Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:2.7.1.1 (hexokinase)
5,274 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Complexes made up of the kinases, hexokinase and glycerol kinase, together with the outer mitochondrial membrane voltage-dependent anion channel (VDAC) protein, porin, and the inner mitochondrial membrane protein, the adenine nucleotide translocator, are involved in tumorigenesis, diabetes mellitus, and central nervous system function. Identification of these two mitochondrial membrane proteins, along with an 18 kD protein, as components of the peripheral benzodiazepine receptor, provides independent confirmation of the interaction of porin and the adenine nucleotide translocator to form functional contact sites between the inner and outer mitochondrial membranes. We suggest that these are dynamic structures, with channel conductances altered by the presence of ATP, and that ligand-mediated conformational changes in the porin-adenine nucleotide translocator complexes may be a general mechanism in signal transduction.
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PMID:Microcompartmentation of energy metabolism at the outer mitochondrial membrane: role in diabetes mellitus and other diseases. 807 85

Hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) is associated with mitochondria from brain of various species. The fraction of the bound activity that can be released in soluble form after incubation of the mitochondria with glucose-6-phosphate (Glc-6-P) has been shown to vary markedly among species (F. Kabir and J.E. Wilson, Arch. Biochem. Biophys. 300, 641-650, 1993). A method has been developed by which mitochondria bearing significant amounts of bound hexokinase can be selectively immunoprecipitated by Staphylococcus aureus cells coated with anti-Type I hexokinase antibodies. Treatment of mitochondria from guinea pig, bovine, and human brain with Glc-6-P solubilized approximately 60, 40, and 20% of the bound hexokinase activity, respectively, but had no effect on the extent to which the mitochondria could be immunoprecipitated. This is consistent with the view that the residual hexokinase, resistant to solubilization with Glc-6-P, coexists on the same mitochondria bearing the Glc-6-P-sensitive form, i.e., removal of the latter does not prevent immunoprecipitation mediated by the Glc-6-P-resistant form. Mitochondrial porin has previously been shown to be involved in binding of hexokinase to mitochondria. Four porin species, having a common molecular weight but differing in isoelectric point, were detected, in the same relative amounts, in mitochondria from bovine and rat brain. The extent to which Glc-6-P solubilizes hexokinase from rat and bovine brain mitochondria is approximately 90 and approximately 40%, respectively. Thus the marked difference in sensitivity to solubilization with Glc-6-P cannot be attributed to differences in the relative amounts of different porin species present in rat and bovine mitochondria.
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PMID:Mitochondrial hexokinase in brain: coexistence of forms differing in sensitivity to solubilization by glucose-6-phosphate on the same mitochondria. 817 26

The surface distribution of several proteins (porin, hexokinase, and two proteins associated with microtubules or actin filaments) on the outer membrane of brain mitochondria was analyzed by immunogold labelling of purified mitochondria in vitro. The results suggest the existence of specialized domains for the distribution of porin in the outer mitochondrial membrane. Similarities between the distribution of porin and the distribution of microtubule-associated proteins bound in vitro to mitochondria suggested that mitochondria and microtubules interact by binding microtubule-associated proteins to porin-containing domains of the outer membrane. This hypothesis was supported by biochemical studies on outer mitochondrial proteins involved in in vitro binding of cytoskeleton elements. In vitro interactions between mitochondria and microtubules or neurofilaments were analyzed by electron microscopy. These studies revealed cross-bridging between the outer membrane of mitochondria and the two cytoskeleton elements. Cross-bridging was influenced by ATP hydrolysis and by several proteins associated with the surface of mitochondria or with microtubules. In addition, unidentified proteins which were recognized by antibodies to all intermediate filaments subunits were associated either with the mitochondrial surface or with microtubules. This data suggest the participation of additional cytoplasmic proteins in the interactions between cytoskeleton elements and mitochondria.
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PMID:Interactions between brain mitochondria and cytoskeleton: evidence for specialized outer membrane domains involved in the association of cytoskeleton-associated proteins to mitochondria in situ and in vitro. 820 13

