Hexokinase I, also known ashexokinase A andHK1, is anenzyme that in humans is encoded by theHK1gene on chromosome 10.Hexokinasesphosphorylateglucose to produceglucose-6-phosphate (G6P), the first step in most glucose metabolism pathways. This gene encodes a ubiquitous form of hexokinase which localizes to theouter membrane of mitochondria. Mutations in this gene have been associated withhemolytic anemia due to hexokinase deficiency.Alternative splicing of this gene results in five transcript variants which encode differentisoforms, some of which are tissue-specific. Each isoform has a distinctN-terminus; the remainder of the protein is identical among all the isoforms. A sixth transcript variant has been described, but due to the presence of severalstop codons, it is not thought to encode a protein. [provided by RefSeq, Apr 2009][5]
TheHK1 gene spans approximately 131kb and consists of 25exons.Alternative splicing of its 5’ exons produces different transcripts in different cell types: exons 1-5 and exon 8 (exons T1-6) are testis-specific exons; exon 6, located approximately 15 kb downstream of the testis-specific exons, is theerythroid-specific exon (exon R); and exon 7, located approximately 2.85 kb downstream of exon R, is the first 5’ exon for the ubiquitously expressed hexokinase I isoform. Moreover, exon 7 encodes the porin-binding domain (PBD) conserved in mammalianHK1 genes. Meanwhile, the remaining 17 exons are shared among all hexokinase I isoforms.
In addition to exon R, a stretch of the proximalpromoter that contains a GATA element, an SP1 site, CCAAT, and an Ets-binding motif is necessary for expression of HK-R in erythroid cells.[6]
This gene encodes a 100 kDahomodimer with a regulatoryN-terminal domain (1-475),catalyticC-terminal domain (residues 476-917), and anα-helix connecting its two subunits.[6][8][9][10] Both terminal domains are composed of a large subdomain and a small subdomain. The flexible region of theC-terminal large subdomain (residues 766–810) can adopt various positions and is proposed to interact with thebase of ATP. Moreover, glucose and G6P bind in close proximity at theN- andC-terminal domains and stabilize a common conformational state of theC-terminal domain.[8][9] According to one model, G6P acts as anallosteric inhibitor which binds theN-terminal domain to stabilize its closed conformation, which then stabilizes a conformation of theC-terminal flexible subdomain that blocks ATP. A second model posits that G6P acts as an active inhibitor that stabilizes the closed conformation and competes with ATP for theC-terminal binding site.[8] Results from several studies suggest that theC-terminal is capable of both catalytic and regulatory action.[11] Meanwhile, the hydrophobicN-terminal lacks enzymatic activity by itself but contains the G6P regulatory site and the PBD, which is responsible for the protein's stability and binding to theouter mitochondrial membrane (OMM).[6][12][10][13]
As one of two mitochondrial isoforms of hexokinase and a member of the sugar kinase family, hexokinase Icatalyzes therate-limiting and first obligatory step of glucose metabolism, which is the ATP-dependent phosphorylation of glucose to G6P.[8][7][10][14] Physiological levels of G6P can regulate this process by inhibiting hexokinase I asnegative feedback, thoughinorganic phosphate (Pi) can relieve G6P inhibition.[8][12][10] However, unlikeHK2 andHK3, hexokinase I itself is not directly regulated by Pi, which better suits its ubiquitouscatabolic role.[7] By phosphorylating glucose, hexokinase I effectively prevents glucose from leaving the cell and, thus, commits glucose to energy metabolism.[8][13][12][10] Moreover, its localization and attachment to the OMM promotes the coupling of glycolysis to mitochondrialoxidative phosphorylation, which greatly enhances ATP production by direct recycling of mitochondrial ATP/ADP to meet the cell's energy demands.[14][10][15] Specifically, OMM-bound hexokinase I bindsVDAC1 to trigger opening of themitochondrial permeability transition pore and release mitochondrial ATP to further fuel the glycolytic process.[10][7]
Another critical function for OMM-bound hexokinase I is cell survival and protection againstoxidative damage.[14][7] Activation ofAktkinase is mediated by hexokinase I-VDAC1 coupling as part of the growth factor-mediated phosphatidyl inositol 3-kinase (PI3)/Akt cell survival intracellular signaling pathway, thus preventingcytochrome c release and subsequent apoptosis.[14][6][10][7] In fact, there is evidence that VDAC binding by the anti-apoptotic hexokinase I and by the pro-apoptoticcreatine kinase are mutually exclusive, indicating that the absence of hexokinase I allows creatine kinase to bind and open VDAC.[7] Furthermore, hexokinase I has demonstrated anti-apoptotic activity by antagonizingBcl-2 proteins located at the OMM, which then inhibitsTNF-induced apoptosis.[6][13]
