Pyruvate kinase isoenzyme type M2 (alias M2-PK, alias PKM2) is one of four pyruvate kinase isoenzymes which differ widely in their occurrence according to the type of tissue, their kinetic characteristics and regulation mechanisms. The three other pyruvate kinase isoenzymes are type M1, type L and type R. The PKM-gene encodes for pyruvate kinase isoenzyme type M2 as well as pyruvate kinase isoenzyme type M1.

The human PKM gene is 32,315 kb long and consists of 12 exons and 11 introns.

Pyruvate kinase isoenzymes type M1 and type M2 are different splicing products of the PKM gene (exon 9 for M1-PK and exon 10 for M2-PK). Both mRNAs are 1593 base pairs long and differ from another within 160 nucleotide residues from 1143-1303. The PKM gene is induced by hormones, mitogenic pathways and nutrients. The thyroid gland hormone triiodothyronine (T3) induces PKM gene expression in rat pituitary cells and the monomeric form of the PKM protein has been identified as T3-receptor. Interleukin 2 stimulates PKM transcription in proliferating thymocytes, resulting in increased PKM2 mRNA and protein levels in the S phase of the cell cycle. In NIH3T3 L1 adipocytes PKM gene expression is induced by insulin. Evidence for a role of hypoxia, and the key nutrients glucose and glutamine, in the regulation of PKM gene expression has also been reported. The regulation of the PKM gene at the promoter level is, however, not well understood. The PKM gene contains putative DNA-consensus binding sites for the transcription factors Sp1 and Sp3 (GC-boxes) and Sp1/Sp3-dependent stimulation of PKM gene transcription has been demonstrated. The GC-boxes appear to also play a role in glucose-dependent PKM-gene induction. A carbohydrate-response element (ChoRE), which integrates regulation of many glycolytic genes in response to changes in glucose concentration, has not yet been precisely localized in the PKM promoter region. However, putative consensus DNA-binding elements for USF (Upstream stimulating factor), a transcription factor which is involved in glucose-response, HIF-1alpha (Hypoxia-inducible factor) and the oncogenic transcription factor Myc are present within the PKM promoter region. The USF-box (5-CACGTG-3), the HIF-1alpha DNA-binding consensus element (5-RCGTG-3) as well as the MYC consensus element (E-box; 5-CACGTG-3) match the consensus core DNA-binding sites (5-CACGTG-3) of the ChoRE. However, direct evidence for a role of these transcription factors in the stimulation of PKM gene expression has bot been obtained.

No Pseudogenes.

In various references pyruvate kinase isoenzyme type M2 (abbreviations M2-PK or PKM2) has been termed type III, type A, type B, type K or type K4.

Each monomer of PKM2 consists of 531 amino acids and can be subdivided into four domains: the N-domain (aa 1-43), the A-domain (aa 44-116 and 219-389), the B-domain (aa 117-218) and the C-domain (aa 390-531). The molecular weight of the M2-PK monomer is 58 kD. In contrast to the other PK isoenzymes which are characterized by a tetrameric quaternary structure, M2-PK occurs in a tetrameric as well as dimeric form. The dimeric form of M2-PK is the result of intracellular contact between the A-domain of two monomers. The tetrameric form occurs by association of the interface of the C-domains of two dimers. The C-domain contains 44 amino acids of the 56 amino acid stretch (aa 378-434) which differs between M1 and M2-PK-isoenzymes and is responsible for the different kinetic characteristics and regulation mechanisms found for M1 and M2-PK, i.e. fructose 1,6-P2 activation and interaction with different oncoproteins. The cleft formed between the A- and B-domain is the location of the active site of the enzyme. The C-domain (aa 393-531) comprises an inducible nuclear translocation signal.

Pyruvate kinase isoenzyme type M2 is expressed in some differentiated tissues, such as lung, fat tissue, retina, pancreatic islets as well as in all cells with a high rate of nucleic acid synthesis, which include all proliferating cells, such as normal proliferating cells, embryonic cells, adult stem cells and especially tumor cells. In healthy tissues all pyruvate kinase isoenzymes consist of four subunits whereby hybrids of the different forms can also occur. Hybrids between M1 and M2-PK were found in the oesophagus and the stomach. L-PK and M2-PK hybrids were found in the jejunum, colon and rectum. During differentiation of embryonic cells M2-PK is progressively replaced by the respective tissue specific isoenzyme. Conversely, during tumorigenesis the tissue specific isoenzymes disappear and M2-PK is expressed.

