HK2

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This article is about the enzyme. For the dependency injection framework, see HK2 DI Kernel.
Hexokinase 2
PDB 2nzt EBI.png
PDB rendering based on 2nzt.
Available structures
PDB Ortholog search: PDBe, RCSB
Identifiers
Symbols HK2 ; HKII; HXK2
External IDs OMIM601125 MGI1315197 HomoloGene37273 ChEMBL: 2640 GeneCards: HK2 Gene
EC number 2.7.1.1
Orthologs
Species Human Mouse
Entrez 3099 15277
Ensembl ENSG00000159399 ENSMUSG00000000628
UniProt P52789 O08528
RefSeq (mRNA) NM_000189 NM_013820
RefSeq (protein) NP_000180 NP_038848
Location (UCSC) Chr 2:
74.83 – 74.89 Mb
Chr 6:
82.73 – 82.77 Mb
PubMed search [1] [2]

Hexokinase 2 also known as HK2 is an enzyme which in humans is encoded by the HK2 gene on chromosome 2.[1][2] Hexokinases phosphorylate glucose to produce glucose-6-phosphate (G6P), the first step in most glucose metabolism pathways. This gene encodes hexokinase 2, the predominant form found in skeletal muscle. It localizes to the outer membrane of mitochondria. Expression of this gene is insulin-responsive, and studies in rat suggest that it is involved in the increased rate of glycolysis seen in rapidly growing cancer cells. [provided by RefSeq, Apr 2009][2]

Structure[edit]

HK2 is one of four highly homologous hexokinase isoforms in mammalian cells.[3][4][5][6][7]

Gene[edit]

The HK2 gene spans approximately 50 kb and consists of 18 exons. There is also an HK2 pseudogene integrated into a long interspersed nuclear repetitive DNA element located on the X chromosome. Though its DNA sequence is similar to the cDNA product of the actual HK2 mRNA transcript, it lacks an open reading frame for gene expression.[6]

Protein[edit]

This gene encodes a 100-kDa, 917-residue enzyme with highly similar N- and C-terminal domains that each form half of the protein.[6][6][8] 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 via gene duplication and tandem ligation.[6][7] Both N- and C-terminal domains possess catalytic ability and can be inhibited by G6P, though the C-terminal domain demonstrates lower affinity for ATP and is only inhibited at higher concentrations of G6P.[6][6] 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.[9] Meanwhile, the first 12 amino acids of the highly hydrophobic N-terminal serve to bind the enzyme to the mitochondria, while the first 18 amino acids contribute to the enzyme’s stability.[5][7]

Function[edit]

As an isoform of hexokinase and a member of the sugar kinase family, HK2 catalyzes the rate-limiting and first obligatory step of glucose metabolism, which is the ATP-dependent phosphorylation of glucose to G6P.[7] Physiological levels of G6P can regulate this process by inhibiting HK2 as negative feedback, though inorganic phosphate (Pi) can relieve G6P inhibition.[4][6][6][7] Pi can also directly regulate HK2, and the double regulation may better suit its anabolic functions.[4] By phosphorylating glucose, HK2 effectively prevents glucose from leaving the cell and, thus, commits glucose to energy metabolism.[6][6][8] Moreover, its localization and attachment to the OMM promotes the coupling of glycolysis to mitochondrial oxidative phosphorylation, which greatly enhances ATP production to meet the cell’s energy demands.[10][11] Specifically, HK2 binds VDAC to trigger opening of the channel and release mitochondrial ATP to further fuel the glycolytic process.[4][11]

Another critical function for OMM-bound HK2 is mediation of cell survival.[4][5] Activation of Akt kinase maintains HK2-VDAC coupling, which subsequently prevents cytochrome c release and apoptosis, though the exact mechanism remains to be confirmed.[4] One model proposes that HK2 competes with the pro-apoptotic proteins BAX to bind VDAC, and in the absence of HK2, BAX induces cytochrome c release.[4][11] In fact, there is evidence that HK2 restricts BAX and BAK oligomerization and binding to the OMM. In a similar mechanism, the pro-apoptotic creatine kinase binds and opens VDAC in the absence of HK2.[4] An alternative model proposes the opposite, that HK2 regulates binding of the anti-apoptotic protein Bcl-Xl to VDAC.[11]

In particular, HK2 is ubiquitously expressed in tissues, though it is majorly found in muscle and adipose tissue.[4][6][11] In cardiac and skeletal muscle, HK2 can be found bound to both the mitochondrial and sarcoplasmic membrane.[12] HK2 gene expression is regulated by a phosphatidylinositol 3-kinaselp70 S6 protein kinase-dependent pathway and can be induced by factors such as insulin, hypoxia, cold temperatures, and exercise.[6][13] Its inducible expression indicates its adaptive role in metabolic responses to changes in the cellular environment.[13]

Clinical significance[edit]

Cancer[edit]

HK2 is highly expressed in several cancers, including breast cancer and colon cancer.[5][11][14] Its role in coupling ATP from oxidative phosphorylation to the rate-limiting step of glycolysis may help drive the tumor cells’ growth.[11] Notably, inhibition of HK2 has demonstrably improved the effectiveness of anticancer drugs.,[14] Thus, HK2 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.[11][14]

