TheHK2 gene spans approximately 50kb and consists of 18exons. There is also anHK2pseudogene integrated into a long interspersed nuclear repetitive DNA element located on the X chromosome. Though itsDNA sequence is similar to the cDNA product of the actualHK2mRNA transcript, it lacks anopen reading frame for gene expression.[10]
This gene encodes a 100-kDa, 917-residueenzyme with highly similarN-terminal andC-terminal domains that each form half of the protein.[10][12] This high similarity, along with the existence of a 50-kDa hexokinase (HK4), suggests that the 100-kDa hexokinases originated from a 50-kDa precursor viagene duplication and tandem ligation.[10][11] BothN- andC-terminal domains possesscatalytic ability and can be inhibited by glucose 6-phosphate, though theC-terminal domain demonstrates loweraffinity forATP and is only inhibited at higher concentrations of glucose 6-phosphate.[10] Despite there being two binding sites for glucose, it is proposed that glucose binding at one site induces a conformational change which prevents a second glucose from binding the other site.[13] Meanwhile, the first 12 amino acids of the highlyhydrophobicN-terminal serve to bind the enzyme to themitochondria, while the first 18 amino acids contribute to the enzyme’s stability.[9][11]
As an isoform of hexokinase and a member of the sugar kinase family, hexokinase IIcatalyzes therate-limiting and first obligatory step of glucose metabolism, which is the ATP-dependent phosphorylation of glucose to glucose 6-phosphate.[11] Physiological levels of glucose 6-phosphate can regulate this process by inhibiting hexokinase II asnegative feedback, thoughinorganic phosphate (Pi) can relieve glucose 6-phosphate inhibition.[8][10][11] Pi can also directly regulate hexokinase II, and the double regulation may better suit itsanabolic functions.[8] By phosphorylating glucose, hexokinase II effectively prevents glucose from leaving the cell and, thus, commits glucose to energy metabolism.[10][12] Moreover, its localization and attachment to the OMM promotes the coupling of glycolysis to mitochondrialoxidative phosphorylation, which greatly enhances ATP production to meet the cell’s energy demands.[14][15] Specifically, hexokinase II bindsVDAC to trigger opening of the channel and release mitochondrial ATP to further fuel the glycolytic process.[8][15]
Another critical function for OMM-bound hexokinase II is mediation of cell survival.[8][9] Activation ofAktkinase maintains HK2-VDAC coupling, which subsequently preventscytochrome c release and apoptosis, though the exact mechanism remains to be confirmed.[8] One model proposes that hexokinase II competes with the pro-apoptotic proteinsBAX to bind VDAC, and in the absence of hexokinase II, BAX inducescytochrome c release.[8][15] In fact, there is evidence that hexokinase II restrictsBAX andBAK oligomerization and binding to the OMM. In a similar mechanism, the pro-apoptoticcreatine kinase binds and opens VDAC in the absence of hexokinase II.[8] An alternative model proposes the opposite, that hexokinase II regulates binding of the anti-apoptotic proteinBcl-Xl to VDAC.[15]
In particular, hexokinase II is ubiquitously expressed in tissues, though it is majorly found inmuscle andadipose tissue.[8][10][15] Incardiac andskeletal muscle, hexokinase II can be found bound to both the mitochondrial andsarcoplasmic membrane.[16] HK2 gene expression is regulated by a phosphatidylinositol 3-kinaselp70 S6 proteinkinase-dependent pathway and can be induced by factors such asinsulin,hypoxia, cold temperatures, and exercise.[10][17] Its inducible expression indicates its adaptive role in metabolic responses to changes in the cellular environment.[17]
Hexokinase II is highly expressed in severalcancers, includingbreast cancer andcolon cancer.[9][15][18] Its role in coupling ATP fromoxidative phosphorylation to the rate-limiting step of glycolysis may help drive thetumor cells’ growth.[15] Notably, inhibition of hexokinase II has demonstrably improved the effectiveness of anticancer drugs.,[18] Thus, hexokinase II stands as a promising therapeutic target, though considering its ubiquitous expression and crucial role in energy metabolism, a reduction rather than complete inhibition of its activity should be pursued.[15][18]
A study onnon-insulin-dependent diabetes mellitus (NIDDM) revealed low basal glucose 6-phosphate levels in NIDDM patients that failed to increase with the addition of insulin. One possible cause is decreased phosphorylation of glucose due to a defect in hexokinase II, which was confirmed in further experiments. However, the study could not establish any links between NIDDM and mutations in theHK2 gene, indicating that the defect may lie in hexokinase II regulation.[10]
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^abcdefghijOkatsu K, Iemura S, Koyano F, Go E, Kimura M, Natsume T, Tanaka K, Matsuda N (Nov 2012). "Mitochondrial hexokinase HKI is a novel substrate of the Parkin ubiquitin ligase".Biochemical and Biophysical Research Communications.428 (1):197–202.doi:10.1016/j.bbrc.2012.10.041.PMID23068103.
^abcPeng Q, Zhou J, Zhou Q, Pan F, Zhong D, Liang H (2009). "Silencing hexokinase II gene sensitizes human colon cancer cells to 5-fluorouracil".Hepato-Gastroenterology.56 (90):355–60.PMID19579598.
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