Introduction
(1dk4) is a homodimeric phosphatase. This protein structure is derived from the the methanogen Methanococcus jannaschii gene MJ0109. Curiously this protein shows both Inositol Monophosphatase (IMPase) activity as well as Fructose-1,6-Bisphosphatase acivity (FBPase)[1]. This protein has a homolog in other archaeabacteria, such as Archaeoglobus fulgidus[2]. This Archaeal phosphatase is thought to be a precursor of many phophatases of higher organisms in Eukaryotes and Bacteria[1].
Methanococcus jannaschii
M. jannaschii are thermophilc methane producing archaeabacteria that were discovered in 1983 by the manned submersible ALVIN[1]. They were sampled from the base of a deep sea hydrothermal vent in 2600m of water, local tempurature was 85°C. The complete 1.66 mega base pairs of its genome has been sequenced, about 1738 genes were identified. Most of them were determined to be homologous to eukaryotic proteins[3].
Structure
The general structure of this enzyme is a homodimer, composed of 252 amino acid residues per monomer. One subunit has a molecular mass of 28.6kDa. IMPase falls within the metallo-phosphatase super family[1]. This particular structure has been crystalized in complex not with its actual ligand but with a (Red and orange), and three inhibitory Zn2+ ions (grey) in the active site of each subunit. Additionally, each subunit possess its own large hydrophilic active site. Normally a Ca2+ ion is interacting with the active site, not shown here[2].
One subunit of MJ0109 is composed of alternating α helix, β sheet, to α helix from the N to C terminus.
The active site is mostly conserved and is similar in sequence to Human IMPase. However, its overall tertiary structure is similar to a monomer of the Sus scrofa domesticus, FBPase (Fructose-1,6-bisphosphatase).
Function
IMPase
IMPase activity within M. jannaschii is mainly limited to the production pathway of a unique inositol, di-myo-inositol-1,1'-phosphate (DIP), using DL-myo-inositol-1-phosphate as substrate. Intracellular DIP concentrations increase usually in response to supraoptimal growth temperatures, and is involved in maintaining high cellular concentration of K+ which sustains optimal enzyme activity[4]. IMPase Km and Kcat at 85°C; 91±16μM and 4.2±0.2/s, respectively[1].
FBPase
Under normal conditions the MJ0109 gene product preferentially metabolizes the second last major step in gluconeogenesis, fructose-1,6-bisphosphate substrate to fructose-6-phosphate. Relative to its IMPase activity, the FBPase activity of MJ0109 is 1.63 times as active[1]. This particular function was not immediately identified by genomic sequence alignment and was later determined using structual and kinetic analysis[1]. FBPase Km and Kcat at 85°C; 38±9μM and 7.0±0.4/s, respectively[1].
FBPase/IMPase
An unusual phosphatase found in several hyperthermophilic organisms which has dual activity toward inositol monophosphate and fructose 1,6-bisphosphate[5].
Inhibitors
As the dual activity indicates, the substrate specificity of this enzyme is particularly low. Proposed evolutionary cousins of this enzyme are regulated by Adenosine monophosphate and Fructose 2,6-bisphosphate. However, there are no such allosteric regulatory binding sites on the MJ0109 protein. Additionally, M. jannaschii IMPase has a IC50 for Li+ of >150mM, compared to the Li+ IC50 of human IMPase <1mM[1].
Evolution
It is generally well received that hyperthermophiles are near the oldest phyletic group, with a very slow rate of evolution[1]. This is due to the rather unforgiving environment toward mutations of these microbes. Given that the MJ0109 protein lacks substrate specificity and regulatory sites, and possess activity of two separate pathways in Eukaryotes; it is a reasonable assumption that this enzyme is an evolutionary snap shot of an original phosphatase approximation[1].
3D Structures of Inositol Monophosphatase
Inositol monophosphatase 3D structures
