Editing Hexokinase

{{short description|Class of enzymes}}
{{Use dmy dates|date=January 2021}}
{{enzyme
| Name = Hexokinase
| EC_number = 2.7.1.1
| CAS_number = 9001-51-8
| GO_code = 0004396
| image = Hexokinase 3O08 structure.png
| width =
| caption  =Crystal structures of hexokinase 1 from ''[[Kluyveromyces lactis]]''.<ref name="Kuettner_2010">{{PDB|3O08}}; {{cite web|vauthors=Kuettner EB, Kettner K, Keim A, Svergun DI, Volke D | url=https://www.wwpdb.org/pdb?id=pdb_00003o08|title = Crystal structure of dimeric KlHxk1 in crystal form I | doi=10.2210/pdb3o08/pdb | year = 2010}}</ref><ref>{{cite journal | last1=Kuettner | first1=E. Bartholomeus | last2=Kettner | first2=Karina | last3=Keim | first3=Antje | last4=Svergun | first4=Dmitri I. | last5=Volke | first5=Daniela | last6=Singer | first6=David | last7=Hoffmann | first7=Ralf | last8=Müller | first8=Eva-Christina | last9=Otto | first9=Albrecht | last10=Kriegel | first10=Thomas M. | last11=Sträter | first11=Norbert | title=Crystal Structure of Hexokinase KlHxk1 of Kluyveromyces lactis | journal=Journal of Biological Chemistry | publisher=Elsevier BV | volume=285 | issue=52 | year=2010 | issn=0021-9258 | doi=10.1074/jbc.m110.185850 | pages=41019–41033| doi-access=free | pmc=3003401 }}</ref>
}}
{{infobox protein
| Name = [[HK1|hexokinase 1]]
| caption = Hexokinase 1, homodimer, Human
| image = 1hkc.jpg
| width = 270
| HGNCid = 4922
| Symbol = [[HK1]]
| AltSymbols =
| EntrezGene = 3098
| OMIM = 142600
| RefSeq = NM_000188
| UniProt = P19367
| PDB =
| ECnumber =
| Chromosome = 10
| Arm = q
| Band = 22
| LocusSupplementaryData =
}}
{{infobox protein
| Name = [[HK2|hexokinase 2]]
| caption =
| image =
| width =
| HGNCid = 4923
| Symbol = [[HK2]]
| AltSymbols =
| EntrezGene = 3099
| OMIM = 601125
| RefSeq = NM_000189
| UniProt = P52789
| PDB =
| ECnumber =
| Chromosome = 2
| Arm = p
| Band = 13
| LocusSupplementaryData =
}}
{{infobox protein
| Name = [[HK3|hexokinase 3 (white cell)]]
| caption =
| image =
| width =
| HGNCid = 4925
| Symbol = [[HK3]]
| AltSymbols =
| EntrezGene = 3101
| OMIM = 142570
| RefSeq = NM_002115
| UniProt = P52790
| PDB =
| ECnumber =
| Chromosome = 5
| Arm = q
| Band = 35.2
| LocusSupplementaryData =
}}
{{Infobox protein family
| Symbol = Hexokinase_1
| Name = Hexokinase_1
| image = PDB 1v4t EBI.jpg
| width =
| caption = crystal structure of human glucokinase
| Pfam = PF00349
| Pfam_clan = CL0108
| InterPro = IPR022672
| SMART =
| PROSITE = PDOC00370
| MEROPS =
| SCOP = 1cza
| TCDB =
| OPM family =
| OPM protein =
| CAZy =
| CDD =
}}
{{Infobox protein family
| Symbol = Hexokinase_2
| Name = Hexokinase_2
| image = PDB 1bg3 EBI.jpg
| width =
| caption = rat brain hexokinase type i complex with glucose and inhibitor glucose-6-phosphate
| Pfam = PF03727
| Pfam_clan = CL0108
| InterPro = IPR022673
| SMART =
| PROSITE = PDOC00370
| MEROPS =
| SCOP = 1cza
| TCDB =
| OPM family =
| OPM protein =
| CAZy =
| CDD =
}}
A '''hexokinase''' is an [[enzyme]] that irreversibly [[phosphorylation|phosphorylates]] [[hexoses]] (six-carbon [[sugar]]s), forming hexose phosphate. In most organisms, [[glucose]] is the most important [[substrate (biochemistry)|substrate]] for hexokinases, and [[glucose-6-phosphate]] is the most important product. Hexokinase possesses the ability to transfer an inorganic phosphate group from ATP to a substrate.