In particular, hexokinase I is the most ubiquitously expressed isoform out of the four hexokinases, and constitutively expressed in most tissues, though it is majorly found inbrain,kidney, andred blood cells (RBCs).[6][8][13][7][15][10][16] Its high abundance in theretina, specifically the photoreceptor inner segment, outer plexiform layer, inner nuclear layer, inner plexiform layer, and ganglion cell layer, attests to its crucial metabolic purpose.[17] It is also expressed in cells derived fromhematopoieticstem cells, such as RBCs,leukocytes, andplatelets, as well as from erythroid-progenitor cells.[6] Of note, hexokinase I is the sole hexokinase isoform found in the cells and tissues which rely most heavily on glucose metabolism for their function, including brain, erythrocytes, platelets, leukocytes, andfibroblasts.[18] In rats, it is also the predominant hexokinase in fetal tissues, likely due to their constitutive glucose utilization.[12][16]
Mutations in this gene are associated with type 4H ofCharcot–Marie–Tooth disease, also known as Russe-type hereditary motor and sensory neuropathy (HMSNR).[19] Changes in hexokinase I have also been identified to cause both mild and severe forms of congenital hyperinsulinism.[20][21][22] Due to the crucial role of hexokinase I in glycolysis, hexokinase deficiency has been identified as a cause of erythroenzymopathies associated withhereditary non-spherocytic hemolytic anemia (HNSHA). Likewise, hexokinase I deficiency has resulted incerebralwhite matter injury, malformations, and psychomotor retardation, as well as latentdiabetes mellitus and panmyelopathy.[6] Meanwhile, hexokinase I is highly expressed incancers, and its anti-apoptotic effects have been observed in highly glycolytichepatoma cells.[13][6]
Hexokinase I may be causally linked tomood andpsychotic disorders, includingunipolar depression (UPD),bipolar disorder (BPD), andschizophrenia via both its roles in energy metabolism and cell survival. For instance, the accumulation of lactate in the brains of BPD and SCHZ patients potentially results from the decoupling of hexokinase I from the OMM, and by extension, glycolysis from mitochondrial oxidative, phosphorylation. In the case of SCHZ, decreasing hexokinase I attachment to the OMM in theparietal cortex resulted in decreased glutamate reuptake capacity and, thus, glutamate spillover from thesynapses. The released glutamate activates extrasynaptic glutamate receptors, leading to altered structure and function of glutamate circuits,synaptic plasticity, frontal cortical dysfunction, and ultimately, the cognitive deficits characteristic of SCHZ.[15] Similarly, hexokinase I mitochondrial detachment has been associated withhypothyroidism, which involves abnormal brain development and increased risk fordepression, while its attachment leads toneural growth.[14] InParkinson's disease, hexokinase I detachment from VDAC viaParkin-mediatedubiquitylation and degradation disrupts the MPTP ondepolarized mitochondria, consequently blocking mitochondrial localization of Parkin and halting glycolysis.[7] Further research is required to determine the relative hexokinase I detachment needed in various cell types for different psychiatric disorders. This research can also contribute to developing therapies to target causes of the detachment, from gene mutations to interference by factors such asbeta-amyloid peptide andinsulin.[14]
Aheterozygousmissense mutation in theHK1 gene (a change at position 847 from glutamate to lysine) has been linked toretinitis pigmentosa.[23][17] Since thissubstitution mutation is located far from known functional sites and does not impair the enzyme's glycolytic activity, it is likely that the mutation acts through another biological mechanism unique to the retina.[23] Notably, studies in mouse retina reveal interactions between hexokinase I, the mitochondrial metallochaperone Cox11, and the chaperone protein Ranbp2, which serve to maintain normal metabolism and function in the retina. Thus, the mutation may disrupt these interactions and lead to retinal degradation.[17] Alternatively, this mutation may act through the enzyme's anti-apoptotic function, as disrupting the regulation of the hexokinase-mitochondria association by insulin receptors could trigger photoreceptor apoptosis and retinal degeneration.[23][17] In this case, treatments that preserve the hexokinase–mitochondria association may serve as a potential therapeutic approach.[17]
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