Глюкозо-6-фосфат изомераза: структура, функции и клиническое значение.

Mammalian protein found in Homo sapiens

Glucose 6 phosphate isomerase (GPI), alternatively known as phosphoglucose isomerase/phosphoglucoisomerase (PGI) or phosphohexose isomerase (PHI), is an enzyme ( ) that in humans is encoded by the GPI gene on chromosome 19. 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 the cytoplasm, the gene product functions as a glycolytic enzyme (glucose 6 phosphate isomerase) that interconverts glucose 6 phosphate (G6P) and fructose 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 induces immunoglobulin secretion. The encoded protein is also referred to as autocrine motility factor (AMF) based on an additional function as a tumor secreted cytokine and angiogenic 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]

Functional GPI is a 64 kDa dimer composed of two identical monomers. 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. 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. The large domain, located at the N terminal, and the C terminal of each monomer also contain "arm like" protrusions. Several residues 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.

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 a cis 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. When going from fructose 6 phosphate toward glucose 6 phosphate, the result could be mannose 6 phosphate if carbon C2 is given the wrong chirality, but the enzyme does not permit that result except at a very low, non physiological, rate. Since the reaction is reversible, its direction is determined by G6P and F6P concentrations. and stimulates metastasis. Extracellular GPI is also known to function as a maturation factor. Neuroleukin is a neurotrophic factor for spinal and sensory neurons. It is found in large amounts in muscle, brain, heart, and kidneys. Neuroleukin also acts as a lymphokine secreted by T cells stimulated by lectin. It induces immunoglobulin secretion in B cells as part of a response that activates antibody secreting cells.

Cloning experiments also revealed that GPI is identical to the protein known as autocrine motility factor (AMF). AMF produced and secreted by cancer cells and stimulates cell growth and motility as a growth factor. AMF is thought to play a key role in cancer metastasis by activating the MAPK/ERK or PI3K/AKT pathways. In the PI3K/AKT pathway, AMF interacts with gp78/AMFR to regulate ER calcium release, and therefore protect against apoptosis in response to ER stress.

In some archaea and bacteria glucose 6 phosphate isomerase activity occurs via a bifunctional enzyme that also exhibits phosphomannose isomerase (PMI) activity. Though not closely related to eukaryotic GPIs, the bifunctional enzyme is similar enough that the sequence includes the cluster of threonines and serines that forms the sugar phosphate binding site in conventional GPI. The enzyme is thought to use the same catalytic mechanisms for both glucose ring opening and isomerization for the interconversion of G6P to F6P.

A deficiency of GPI is responsible for 4% of the hemolytic anemias due to glycolytic enzyme deficiencies. Several cases of GPI deficiency have recently been identified. Elevated serum GPI levels have been used as a prognostic biomarker for colorectal, breast, lung, kidney, gastrointestinal, and other cancers. GPI also participates in a positive feedback loop with HER2, 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 using Herceptin/Trastuzumab, and should be considered as an additional target when treating patients.