Phospholysine phosphohistidine inorganic pyrophosphate phosphatase **(LHPP)** is a hydrolase enzyme which is known to be expressed in the liver, kidney, and at moderate levels in the brain[1]. It belongs to the haloacid dehalogenase (HAD) superfamily of hydrolases with hydrolyze a wide variety of substrates[2]. LHPP, specifically, hydrolyzes both oxygen-phosphorous bonds in inorganic phosphate and nitrogen-phosphorous bonds in **phospholysine**, **phosphohistidine**, and **imidodiphosphate**. LHPP has been of particular interest to researchers in recent years due to its hypothesized function as a tumor suppressor and thus its role in various cancers[3].
The HAD Superfamily
The haloacid dehalogenase superfamily contains over 79,000 unique sequences of enzymes and is largely made up of enzymes that catalyze phosphoryl transfer[1]. **Phosphatases** (phosphate monoester hydrolases) make up the majority of these enzymes at ~79%, with many of the rest be **ATPases** (phosphoanhydride hydrolases)[2]. While many members of the enzyme family are found predominantly in prokaryotes, 183 human HAD enzymes have been identified, with at least 40 HAD-type phosphatases. This ancient group of enzymes has evolved over time to dephosphorylate a wide variety of substituents including carbohydrates, lipids, DNA, and various amino acid-phosphorylated proteins in humans, though many target small metabolites in intermediary metabolic reactions. These enzyme were originally thought to carry out simple regulatory tasks, but recent research has shown that some of these enzymes play roles in various genetic disorders[1].
Sequentially, there is very low similarity across the HAD phosphatases, so members of the family are instead identified using alignments of amino acid sequences that are based on the presence of **four short signature motifs** that contain conserved catalytic residues present in HAD enzymes. Another similarity between the HAD phosphatase superfamily is that all the enzymes share the same active core structural arrangement, consisting of catalytic machinery residues positioned in a **Rossmann fold**. This super-secondary structure is characterized by an alternating motif of repeating β-α units arranged in three stacked α/β sandwiches. The Rossmann fold of HAD phosphatases also contains three unique structural signatures including the **squiggle**, **flap**, and **cap** domains. These domains allow HAD phosphatases to form different conformational states as well as influence substrate specificity[2].
HAD Phosphatases: Mechanism & Structure
The catalysis mechanism of HAD phosphatases is unique in comparison to other phosphatases and requires the use of an **aspartate residue in the active site**. This residue facilitates a nucleophilic attack and also contributes to these enzymes' lack of sensitivity to common phosphatase inhibitors. This attack is carried out in a two-step phosphoaspartyl transferase mechanism. As previously mentioned, the aspartate residue initiates the nucleophilic attack on the substrate's phosphoryl group, displacing the substrate's leaving group and forming a phosphoaspartyl enzyme intermediate. In the second step, a water molecule initiates a nucleophilic attack on the previously formed intermediate, releasing free phosphate and regenerating the aspartate catalyst. There is also a second Asp residue, designate **Asp+2**, which functions as a general acid/base to protonate the leaving group of the substrate in the first reaction and deprotonate the water molecule in the second reaction[2].
It is also important to mention that all HAD phosphoaspartyl transferases require **Mg2+** as an obligatory cofactor. This cofactor has multiple functions, including positioning of the substrate phosphoryl group in relation to the Asp nucleophile, providing electrostatic stabilization and charge neutralization in the transition state[2].
LHPP-Specific Mechanisms & Structure
Role in Disease
Major Depressive Disorder
Cancer
Thyroid Diseases
References
- ↑ 1.0 1.1 1.2 Gohla A. Do metabolic HAD phosphatases moonlight as protein phosphatases? Biochim Biophys Acta Mol Cell Res. 2019 Jan;1866(1):153-166. doi:, 10.1016/j.bbamcr.2018.07.007. Epub 2018 Jul 18. PMID:30030002 doi:http://dx.doi.org/10.1016/j.bbamcr.2018.07.007
- ↑ 2.0 2.1 2.2 2.3 2.4 Seifried A, Schultz J, Gohla A. Human HAD phosphatases: structure, mechanism, and roles in health and disease. FEBS J. 2013 Jan;280(2):549-71. doi: 10.1111/j.1742-4658.2012.08633.x. Epub 2012 , Jun 13. PMID:22607316 doi:http://dx.doi.org/10.1111/j.1742-4658.2012.08633.x
- ↑ Hindupur SK, Colombi M, Fuhs SR, Matter MS, Guri Y, Adam K, Cornu M, Piscuoglio S, Ng CKY, Betz C, Liko D, Quagliata L, Moes S, Jenoe P, Terracciano LM, Heim MH, Hunter T, Hall MN. The protein histidine phosphatase LHPP is a tumour suppressor. Nature. 2018 Mar 29;555(7698):678-682. doi: 10.1038/nature26140. Epub 2018 Mar, 21. PMID:29562234 doi:http://dx.doi.org/10.1038/nature26140
