The HK2 gene spans approximately 50
kb and consists of 18
exons. There is also an HK2pseudogene 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 HK2mRNA transcript, it lacks an
open reading frame for gene expression.[10]
Protein
This gene encodes a 100-kDa, 917-
residueenzyme with highly similar
N- and
C-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 via
gene duplication and tandem ligation.[10][11] 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.[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 highly
hydrophobic N-terminal serve to bind the enzyme to the
mitochondria, while the first 18 amino acids contribute to the enzyme’s stability.[9][11]
Function
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.[11] Physiological levels of G6P can regulate this process by inhibiting HK2 as
negative feedback, though
inorganic phosphate (Pi) can relieve G6P inhibition.[8][10][11] Pi can also directly regulate HK2, and the double regulation may better suit its
anabolic functions.[8] By phosphorylating glucose, HK2 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 mitochondrial
oxidative phosphorylation, which greatly enhances ATP production to meet the cell’s energy demands.[14][15] Specifically, HK2 binds
VDAC to trigger opening of the channel and release mitochondrial ATP to further fuel the glycolytic process.[8][15]
Another critical function for OMM-bound HK2 is mediation of cell survival.[8][9] Activation of
Aktkinase maintains HK2-VDAC coupling, which subsequently prevents
cytochrome c release and apoptosis, though the exact mechanism remains to be confirmed.[8] 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.[8][15] 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.[8] An alternative model proposes the opposite, that HK2 regulates binding of the anti-apoptotic protein
Bcl-Xl to VDAC.[15]
In particular, HK2 is ubiquitously expressed in tissues, though it is majorly found in
muscle and
adipose tissue.[8][10][15] In
cardiac and
skeletal muscle, HK2 can be found bound to both the mitochondrial and
sarcoplasmic membrane.[16] 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.[10][17] Its inducible expression indicates its adaptive role in metabolic responses to changes in the cellular environment.[17]
Clinical significance
Cancer
HK2 is highly expressed in several
cancers, including
breast cancer and
colon cancer.[9][15][18] Its role in coupling ATP from
oxidative phosphorylation to the rate-limiting step of glycolysis may help drive the
tumor cells’ growth.[15] Notably, inhibition of HK2 has demonstrably improved the effectiveness of anticancer drugs.,[18] 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.[15][18]
Non-insulin-dependent diabetes mellitus
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.[10]
^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.
PMID12432216.
S2CID23521290.
^
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.
Further reading
Rual JF, Venkatesan K, Hao T, Hirozane-Kishikawa T, Dricot A, Li N, Berriz GF, Gibbons FD, Dreze M, Ayivi-Guedehoussou N, Klitgord N, Simon C, Boxem M, Milstein S, Rosenberg J, Goldberg DS, Zhang LV, Wong SL, Franklin G, Li S, Albala JS, Lim J, Fraughton C, Llamosas E, Cevik S, Bex C, Lamesch P, Sikorski RS, Vandenhaute J, Zoghbi HY, Smolyar A, Bosak S, Sequerra R, Doucette-Stamm L, Cusick ME, Hill DE, Roth FP, Vidal M (Oct 2005). "Towards a proteome-scale map of the human protein-protein interaction network". Nature. 437 (7062): 1173–8.
Bibcode:
2005Natur.437.1173R.
doi:
10.1038/nature04209.
PMID16189514.
S2CID4427026.
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.
PMID9056853.
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.
PMID19579598.
He HC, Bi XC, Zheng ZW, Dai QS, Han ZD, Liang YX, Ye YK, Zeng GH, Zhu G, Zhong WD (2009). "Real-time quantitative RT-PCR assessment of PIM-1 and hK2 mRNA expression in benign prostate hyperplasia and prostate cancer". Medical Oncology. 26 (3): 303–8.
doi:
10.1007/s12032-008-9120-9.
PMID19003546.
S2CID44560397.
Foster LJ, Rudich A, Talior I, Patel N, Huang X, Furtado LM, Bilan PJ, Mann M, Klip A (Jan 2006). "Insulin-dependent interactions of proteins with GLUT4 revealed through stable isotope labeling by amino acids in cell culture (SILAC)". Journal of Proteome Research. 5 (1): 64–75.
doi:
10.1021/pr0502626.
PMID16396496.
Rodríguez-Enríquez S, Marín-Hernández A, Gallardo-Pérez JC, Moreno-Sánchez R (Dec 2009). "Kinetics of transport and phosphorylation of glucose in cancer cells". Journal of Cellular Physiology. 221 (3): 552–9.
doi:
10.1002/jcp.21885.
PMID19681047.
S2CID45600187.
Fonteyne P, Casneuf V, Pauwels P, Van Damme N, Peeters M, Dierckx R, Van de Wiele C (Aug 2009). "Expression of hexokinases and glucose transporters in treated and untreated oesophageal adenocarcinoma". Histology and Histopathology. 24 (8): 971–7.
PMID19554504.
Peng QP, Zhou JM, Zhou Q, Pan F, Zhong DP, Liang HJ (2008). "Downregulation of the hexokinase II gene sensitizes human colon cancer cells to 5-fluorouracil". Chemotherapy. 54 (5): 357–63.
doi:
10.1159/000153655.
PMID18772588.
S2CID32344187.