Pyruvate kinase type M2 is found predominantly in the cytosol and to a minor extent in the nucleus. Cytosolic M2-PK is associated with other glycolytic enzymes, i.e. hexokinase, glyceraldehyde 3-P dehydrogenase, phosphoglycerate kinase, phosphoglyceromutase, enolase and lactate dehydrogenase in a so-called glycolytic enzyme complex.

Pyruvate kinase (ATP: pyruvate O²-phosphotransferase; EC 2.7.1.40) catalyzes the last step within glycolysis, the dephosphorylation of phosphoenolpyruvate (PEP) to pyruvate while producing one mole of ATP per mole of PEP. Depending upon the tetramer to dimer ratio M2-PK plays a bi-functional role within tumor metabolism. The tetrameric form of M2-PK favors the degradation of glucose to pyruvate and lactate with regeneration of energy due to a high affinity to its substrate PEP. The dimeric form is characterized by a low PEP affinity and is nearly inactive at physiological PEP concentrations. This leads to an expansion of all phosphometabolites above the pyruvate kinase reaction and an increased channeling of glucose carbons into synthetic processes, i.e. DNA, phospholipid and amino acid synthesis. Tumor cells contain high levels of dimeric M2-PK, which has therefore been termed Tumor M2-PK.
The M2-PK tetramer to dimer ratio fluctuates in tumor cells depending upon the concentrations of signal metabolites. High fructose 1,6-P2 levels induce the association of the inactive dimeric form of M2-PK to the highly active tetrameric form. When FBP levels drop below a critical value the tetrameric form dissociates to the dimeric form. Dimerization of M2-PK is induced by direct interaction with different oncoproteins, i.e. pp60v-src, A-Raf and HPV-16 E7. The importance of M2-PK for oncogenesis is further underlined by the impairment of the oncogenic activity of activated A-Raf (gag-A-Raf) by a kinase-dead mutant of M2-PK and the enhancement of the transforming activity of gag-A-Raf by ectopically expressed wild type M2-PK. Similarly, a knockdown of M2-PK expression by short hairpin RNA and replacement with M1-PK led to a reduction in tumor growth rate. Peptide aptamers which specifically bind to M2-PK and not to the 96% homologous PK isoenzyme type M1 were found to avoid re-association of M2-PK to the tetrameric form thereby reducing ATP levels and decelerating tumor cell proliferation.
Recent work has shown that the binding of cytosolic promyelocytic leukemia (PML) tumor suppressor protein to M2-PK leads to inhibition of the activity of the tetrameric form of M2-PK which results in a suppression of lactate production. The interaction of M2-PK with HERC-1, PKCdelta and tumor endothelial marker TEM8 has also been reported; however, the physiological functions of these findings are not yet well understood.
Regarding the function of M2-PK in the nucleus both pro-proliferative, but also pro-apoptotic stimuli have been described. Thus, interleukin-3-induced nuclear translocation of M2-PK stimulated cell proliferation, whereas nuclear translocation of M2-PK induced by TT232, H2O2 or UV-irradiation was linked to the induction of caspase independent programmed cell death. Nuclear M2-PK was found to participate in the phosphorylation of histone H1 by direct phosphate transfer from PEP to histone H1. Furthermore, M2-PK was shown to interact with Oct-4 and stimulates transactivation by the transcription factor; however, the functional consequences of these findings have not been elucidated.
The interaction between PKM2 with gonococcal Opa proteins points to a physiological role of M2-PK in bacterial pathogenesis.

It is assumed that M1 and M2-PK diverged shortly before the evolution of fish. The pyruvate kinase amino acid sequence is highly conserved. The homology between human M1-PK and human M2-PK is 96%. Comparison of the M2-PK amino acid sequence between different species revealed the following homologies: human and rat: 93%; human and mus musculus: 93 %; rat and mus musculus 98%; human and S. cerevisiae: 50%.

There is one report which describes a missense mutation and a frame shift mutation in exon 10 of the M gene in three B-lymphoblastoid cell lines established from three Bloom syndrome patients. Exon 10 encodes for the intersubunit contact domain of the M2-PK protein. These mutations have a dominant negative effect leading to inactivation of M2-PK. However, the relevance of these mutations has not yet been determined.