Glucose-6-phosphate isomerase (GPI), alternatively known asphosphoglucose isomerase/phosphoglucoisomerase (PGI) orphosphohexose isomerase (PHI), is anenzyme (EC5.3.1.9) that in humans is encoded by theGPIgene on chromosome 19.[4]This gene encodes a member of the glucose phosphate isomerase protein family. The encoded protein has been identified as a moonlighting protein based on its ability to perform mechanistically distinct functions. In thecytoplasm, the gene product functions as a glycolytic enzyme (glucose-6-phosphate isomerase) that interconvertsglucose-6-phosphate (G6P) andfructose-6-phosphate (F6P). Extracellularly, the encoded protein (also referred to as neuroleukin) functions as a neurotrophic factor that promotes survival of skeletal motor neurons and sensory neurons, and as a lymphokine that inducesimmunoglobulin secretion. The encoded protein is also referred to as autocrine motility factor (AMF) based on an additional function as atumor-secretedcytokine andangiogenic factor. Defects in this gene are the cause of nonspherocytic hemolytic anemia, and a severe enzyme deficiency can be associated with hydrops fetalis, immediate neonatal death and neurological impairment.Alternative splicing results in multiple transcript variants. [provided by RefSeq, Jan 2014][5]
Functional GPI is a 64-kDa dimer composed of two identical monomers.[6][7] The two monomers interact notably through the two protrusions in a hugging embrace. The active site of each monomer is formed by a cleft between the two domains and the dimer interface.[6]
GPI monomers are made of two domains, one made of two separate segments called the large domain and the other made of the segment in between called the small domain.[8] The two domains are each αβα sandwiches, with the small domain containing a five-strand β-sheet surrounded by α-helices while the large domain has a six-stranded β-sheet.[6] The large domain, located at theN-terminal, and theC-terminal of each monomer also contain "arm-like" protrusions.[8][9] Severalresidues in the small domain serve to bind phosphate, while other residues, particularly His388, from the large and C-terminal domains are crucial to the sugar ring-opening step catalyzed by this enzyme. Since the isomerization activity occurs at the dimer interface, the dimer structure of this enzyme is critical to its catalytic function.[9]
It is hypothesized that serine phosphorylation of this protein induces a conformational change to its secretory form.[7]
The mechanism that GPI uses to interconvert glucose 6-phosphate and fructose 6-phosphate (aldose to ketose) consists of three major steps: opening the glucose ring, isomerizing glucose into fructose through an enediol intermediate, and closing the fructose ring.[10]
Glucose 6-phosphate binds to GPI in its pyranose form. The ring is opened in a "push-pull" mechanism by His388, which protonates the C5 oxygen, and Lys518, which deprotonates the C1 hydroxyl group. This creates an open chain aldose. Then, the substrate is rotated about the C3-C4 bond to position it for isomerization. At this point, Glu357 deprotonates C2 to create acis-enediolate intermediate stabilized by Arg272. To complete the isomerization, Glu357 donates its proton to C1, the C2 hydroxyl group loses its proton and the open-chain ketose fructose 6-phosphate is formed. Finally, the ring is closed by rotating the substrate about the C3-C4 bond again and deprotonating the C5 hydroxyl with Lys518.[11]
When going from fructose-6-phosphate toward glucose-6-phosphate, the result could bemannose-6-phosphate if carbon C2 is given the wrongchirality, but the enzyme does not permit that result except at a very low, non-physiological, rate.[11]
This gene belongs to the GPI family.[5] The protein encoded by this gene is a dimeric enzyme that catalyzes the reversible isomerization of G6P and F6P.[12][13] Since the reaction is reversible, its direction is determined by G6P and F6P concentrations.[9]
The protein has different functions inside and outside the cell. In thecytoplasm, the protein is involved inglycolysis andgluconeogenesis, as well as the pentose phosphate pathway.[9] Outside the cell, it functions as aneurotrophic factor for spinal and sensory neurons, calledneuroleukin.[13] The same protein is also secreted bycancer cells, where it is calledautocrine motility factor[14] and stimulatesmetastasis.[15] Extracellular GPI is also known to function as a maturation factor.[9][13]