The binding of hexo-/glucokinase and glycerol kinase to mitochondria via the channel forming protein, porin, in pancreatic islet beta-cells and adipocytes, was recently proposed to participate in nutritional signaling, glucose sensing, and the control of high-energy phosphate distribution and oxidative phosphorylation. In this study we demonstrate that polyclonal antisera against purified rat liver porin recognize unique proteins in rat pancreatic islets, adipocytes, and RINm5F cells, each with an apparent M(r) about 2000 smaller than that of liver porin. Immunoblotting of subcellular fractions, the purity of which has been controlled by the distribution of marker proteins, revealed the mitochondrial localization of the cross-reacting proteins. Their enrichment with a method used for the purification of porin proteins, the characteristic behavior during isoelectric focusing, and the specific binding of rat liver hexokinase and glycerol kinase to phospholipid vesicles containing the purified cross-reacting beta-cell or adipocyte proteins strongly suggest their identity with mitochondrial porin. The subtle differences in the apparent M(r) and charge heterogeneity raise the possibility of the existence of porin isoforms expressed in a tissue-specific manner. Anti-porin antisera coimmunoprecipitated hexo-/glucokinase from rat insulinoma cell (RINm5F) and adipocyte mitochondria as determined by subsequent immunoblotting of the immunoprecipitates with polyclonal antisera against yeast hexokinase and rat liver glucokinase, respectively. This indicates that some rat pancreatic glucokinase (54 kDa) and liver hexokinase (102 kDa), respectively, is bound to mitochondrial porin. The major portion of the bound fraction is released from mitochondria after treatment with glucose 6-phosphate. Incubation of RINm5F and fat cells with the insulin releasing sulfonylurea drug, glimepiride (20 nM and 5 microM, respectively) for 30 min reduces the amount of hexo-/glucokinase associated with mitochondria and porin to about 50-30%. The reduced kinase binding activity of porin is preserved after isolation of porin from glimepiride-treated cells, reconstitution into phospholipid vesicles and assaying for glucose 6-phosphate inhibitable binding of rat liver hexokinase. The sulfonylurea tolbutamide (20 microM and 5 mM) is significantly less effective. The sulfonylurea-induced inhibition of hexo-/glucokinase binding to mitochondrial porin does not require glucose metabolism or Ca2+ influx into the cells. These data suggest that the sulfonylurea glimepiride, which is thought to inhibit the ATP-regulated K(+)-channel in beta-cells, may have, in addition, an intracellular site of action in pancreatic islet and adipocyte cells at the level of regulation of gluco-/hexokinase binding to mitochondrial porin.
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PMID:Porin proteins in mitochondria from rat pancreatic islet cells and white adipocytes: identification and regulation of hexokinase binding by the sulfonylurea glimepiride. 831 78

We have identified cDNAs representing three hexokinase mRNAs (Hk1-sa, Hk1-sb, Hk1-sc) by screening mouse spermatogenic cell cDNA libraries with a mouse hepatoma cell line hexokinase (Hk1) cDNA [Arora KK, Fanciulli M, Pederson PL. J Biol Chem 1990; 265:6481-6488]. Although all three cDNAs show 99% identity to the somatic Hk1 cDNA sequence throughout most of their coding region, they differ from this sequence at the 5' end. They contain a common spermatogenic cell-specific sequence and a sequence unique to each cDNA immediately 5' to the common domain. However, they lack the porin-binding domain (PBD) present in this region of Hk1, used for binding to a pore-forming protein in the outer mitochondrial membrane. These observations appear to support a model proposed by others for hexokinase gene evolution in mammals. In addition, we found that Hk1-sb has an internal sequence that is not present in Hk1, Hk1-sa, or Hk1-sc. Moreover, Hk1-sa and Hk1-sb transcripts are developmentally expressed in mouse spermatogenic cells. Hk1-sa mRNA is first expressed during meiosis and continues to be present in postmeiotic germ cells, while the more abundant Hk1-sb mRNA is detected only in postmeiotic germ cells. These and other findings suggest that enzymes encoded by Hk1-sa, Hk1-sb, and Hk1-sc are present only in spermatogenic cells.
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PMID:Unique hexokinase messenger ribonucleic acids lacking the porin-binding domain are developmentally expressed in mouse spermatogenic cells. 839 93