Hexokinases should not be confused with [[glucokinase]], which is a specific hexokinase found in the liver.  All hexokinases are capable of phosphorylating several hexoses but hexokinase IV(D) is often misleadingly called glucokinase, though it is no more specific for glucose than the other mammalian isoenzymes.<ref name="Cardenas1984">{{cite journal | doi= 10.1042/bj2220363 | journal = Biochemical Journal  | last1 = Cárdenas | first1 = María Luz | last2 = Rabajille | first2 = E. |  last3 = Niemeyer | first3 = H. | title = Fructose is a good substrate for rat liver glucokinase (hexokinase D) | volume =222 | number=2 | year =1984 | pages=363–370| pmid = 6477520 | pmc = 1144187 }}</ref>

==Variation==
[[Gene]]s that encode hexokinase have been discovered in every domain of life, and exist among a variety of species that range from [[bacteria]], [[yeast]], and [[plant]]s to humans and other [[vertebrate]]s. The enzymes from yeast, plants and vertebrates all show clear sequence evidence of homology, but those of bacteria may not be related.<ref name="Cardenas1998">{{cite journal | doi= 10.1016/S0167-4889(97)00150-X | journal = Biochimica et Biophysica Acta | volume = 1401 | number = 3 | pages= 242–264 | title = Evolution and regulatory role of the hexokinases  | last1 = Cárdenas | first1= María Luz | last2 = Cornish-Bowden | first2 = A. | last3 = Ureta | first3= T. | year=1998| pmid = 9540816 }}</ref>

They are categorized as ''actin fold'' proteins, sharing a common [[adenosine triphosphate|ATP]] binding site core that is surrounded by more variable sequences which determine substrate affinities and other properties.

Several hexokinase isoenzymes that provide different functions can occur in a single [[species]].

==Reaction==
The intracellular reactions mediated by hexokinases can be typified as:
:Hexose-CH<sub>2</sub>OH + MgATP{{su|p=2&minus;}} → Hexose-CH<sub>2</sub>O-PO{{su|b=3|p=2&minus;}} + MgADP{{su|p=&minus;}} + H<sup>+</sup>
where hexose-CH<sub>2</sub>OH represents any of several hexoses (like glucose) that contain an accessible -CH<sub>2</sub>OH moiety.
[[File:Hexokinase-glucose.png|class=skin-invert-image|Action of Hexokinase on Glucose|494x494px]]

==Consequences of hexose phosphorylation==
Phosphorylation of a hexose such as glucose often limits it to a number of intracellular metabolic processes, such as [[glycolysis]] or [[glycogen]] synthesis. This is because phosphorylated hexoses are charged, and thus more difficult to transport out of a cell.

In patients with [[essential fructosuria]], metabolism of fructose by hexokinase to [[fructose-6-phosphate]] is the primary method of metabolizing dietary fructose; this pathway is not significant in normal individuals.

==Size of different isoforms==
Most bacterial hexokinases are approximately 50&nbsp;kDa in size. Multicellular organisms including  plants and animals often have more than one hexokinase isoform. Most are about 100&nbsp;kDa in size and consist of two halves (N and C terminal), which share much sequence homology. This suggests an evolutionary origin by duplication and fusion of a 50&nbsp;kDa ancestral hexokinase similar to those of bacteria.

==Types of mammalian hexokinase==
There are four important [[mammal]]ian hexokinase isozymes ({{EC number|2.7.1.1}}) that vary in subcellular locations and kinetics with respect to different substrates and conditions, and physiological function. They were designated hexokinases A, B, C, and D on the basis of their electrophoretic mobility.<ref>{{cite journal | doi= 10.1016/0006-291X(64)90038-5 | title = Multiple molecular forms of ATP:hexose 6-phosphotransferase from rat liver | last1=González | first1 = C. | last2 = Sánchez  | first2 = R. | last3 = Ureta | first3 = T. | last4 = Niemeyer | first4 = H. |journal = Biochemical and Biophysical Research Communications  | volume= 16 | number = 4 | year=1964 | pages=347-352| pmid = 5871820 }}</ref> The alternative names hexokinases I, II, III, and IV (respectively)<ref>{{cite journal | doi= 10.1016/0006-291X(65)90472-9 |journal = Biochemical and Biophysical Research Communications  | last1 = Katzen  | first1 = H. M. | last2 = Sodermann |first2 = D. D.  | last3 = Nitowsky | first3 = H. M. | volume=19  | number = 3 | pages = 377–382 | title = Kinetic and electrophoretic evidence for multiple forms of glucose–ATP phosphotransferase activity from human cell cultures and rat liver|year = 1965 |pmid = 14317406 }}</ref> proposed later are widely used.

