosphorylates the isomers of inositol monophosphate to produce inositol, mostly in the form of the stereoisomer, myo-inositol. Inositol monophosphatase is able to regulate inositol homeostasis because it lies at the convergence of two pathways that generate inositol:

IMPase in the phosphatidylinositol signaling pathway

In this pathway, G-coupled protein receptors and tyrosine kinase receptors are activated, resulting in the activation of phospholipase C (PLC). PLC hydrolyzes phosphatidylinositol bisphosphate (PIP2), resulting in a membrane associated product, diacylglycerol (DAG), and a water-soluble product, inositol triphosphate (IP3). DAG acts as a second messenger, activating several protein kinases and produces extended downstream signaling. IP3 is also a second messenger which activates receptors on the endoplasmic reticulum to release calcium ion stores into the cytoplasm, creating a complex signaling system that can be involved in modulating fertilization, proliferation, contraction, cell metabolism, vesicle and fluid secretion, and information processing in neuronal cells. Overall, DAG and IP3 signaling has implications for neuronal plasticity, impacting hippocampal long term potentiation, stress-induced cognitive impairment, and neuronal growth cone spreading. Furthermore, not only is PIP2 a precursor to several signaling molecules, it can be phosphorylated at the 3’ position to become PIP3, which is involved in cell proliferation, apoptosis and cell movement.

In this pathway, IMPase is the common, final step in recycling IP3 to produce PIP2. IMPase does this by dephosphorylating inositol monophosphate to produce inorganic phosphate and myo-inositol, the precursor to PIP2. Because of IMPase's crucial role in this signaling pathway, it is a potential drug target for inhibition and modulation.

IMPase in the de novo synthesis of myo-inositol

There are at least 2 known steps in the de novo synthesis of myo-inositol from glucose 6-phosphate. In the first step, glucose 6-phosphate is converted to D-inositol 1 monophosphate by the enzyme glucose 6 phosphate cyclase. Inositol monophosphatase catalyzes the final step in which D-inositol 1 monophosphate is dephosphorylated to form myo-inositol.

Clinical significance

Inositol monophosphatase has historically been believed to be a direct target of lithium, the primary treatment for bipolar disorder. It is thought that lithium acts according to the inositol depletion hypothesis: lithium produces its therapeutic effect by inhibiting IMPase and therefore decreasing levels of myo-inositol. Scientific support for this hypothesis exists but is limited; the complete role of lithium and inositol monophosphatase in treating bipolar disorder or reducing myo-inositol levels is not well understood.

In support of the inositol depletion hypothesis, researchers have shown that lithium binds uncompetitively to purified bovine IMPase at the site of one of the magnesium ions. Rodents administered lithium showed a decrease in inositol levels, in line with the hypothesis. Valproate, another mood-stabilizing drug given to bipolar disorder patients, has also been shown to mimic the effects of lithium on myo-inositol.

However, some clinical studies have found that bipolar disorder patients that had been administered lithium showed lower myo-inositol levels, while others found no effect on myo-inositol levels. Furthermore, lithium also binds to inositol polyphosphate 1-phosphatase (IPP), an enzyme also present in the phosphoinositide pathway, and could lower inositol levels through this mechanism More research is required to fully explain the role that lithium and IMPase play in bipolar disorder patients.

Despite the fact that lithium is effective in treating bipolar disorder, it is extremely toxic metal and the toxic dose is only marginally greater than the therapeutic dose. A novel inhibitor of IMPase that is less toxic could be a more desirable treatment for bipolar disorder. Such an inhibitor would need to cross the blood–brain barrier in order to reach the IMPase in neurons.