Though originally treated as separate proteins, cloning technology demonstrated that GPI is almost identical to the proteinneuroleukin.[16] Neuroleukin is aneurotrophic factor for spinal and sensory neurons. It is found in large amounts in muscle, brain, heart, and kidneys.[17] Neuroleukin also acts as alymphokine secreted by T cells stimulated by lectin. It induces immunoglobulin secretion inB cells as part of a response that activates antibody-secreting cells.[18]
Cloning experiments also revealed that GPI is identical to the protein known asautocrine motility factor (AMF).[19] AMF produced and secreted by cancer cells and stimulates cell growth and motility as agrowth factor.[20] AMF is thought to play a key role in cancermetastasis by activating theMAPK/ERK orPI3K/AKT pathways.[21][22][23] In the PI3K/AKT pathway, AMF interacts with gp78/AMFR to regulateER calcium release, and therefore protect againstapoptosis in response to ER stress.[21]
In somearchaea andbacteria glucose-6-phosphate isomerase activity occurs via a bifunctionalenzyme that also exhibitsphosphomannose isomerase (PMI) activity. Though not closely related toeukaryotic GPIs, the bifunctional enzyme is similar enough that thesequence includes the cluster ofthreonines andserines that forms the sugar phosphate-binding site in conventional GPI. The enzyme is thought to use the samecatalytic mechanisms for bothglucose ring-opening andisomerization for the interconversion of G6P to F6P.[24]
A deficiency of GPI is responsible for 4% of thehemolytic anemias due to glycolytic enzyme deficiencies.[12][13][25][26] Several cases of GPI deficiency have recently been identified.[27]
Elevated serum GPI levels have been used as a prognosticbiomarker forcolorectal,breast,lung,kidney,gastrointestinal, and othercancers.[7][13] As AMF, GPI is attributed with regulating cell migration during invasion andmetastasis.[7] One study showed that the external layers ofbreast tumor spheroids (BTS) secrete GPI, which inducesepithelial–mesenchymal transition (EMT), invasion, and metastasis in BTS. The GPI inhibitors ERI4P and 6PG were found to block metastasis of BTS but not BTS glycolysis or fibroblast viability. In addition, GPI is secreted exclusively by tumor cells and not normal cells. For these reasons, GPI inhibitors may be a safer, more targeted approach for anti-cancer therapy.[28] GPI also participates in apositive feedback loop withHER2, a major breast cancer therapeutic target, as GPI enhances HER2 expression and HER2 overexpression enhances GPI expression, and so on. As a result, GPI activity likely confers resistance in breast cancer cells against HER2-based therapies usingHerceptin/Trastuzumab, and should be considered as an additional target when treating patients.[23]
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Haga A, Niinaka Y, Raz A (July 2000). "Phosphohexose isomerase/autocrine motility factor/neuroleukin/maturation factor is a multifunctional phosphoprotein".Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology.1480 (1–2):235–244.doi:10.1016/s0167-4838(00)00075-3.PMID11004567.
1dqr: CRYSTAL STRUCTURE OF RABBIT PHOSPHOGLUCOSE ISOMERASE, A GLYCOLYTIC ENZYME THAT MOONLIGHTS AS NEUROLEUKIN, AUTOCRINE MOTILITY FACTOR, AND DIFFERENTIATION MEDIATOR
1g98: CRYSTAL STRUCTURE ANALYSIS OF RABBIT PHOSPHOGLUCOSE ISOMERASE COMPLEXED WITH 5-PHOSPHOARABINONATE, A TRANSITION STATE ANALOGUE
1gzd: CRYSTAL STRUCTURE OF PIG PHOSPHOGLUCOSE ISOMERASE
1gzv: THE CRYSTAL STRUCTURE OF PHOSPHOGLUCOSE ISOMERASE FROM PIG MUSCLE COMPLEXED WITH 5-PHOSPHOARABINONATE
1hm5: CRYSTAL STRUCTURE ANALYSIS OF THE RABBIT D-GLUCOSE 6-PHOSPHATE ISOMERASE (NO LIGAND BOUND)
1hox: CRYSTAL STRUCTURE OF RABBIT PHOSPHOGLUCOSE ISOMERASE COMPLEXED WITH FRUCTOSE-6-PHOSPHATE
1iat: CRYSTAL STRUCTURE OF HUMAN PHOSPHOGLUCOSE ISOMERASE/NEUROLEUKIN/AUTOCRINE MOTILITY FACTOR/MATURATION FACTOR
1iri: Crystal structure of human autocrine motility factor complexed with an inhibitor
1jiq: Crystal Structure of Human Autocrine Motility Factor
1jlh: Human Glucose-6-phosphate Isomerase
1koj: Crystal structure of rabbit phosphoglucose isomerase complexed with 5-phospho-D-arabinonohydroxamic acid
1n8t: The crystal structure of phosphoglucose isomerase from rabbit muscle
1nuh: The crystal structure of human phosphoglucose isomerase complexed with 5-phosphoarabinonate
1xtb: Crystal Structure of Rabbit Phosphoglucose Isomerase Complexed with Sorbitol-6-Phosphate