Non-insulin-dependent diabetes mellitus[edit]

A study on non-insulin-dependent diabetes mellitus (NIDDM) revealed low basal G6P 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 HK2, which was confirmed in further experiments. However, the study could not establish any links between NIDDM and mutations in the HK2 gene, indicating that the defect may lie in HK2 regulation.[6]

Interactions[edit]

HK2 is known to interact with:

Interactive pathway map[edit]

Click on genes, proteins and metabolites below to link to respective articles. [§ 1]

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GlycolysisGluconeogenesis_WP534 go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to Entrez go to article go to article go to article go to article go to article go to WikiPathways go to article go to Entrez go to article
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Glycolysis and Gluconeogenesis edit
  1. ^ The interactive pathway map can be edited at WikiPathways: "GlycolysisGluconeogenesis_WP534". 

See also[edit]

References[edit]

  1. ^ Lehto M, Xiang K, Stoffel M, Espinosa R, Groop LC, Le Beau MM, Bell GI (Dec 1993). "Human hexokinase II: localization of the polymorphic gene to chromosome 2". Diabetologia 36 (12): 1299–302. doi:10.1007/BF00400809. PMID 8307259. 
  2. ^ a b "Entrez Gene: HK2 hexokinase 2". 
  3. ^ Murakami K, Kanno H, Tancabelic J, Fujii H (2002). "Gene expression and biological significance of hexokinase in erythroid cells". Acta Haematologica 108 (4): 204–9. doi:10.1159/000065656. PMID 12432216. 
  4. ^ a b c d e f g h i j Okatsu 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. PMID 23068103. 
  5. ^ a b c d Schindler A, Foley E (Dec 2013). "Hexokinase 1 blocks apoptotic signals at the mitochondria". Cellular Signalling 25 (12): 2685–92. doi:10.1016/j.cellsig.2013.08.035. PMID 24018046. 
  6. ^ a b c d e f g h i j k l m n Printz RL, Osawa H, Ardehali H, Koch S, Granner DK (Feb 1997). "Hexokinase II gene: structure, regulation and promoter organization". Biochemical Society Transactions 25 (1): 107–12. doi:10.1042/bst0250107. PMID 9056853. 
  7. ^ a b c d e Ahn KJ, Kim J, Yun M, Park JH, Lee JD (Jun 2009). "Enzymatic properties of the N- and C-terminal halves of human hexokinase II". BMB Reports 42 (6): 350–5. doi:10.5483/bmbrep.2009.42.6.350. PMID 19558793. 
  8. ^ a b Aleshin AE, Zeng C, Bourenkov GP, Bartunik HD, Fromm HJ, Honzatko RB (Jan 1998). "The mechanism of regulation of hexokinase: new insights from the crystal structure of recombinant human brain hexokinase complexed with glucose and glucose-6-phosphate". Structure 6 (1): 39–50. doi:10.1016/s0969-2126(98)00006-9. PMID 9493266. 
  9. ^ Cárdenas, ML; Cornish-Bowden, A; Ureta, T (5 March 1998). "Evolution and regulatory role of the hexokinases.". Biochimica et Biophysica Acta 1401 (3): 242–64. doi:10.1016/s0167-4889(97)00150-x. PMID 9540816. 
  10. ^ Shan D, Mount D, Moore S, Haroutunian V, Meador-Woodruff JH, McCullumsmith RE (Apr 2014). "Abnormal partitioning of hexokinase 1 suggests disruption of a glutamate transport protein complex in schizophrenia". Schizophrenia Research 154 (1-3): 1–13. doi:10.1016/j.schres.2014.01.028. PMID 24560881. 
  11. ^ a b c d e f g h Palmieri D, Fitzgerald D, Shreeve SM, Hua E, Bronder JL, Weil RJ, Davis S, Stark AM, Merino MJ, Kurek R, Mehdorn HM, Davis G, Steinberg SM, Meltzer PS, Aldape K, Steeg PS (Sep 2009). "Analyses of resected human brain metastases of breast cancer reveal the association between up-regulation of hexokinase 2 and poor prognosis". Molecular Cancer Research 7 (9): 1438–45. doi:10.1158/1541-7786.MCR-09-0234. PMC 2746883. PMID 19723875. 
  12. ^ Reid, S; Masters, C (1985). "On the developmental properties and tissue interactions of hexokinase.". Mechanisms of ageing and development 31 (2): 197–212. doi:10.1016/s0047-6374(85)80030-0. PMID 4058069. 
  13. ^ a b Wyatt, E; Wu, R; Rabeh, W; Park, HW; Ghanefar, M; Ardehali, H (3 November 2010). "Regulation and cytoprotective role of hexokinase III.". PLOS ONE 5 (11): e13823. doi:10.1371/journal.pone.0013823. PMID 21072205. 
  14. ^ a b c Peng 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. PMID 19579598. 

Further reading[edit]

This article incorporates text from the United States National Library of Medicine, which is in the public domain.