Glycerol transport is commonly cited as the only example of facilitated diffusion across the Escherichia coli cytoplasmic membrane. Two proteins, the glycerol facilitator and glycerol kinase, are involved in the entry of external glycerol into cellular metabolism. The glycerol facilitator is thought to act as a carrier or to form a selective pore in the cytoplasmic membrane, whereas the kinase traps the glycerol inside the cell as sn-glycerol-3-phosphate. We found that the kinetics of glycerol uptake in a facilitator-minus strain are significantly different from the kinetics of glycerol uptake in the wild type. Free glycerol was not observed inside wild-type cells transporting glycerol, and diffusion of glycerol across the cytoplasmic membrane was not the rate-limiting step for phosphorylation in facilitator-minus mutants. Therefore, the kinetics of glycerol phosphorylation are different, depending on the presence or absence of the facilitator protein. We conclude that there is an interaction between the glycerol facilitator protein and glycerol kinase that stimulates kinase activity, analogous to the hexokinase- and glycerol kinase-porin interactions in mitochondria.
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PMID:Glycerol kinase of Escherichia coli is activated by interaction with the glycerol facilitator. 843 2

Several enzymes in the glycolytic pathway are reported to have spermatogenic cell-specific isozymes. We reported recently the cloning of cDNAs representing three unique type 1 hexokinase mRNAs (mHk1-sa, mHk1-sb, and mHk1-sc) present only in mouse spermatogenic cells and the patterns of expression of these mRNAs (Mori et al., 1993: Biol Reprod 49:191-203). The mRNAs contain a spermatogenic cell-specific sequence, but lack the sequence for the porin-binding domain that somatic cell hexokinases use to bind to a pore-forming protein in the outer mitochondrial membrane. We now report the cloning of cDNAs representing three unique human type 1 hexokinase mRNAs (hHK1-ta, hHK1-tb, and hHK1-tc) expressed in testis, but not detected by Northern analysis in other human tissues. These mRNAs also contain a testis-specific sequence not present in somatic cell type 1 hexokinase, but lack the sequence for the porin-binding domain. The hHK1-tb and hHK1-tc mRNAs each contain an additional unique sequence. The testis-specific sequence of the human mRNAs is similar to the spermatogenic cell-specific sequence of the mouse mRNAs. Furthermore, Northern analysis of RNA from mouse, hamster, guinea pig, rabbit, ram, human, and rat demonstrated expression of type 1 hexokinase mRNAs lacking the porin-binding domain in the testes of these mammals. These results suggest that hexokinase may have unique structural or functional features in spermatogenic cells and support a model proposed by others for hexokinase gene evolution in mammals.
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PMID:Testis-specific expression of mRNAs for a unique human type 1 hexokinase lacking the porin-binding domain. 872 88