===Hexokinases I, II, and III===
Hexokinases I, II, and III are referred to as low-''K''<sub>m</sub> isoenzymes because of a high affinity for glucose (below 1 mM). Hexokinases I and II follow [[Michaelis-Menten kinetics]] at physiological concentrations of substrates.{{citation needed|date=April 2016}} All three are strongly [[Enzyme inhibitor|inhibited]] by their product, [[glucose-6-phosphate]]. [[Molecular mass]]es are around 100 kDa. Each consists of two similar 50kDa halves, but only in hexokinase II do both halves have functional active sites.

* Hexokinase I/A is found in all mammalian tissues, and is considered a "housekeeping enzyme," unaffected by most physiological, hormonal, and metabolic changes.
* Hexokinase II/B constitutes the principal regulated isoenzyme in many cell types and is increased in many cancers. It is the hexokinase found in muscle and heart. Hexokinase II is also located at the mitochondria outer membrane so it can have direct access to ATP.<ref>{{cite web|url=https://www.uniprot.org/uniprot/P52789#subcellular_location|website=uniprot.org|title=Hexokinase data on Uniprot}}</ref> The relative specific activity of hexokinase II increases with pH at least in a pH range from 6.9 to 8.5.<ref name = "Šimčíková_2019">{{cite journal | vauthors = Šimčíková D, Heneberg P | title = Identification of alkaline pH optimum of human glucokinase because of ATP-mediated bias correction in outcomes of enzyme assays | journal = Scientific Reports | volume = 9 | issue = 1 | pages = 11422 | date = August 2019 | pmid = 31388064 | pmc = 6684659 | doi = 10.1038/s41598-019-47883-1 | bibcode = 2019NatSR...911422S }}</ref>
* Hexokinase III/C is substrate-inhibited by glucose at physiological concentrations. Little is known about the regulatory characteristics of this isoenzyme.

===Hexokinase IV ("glucokinase")===
{{Main|Glucokinase}}

Mammalian hexokinase IV, also referred to as [[glucokinase]], differs from other hexokinases in kinetics and functions.

The location of the [[phosphorylation]] on a subcellular level occurs when glucokinase translocates between the [[cytoplasm]] and [[Cell nucleus|nucleus]] of [[liver]] cells.  Glucokinase can only phosphorylate glucose if the concentration of this substrate is high enough; it does not follow [[Michaelis–Menten kinetics|Henri–Michaelis–Menten kinetics]], and has no ''K''<sub>m</sub>; It is half-saturated at glucose concentrations 100 times higher than those of hexokinases I, II, and III.

Hexokinase IV is monomeric, about 50kDa, displays positive cooperativity with glucose, and is not [[allosterically]] inhibited by its product, glucose-6-phosphate.<ref name="Cardenas1998" />

Hexokinase IV is present in the [[liver]], [[pancreas]], [[hypothalamus]], [[small intestine]], and perhaps certain other [[neuroendocrine]] cells, and plays an important regulatory role in [[carbohydrate metabolism]]. In the [[beta cell|β cells]] of the pancreatic [[islets of Langerhans|islet]]s, it serves as a glucose sensor to control [[insulin]] release, and similarly controls [[glucagon]] release in the [[alpha cell|α cells]]. In [[hepatocyte]]s of the liver, glucokinase responds to changes of ambient glucose levels by increasing or reducing glycogen synthesis.

==In glycolysis==
Glucose is unique in that it can be used to produce ATP by all cells in both the presence and absence of molecular oxygen (O<sub>2</sub>). The first step in [[glycolysis]] is the [[phosphorylation]] of glucose by hexokinase.