In vitro incubation of isolated hexokinase isozyme I or isolated dimer of mitochondrial creatine kinase with the outer mitochondrial membrane pore led to high molecular weight complexes of enzyme oligomers. Similar complexes of hexokinase and mitochondrial creatine kinase could be extracted by 0.5% Triton X-100 from homogenates of rat brain. Hexokinase and creatine kinase complexes could be separated by subsequent chromatography on DEAE anion exchanger. The molecular weight, as determined by gel-permeation chromatography, was approximately 400 kDa for both complexes. The Mr suggested tetramers of hexokinase (monomer 100 kDa) and creatine kinase (active enzyme is a dimer of 80 kDa). The composition of the complexes was further characterised by specific antibodies. Besides either hexokinase or creatine kinase molecules the complexes contained porin and adenylate translocator. It was possible to incorporate the complexes into artificial bilayer membranes and to measure conductance in 1 M KCI. The incorporating channels had a high conductance of 6 nS that was asymmetrically voltage dependent. The complexes were also reconstituted in phospholipid vesicles that were loaded with ATP. Complex containing vesicles retained ATP while vesicles reconstituted with pure porin were leaky. The internal ATP could be used by creatine kinase and hexokinase in the complex to phosphorylate external creatine or glucose. This process was inhibited by atractyloside. The hexokinase complex containing vesicles were furthermore loaded with malate or ATP that was gradually released by addition of Ca2+ between 100 and 600 microM. The liberation of malate or ATP by Ca2+ could be inhibited by N-methylVal-4-cyclosporin, suggesting that the porin translocator complex constitutes the permeability transition pore. The results show the physiological existence of kinase porin translocator complexes at the mitochondrial surface. It is assumed that such complexes between inner and outer membrane components are the molecular basis of contact sites observed by electron microscopy. Kinase complex formation may serve three regulatory functions, firstly regulation of the kinase activity, secondly stimulation of oxidative phosphorylation and thirdly regulation of the permeability transition pore.
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PMID:Complexes between kinases, mitochondrial porin and adenylate translocator in rat brain resemble the permeability transition pore. 891 85

A unique cDNA for hexokinase (HK) was identified from poly(A)+ RNA of human reticulocytes by anchored polymerase chain reaction. This appeared to represent the cDNA for the red blood cell (RBC)-specific HK isozyme (HKR) described in our previous study (Murakami et al: Blood 75:770, 1990). Its nucleotide sequence was identical to HKI cDNA except for the 5' extreme end. It lacked the first 62 nucleotides of the HKI coding region: instead, it contained a unique sequence of 60 nucleotides at the beginning of the coding sequence as well as another unique sequence upstream of the putative translation initiation site. It lacked the porin-binding domain which facilitates binding to the mitochondria, thus explaining the exclusive cytoplasmic localization of HKR. It was the major cDNA derived from reticulocytes, consistent with the observation that HKR activity is predominant in reticulocytes. Northern blot analysis showed that it was expressed in the reticulocytes and in the K562 erythroleukemic cell line, but not in a lymphocytic cell line. In the extract of K562 cells, HKR activity co-eluted with the HKR of human RBCs on a MonoQ column (Pharmacia, Piscataway, NJ) chromatography, using a salt gradient elution. The separate genetic control of the RBC-specific HK isozyme explains the clinical reports of two types of HK deficiency, one in which the HK activity was reduced exclusively in the RBC (HKR defect) and another with general decrease of HK activity in several tissues (HKI defect).
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PMID:Identification of the cDNA for human red blood cell-specific hexokinase isozyme. 941 10

Binding of the Type I isozyme of mammalian hexokinase to mitochondria is mediated by the porin present in the outer mitochondrial membrane. Type I hexokinase from rat brain is avidly bound by rat liver mitochondria while, under the same conditions, there is no significant binding to mitochondria from S. cerevisiae. Previously published work demonstrates the lack of significant interaction of yeast hexokinase with mitochondria from either liver or yeast. Thus, structural features required for the interaction of porin and hexokinase must have emerged during evolution of the mammalian forms of these proteins. If these structural features serve no functional role other than facilitating this interaction of hexokinase with mitochondria, it seems likely that they evolved in synchrony since operation of selective pressures on the hexokinase-mitochondrial interaction would require the simultaneous presence of hexokinase and porin capable of at least minimal interaction, and be responsive to changes in either partner that affected this interaction. Recent studies have indicated that a second type of binding site, which may or may not involve porin, is present on mammalian mitochondria. There are also reports of hexokinase binding to mitochondria in plant tissues, but the nature of the binding site remains undefined.
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PMID:Homologous and heterologous interactions between hexokinase and mitochondrial porin: evolutionary implications. 906 7


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