{{Enzymatic Reaction
|forward_enzyme=Hexokinase
|reverse_enzyme=
|substrate=<small>D</small>-[[Glucose]]
|product=α-<small>D</small>-[[Glucose-6-phosphate]]
|reaction_direction_(forward/reversible/reverse)=forward
|minor_forward_substrate(s)=[[adenosine triphosphate|ATP]]
|minor_forward_product(s)  =[[adenosine diphosphate|ADP]]
|minor_reverse_substrate(s)=
|minor_reverse_product(s)  =
|substrate_image=D-glucose wpmp.svg
|product_image=Alpha-D-glucose-6-phosphate wpmp.png
}}
{{KEGG compound|C00031}} {{KEGG enzyme|2.7.1.1}} {{KEGG compound|C00668}} {{KEGG reaction|R01786}}

By catalyzing the phosphorylation of glucose to yield glucose 6-phosphate, hexokinases maintain the downhill concentration gradient that favors the facilitated transport of glucose into cells. This reaction also initiates all physiologically relevant pathways of glucose utilization, including [[glycolysis]] and the [[pentose phosphate pathway]].<ref>{{cite journal | doi = 10.1038/sj.onc.1209595 | last1 = Robey | first1 = RB | last2 = Hay | first2 = N | year = 2006 | title = Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt | journal = Oncogene | volume = 25 | issue = 34| pages = 4683–96 | pmid = 16892082 | s2cid = 25230246 | doi-access =  }}</ref> The addition of a charged [[phosphate]] group at the 6-position of hexoses also ensures 'trapping' of glucose and 2-deoxyhexose glucose analogs (e.g. 2-deoxyglucose, and 2-fluoro-2-deoxyglucose) within cells, as charged hexose phosphates cannot easily cross the cell membrane.

==Association with mitochondria==
Hexokinases I and II can associate physically to the outer surface of the external membrane of [[mitochondria]] through specific binding to a porin, or voltage dependent anion channel. This association confers hexokinase direct access to ATP generated by mitochondria, which is one of the two substrates of hexokinase. Mitochondrial hexokinase is highly elevated in rapidly growing malignant tumor cells, with levels up to 200 times higher than normal tissues. Mitochondrially bound hexokinase has been demonstrated to be the driving force<ref>{{cite journal |vauthors=Bustamante E, Pedersen P |title=High aerobic glycolysis of rat hepatoma cells in culture: role of mitochondrial hexokinase |journal= Proceedings of the National Academy of Sciences |volume=74 |issue=9 |pages=3735–9 |year=1977 |pmid=198801 |doi=10.1073/pnas.74.9.3735 |pmc=431708 |bibcode=1977PNAS...74.3735B|doi-access=free }}</ref> for the extremely high glycolytic rates that take place aerobically in tumor cells (the so-called Warburg effect described by [[Otto Heinrich Warburg]] in 1930).

==Deficiency==
[[Hexokinase deficiency]] is a genetic autosomal recessive disease that causes chronic haemolytic anaemia.  Chronic haemolytic anaemia is caused by a mutation in the gene that  codes for hexokinase. The mutation causes a reduction of the hexokinase activity, and hence hexokinase deficiency.<ref>{{cite web|title=Hexokinase deficiency|url=http://www.enerca.org/anaemias/38/hexokinase-deficiency|website=Enerca|access-date=6 April 2017|archive-date=8 August 2020|archive-url=https://web.archive.org/web/20200808010837/https://www.enerca.org/anaemias/38/hexokinase-deficiency|url-status=dead}}</ref>

== See also ==
*{{annotated link|Allostery}}
*{{annotated link|Enzyme catalysis}}
*{{annotated link|Flexible linker}}
*{{annotated link|Fluorescent glucose biosensors}}
*{{annotated link|Glucokinase}}
*{{annotated link|Glycolysis}}
*{{annotated link|Glycogen}}
*{{annotated link|Glucose 6-phosphatase}}
*{{annotated link|Hexose phosphate uptake}}
*{{annotated link|Insulin}}
*{{annotated link|Protein dynamics#Global flexibility: multiple domains|Protein domain dynamics}}
*{{annotated link|Protein domain#Domains and protein flexibility|Protein flexibility}}

==References==
{{reflist}}

{{glycolysis}}
{{Glycolysis enzymes}}
{{Kinases}}
{{Enzymes}}
{{Portal bar|Biology|border=no}}
[[Category:Glycolysis enzymes]]
[[Category:EC 2.7.1]]
[[Category:Moonlighting proteins]]
[[Category:Glycolysis]]